Agilent 0.35T MRI/Radiotherapy Magnet System Service Manual · be used to measure the resistance of...

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Agilent

0.35T MRI/Radiotherapy

Magnet System

Service Manual

Document Reference: Viewray SERVICE MANUAL revF.doc

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Contents

1. Daily Checks by User .............................................................................................. 4 1.1 Helium Levels ............................................................................................................... 4

1.2 Operating Pressure of the Cryostat ............................................................................... 4

1.3 Condensation ................................................................................................................ 5

1.4 Compressor Operation .................................................................................................. 5

1.5 PC Monitoring .............................................................................................................. 5

2. Weekly Checks by User .......................................................................................... 6 2.1 Daily Checks ................................................................................................................. 6

2.2 Emergency Discharge Unit ........................................................................................... 6

2.3 Shield temperatures ...................................................................................................... 6

2.4 Room Temperature Shim Currents ............................................................................... 7

2.5 Compressor Cooling Water Flow Rate ......................................................................... 7

3. Service Call Level 1 - by Agilent Technologies .................................................... 8

3.1 Safety Whilst Performing Service Calls and Decommissioning

the Magnet System ....................................................................................................... 8

3.2 System Grounding ........................................................................................................ 9

3.3 Weekly checks .............................................................................................................. 9

3.4 Filling helium ............................................................................................................... 9

3.4.1 Tools and equipment .................................................................................... 10

3.4.2 Procedure ...................................................................................................... 10

3.5 Monitoring the shield temperatures ............................................................................ 11

3.6 Monitoring the cryogen usage .................................................................................... 12

3.7 Monitoring RT Shim currents ..................................................................................... 12

3.8 Monitoring cryocooler runtime and pressures ............................................................ 12

3.9 Checking for icing ...................................................................................................... 12

3.9.1 Observing ice formation ............................................................................... 12

3.9.2 Leak detecting on the turret .......................................................................... 13

3.9.3 Operation of the valves ................................................................................ 14

3.10 Cleaning

4. Service Call Level 2 - by Agilent Technologies .................................................. 15 4.1 Service call level 1 ...................................................................................................... 15

4.2 Changing the bursting disc ......................................................................................... 15

4.3 Changing the cold heads ............................................................................................. 16

4.3.1 Safety while changing the cold head .............................................................. 16

4.3.2 Tools and equipment ....................................................................................... 17

4.3.3 Procedure ........................................................................................................ 17

4.4 Changing the absorber (every second level 2 service call)......................................... 18

4.4.1 Safety considerations ...................................................................................... 18


4.4.3 Procedure ........................................................................................................ 19

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5. Decommissioning the Magnet ............................................................................. 20 5.1 Safety when Decommissioning the Magnet ............................................................... 20

5.2 De-energising the magnet ........................................................................................... 20


5.2.2 Procedure ........................................................................................................ 21

5.3 Removing the helium .................................................................................................. 25


5.3.2 Procedure ........................................................................................................ 25

5.4 Warming the magnet .................................................................................................. 26


5.4.2 Procedure ........................................................................................................ 26

6. Procedures Required Following a Quench ......................................................... 30 Appendix A – Control Measures in Place ................................................................ 31 Tables ......................................................................................................................... 43

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1. Daily Checks by the User

The checks that are detailed in this section should be carried out every day by the user. It is good

practice to keep a separate log of the data from any computerised system, either manually in a

logbook or electronically (with hard copies produced), so that any deviations from normal

behaviour can be easily detected. This should also allow the early detection of any serious

problems. An example of a typical log is shown in table 1. The user should be aware of the hazards

of working with magnet systems and attention is brought to the “Safety Consideration for the

Installation and Operation of Magnet Systems” document provided in the system manual.

1.1 Helium Levels

There is a permanent helium level probe on each magnet half. Electronic access to the main probe is

via the 10-way connectors on the service turrets that are connected to the helium level meter

(E5083). The front display - of the helium level meter is shown in figure 1. The calibrations for

these can be found in the operating manual.

Figure 1 – Front panel of the E5083 Liquid Helium Monitor

It is important that the helium level meter is set to “Normal” during ordinary operation of the

magnet. Setting the meter to “Fill” will significantly increase the helium boil-off.

The alarm level for the helium level meter should be set by an Agilent Technologies installation

engineer. This is indicated in the operating manual.

The level of helium in the magnet should be monitored and recorded daily from the digital display

of the helium level meter. If the helium level falls below the level requiring refilling (Level 1

Service Call), or in the case of any problems with the helium level meter such as no power; the read

light coming on continuously; or the level displaying the same level continuously, then Agilent

Technologies should be contacted.

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1.2 Operating Pressure of the Cryostat

It is extremely important that the helium reservoir is completely sealed, and that no air is allowed to

enter. The helium reservoir should operate slightly above atmospheric pressure (typically 1040

mB). The cryostat pressure is displayed on the front panel of the E5083 level monitor.

1.3 Condensation

The surface and joints of the cryostat should be monitored daily for any indication of condensation.

The entire magnet cryostat should be checked, particularly the service turret and the cold head

turrets for the presence of localised condensation. The service turret, cold head and cold head turret

are shown in figures 2 and 3. Formation of condensation could be the indication of a more serious

problem (such as helium boil-off or blockage of the helium exhaust leading to a dangerous pressure

build-up in the vessel) and so Agilent Technologies should be contacted immediately.

Figure 2 – View of the service turret.

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Figure 3 Schematic diagram and view of cold head turret.

When performing these checks, the user should be aware of various safety concerns. In order to

inspect the turrets, the user will be in close proximity to high magnetic fields (particularly around

the magnet bore), and so should take precautions as detailed in the safety documentation as

provided in the operating manual.

1.4 Compressor Operation

To ensure the correct operation of the cold heads, the cryocooler compressors should be monitored.

On a daily basis, the digital display on the compressors should be checked to ensure that they are

still running as shown in figures 4. Connections to the compressor are also shown in figure 4. The

run-time should also be monitored daily, so that if the compressors stop running, the time at which

that occurred can easily be determined.

The compressor pressure should also be monitored daily. This can be read from the analogue dial

on the compressor unit as shown in figure 4. The compressors should operate at between 2.1 to

2.3 MPa. When reading the pressure, it should be noted that the actual pressure rises and falls over

the range of 0.05 MPa (0.5 bar or 10 psi) for each compression cycle. This is an operating feature of

the compressors, and the user should be aware of this.

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Figure 4 – Front panel of the cryocooler compressor. The analogue dial located top left shows

compressor pressure.

In the event of a problem, then the type of error should be displayed on the display of the

compressor. This information should be passed on when Agilent Technologies are contacted.

1.5 PC Monitoring

If there is a data acquisition system monitoring the status of the magnet, then a daily check should

be made to ensure that the PC is working correctly, and that the data is being stored as required.

Where possible, the data that is taken manually from the instrumentation during the daily checks

should be cross correlated with the data acquired by the PC. A front panel for PC monitoring can be

seen in figure 5.

Figure 5 – Typical front panel for PC monitoring

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2. Weekly Checks by User

The following section gives details of checks that should be performed by the user on a weekly

basis rather than on a daily basis. As before, it is good practice to keep a manual log of these data to

ensure that any deviations from normal behaviour can be easily detected. A typical log sheet for

recording weekly checks is shown in table 2. The user should be aware of the hazards of working

with magnet systems and attention is brought to the “Safety Consideration for the Installation and

Operation of Magnet Systems” document provided in the system manual.

2.1 Daily Checks

All of the checks that are detailed in section 1 (Daily Checks) should be performed on a weekly

basis as well. A weekly log of the helium levels (section 1.1), operating pressure (1.2) condensation

on the magnet (section 1.3) and details of the compressor operation (section 1.4), should be logged

along with the rest of the weekly checks detailed in this section. This should allow the data to be

easily reviewed and early detection of any possible problems with the magnet system.

2.2 Emergency Discharge Unit

The magnet system is fitted with an emergency run down function for the event of a patient

emergency. This is achieved by heating the outer coils to initiate a quench in the main magnet. The

emergency discharge unit (EDU) uses a mains derived dc voltage to drive current into the heaters,

and has a battery back up in the event of a power failure. The front panel of the EDU (E7007) is

shown in figure 6.

Figure 6 – Front panel of the E7007 Emergency Discharge Unit

In order to test that the quench heater circuit and batteries are in satisfactory condition, an EDU

Monitor (E7002 or E7007) is connected to the EDU, which performs an automated test of the EDU

once an hour. The front panel of the EDU Monitor is shown in figure 6. The EDU Monitor has an

illuminated mains switch to show that power is connected. There are two green LEDs to indicate

whether the Heater Status and Battery Voltage are healthy or not. If an error is detected, then an

audible alarm will sound. This may be muted by a switch, which will illuminate if pressed showing

visibly the alarm is muted.

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An internal back-up battery maintains internal power for 2 hours, during which time normal

monitoring operation is maintained. Within the 2 hour period the Heater and Battery LEDs will

continue to show correct colour status but will blink to indicate power loss in the event that the

alarm has been silenced. In the event of a power fail and the internal power is drained, a period of

approximately 15 hours with power applied will be required to restore the 2 hour battery back up

capability.

A manual test of the condition of the battery and heater circuit can also be observed by pressing the

TEST button on the EDU (E7002 or E7007). The BATTERY OK and HEATER OK LED’s will

only illuminate if the battery capacity is okay and the quench heater circuit is intact. This manual

test should be performed to check that the EDU monitor is functioning correctly, and to ensure that

the EDU will function correctly if required.

2.3 Shield Temperatures

The temperatures of the individual thermometers mounted on the thermal shield should be

measured and recorded directly from either the instruments that are reading them or a PC

monitoring system (Figure 5).

If there are no dedicated instruments for reading the thermometers, then the resistances of the

thermometers should be measured directly. The wiring diagrams and location of the thermometers

are provided in the operating manual for the magnet system. A hand-held digital multimeter should

be used to measure the resistance of the thermometer. If it is not possible to perform a four-terminal

resistance measurement as indicated in the wiring diagram, then a two-terminal measurement

should be performed as follows. The total resistance of both the thermometer and leads should be

determined by measuring across V+ to V-. From this the lead resistance should be subtracted, which

can be found by measuring the resistance across V+ to I+. From the calibrations found in the

operating data for the magnet, the temperature of the thermometer can be calculated and recorded.

If direct measurements have to be taken, then care should be taken if it is necessary to move around

the magnet to fit the ten-pin boxes to the correct ten-pin seals. The user will be in close proximity to

high magnetic fields (particularly around the magnet bore), and so should take precautions as

detailed in the safety documentation as provided in the operating manual. If it is necessary to use

the hand-held digital multimeter in high magnetic fields, then it should be remembered that

components of the multimeter including the batteries are highly magnetic.

2.4 Room Temperature Shim Currents

Monitoring the current in the room temperature resistive shim coils (if used) will provide

information on the stability of the superconducting magnet and superconducting shims. These shim

currents should be measured on a weekly basis, and recorded in a log sheet as shown in table 3.

2.5 Compressor Cooling Water Flow Rate

Measuring the flow rate of the cooling water for the compressors will provide early warning of the

compressor shutting down due to insufficient flow or a flow rate that is too great. This flow rate

should be measured on a weekly basis and recorded on the weekly log sheet as shown in table 2.

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3. Service Call – Level 1 – Performed by Agilent Technologies

The following tests and procedures are to be performed each time that the magnet requires re-filling

with helium. Note the magnet operates at zero boiloff, so helium fill will only be necessary in the

event of prolonged power outages. A log sheet for recording tasks and data during a level 1 service

call is shown in table 5, with reference to the sheets shown in tables 1-4.

3.1 Safety Whilst Performing Service Calls and Decommissioning the Magnet System

There are inherent hazards that occur whilst working on or around the superconducting magnet

system. Safe working practices should therefore be employed during any service calls or during

decommissioning of the magnet. For this purpose, the installation engineer should be aware of the

control measures that have been put in place for working with the magnet system, and are contained

in appendix A of this service manual. In addition to this, attention should be drawn to the “Safety

Consideration for the Installation and Operation of Magnet Systems” documentation, the generic

method for working on site, and the risk assessment (RISK/TEST/004) for installation and

commissioning superconducting magnets on customers’ premises.

Of the hazards that are covered in the documentation detailed in the previous paragraph, particular

notice should be paid to the following hazards:

Care should be taken when using cryogenic liquids and gases. Personal protective equipment as

detailed in section C of the control measures should be used at all times when using liquid

cryogens, or working on or near cryogenic pipes and equipment so that cold burns/frostbite can be

avoided. When dealing with cryogenic liquids and gases, long sleeved trousers and jackets,

protective gloves and goggles should be worn. Non-magnetic safety shoes should be worn because

some work will be performed in areas of high magnetic fields. The pressure gauge or the magnet

monitoring should be checked before any operation that involves opening the cryostat to

atmosphere. If the gauge indicates that the cryostat is under vacuum, then the pressure should be

increased by temporarily increasing the system boil-off by switching the helium level meter to fill

(this must be switched back once the desired pressure has been achieved) otherwise it is possible to

suck air into the cryostat which may lead to the formation of ice. Once the magnet is at a positive

pressure with respect to air pressure, it may be safely opened for filling.

Although every attempt will be made to remove helium to the outside atmosphere (control measure

E), engineers should use personal oxygen monitors (control measure C) in case of failure of quench

ducting or the build-up of helium gas in the magnet room during helium transfers or other such

activities. Engineers should be aware of the dangers of asphyxiation from the displacement of

oxygen in enclosed spaces. Particular attention should be paid to the dangers of asphyxiation and

frostbite in the event of a quench during a service procedure. There should be an easy, identified

escape route at all times from where the service engineer is working. Furthermore, if the doors to

the magnet room open inwards, they must be kept open at all times that work is being carried out.

Working with a superconducting magnet system requires the work to be performed in high

magnetic fields. Engineers should be aware of the dangers caused by ferromagnetic objects that will

be attracted towards the magnet. These objects will become projectiles and/or may trap the engineer

against the side of the magnet. This will cause personal injury and/or compromise the safety of the

magnet. All tools used for work near the magnet are to be non-magnetic, as are any transport

Dewar’s for holding liquid cryogens. Ferromagnetic compressed gas cylinders are not permitted

within the 10 Gauss contour from the magnet (control measure G).

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There are suggested maximum limits for working in high magnetic fields as presented by the

National Radiological Protection Board (NRPB). These are a maximum of 2 T in the tissues of the

head neck and trunk and 200 mT time averaged over 24 hours. Possible effects on the body are

vertigo, nausea, cardiac arrhythmia and impaired mental function. Freestanding signs indicating

high magnetic fields (and no entry if required) should be placed in suitable positions to warn of

possible dangers (control measure N).

The size of the outer vacuum vessel requires the operator to work at height. Care should therefore

be taken to observe all the in force local regulations in the country/state of installation. As a

minimum requirement safe working platforms suitable for working at height must always be

provided. Working from ladders is not satisfactory (see also Control Measures in Place at the end

of this document).

Certain work instructions regarding certain service procedures will require manual handling and

lifting operations. Service engineers should carry out these tasks in a safe way (control measure L).

As well as providing this general overview of safety, specific hazards are highlighted in relevant

sections (such as those encountered whilst performing maintenance on the cryocooler systems). The

service engineer must be aware of all hazards that will be encountered on-site as detailed in control

measures B & M.

3.2 System Grounding

At the start of each service call, the magnet should be checked to see that the cryostat is earthed

correctly. If this is not the case, then there is a danger of electric shock whilst performing work on

the magnet. If the earthing cable is disconnected, it should simply be reconnected.

3.3 Weekly Checks

When performing a level 1 service call, both the daily checks detailed in section 1 and the weekly

checks detailed in section 2 should be performed as directed in each relevant section. Additional

checks will required for the helium level (see section 3.6), shield temperatures (see section 3.5),

cryocooler run-time and operating pressure (see section 3.8).

A detailed check of the magnet cryostat for the presence of ice is described in section 3.9.

Condensation of the magnet cryostat can be an indication of ice inside the cryostat, and so if

condensation has been observed during the weekly checks, extra vigilance should be shown whilst

checking for any ice.

In the EDU (E7007 or E002) and EDU Monitor (E5101), there are batteries that are used as a back-

up voltage source in case of a power failure. The length of time that they have been used should be

noted and the batteries replaced if necessary. The maximum amount of time that any batteries

should be used in any equipment is three years. They must be replaced at least every three

years. In the case of a power failure where the batteries may have run down, then the batteries

should tested and replaced if required.

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3.4 Helium Filling

The engineer should be aware of safe working practices when re-filling the magnet with helium

(control measure K), and of the hazards caused by cryogenic liquids (namely frostbite and

asphyxiation) as detailed in section 3.1.

NOTE: there is danger of quenching, rupturing a bursting disc or contaminating the helium vessel

with ice during the helium refill procedure therefore only personnel trained to preform helium refills

on Viewray type magnets should perform this task

3.4.1 Tools and Equipment

All tools used are to be NON-MAGNETIC near the magnet.

Liquid helium (amount required specified in magnet manual) contained in a NON-MAGNETIC

container. There should be enough liquid helium on-site to re-fill the helium vessel.

½" diameter Helium transfer siphon

Helium gas bottle with pressure regulator

Pressure gauge

NW25 fittings

½" OD, ¼" ID rubber hosing. The length of tube should be long enough to extend between the

liquid helium transport vessel and the helium gas bottle that is placed outside the 10 gauss

line/magnetic shield of the magnet

3.4.2 Procedure

During this procedure, it may be necessary to defrost part of the service turret. Motor driven hot air

guns do not function correctly in high magnetic fields, so a suitable means of melting the ice must

be employed. A small flame torch may be used subject to obtaining permission to use it.

Permission should be obtained both locally and from the Agilent Technologies Service Manager

pending the following safety concerns. There should be no sources of fuel (e.g. flammable

materials), and under no circumstances should the flame torch be used during the transfer of

liquid helium where it is possible that there will be liquid oxygen condensed on the transfer lines.

The transfer must have completed so that there is no chance of the formation of liquid oxygen

before the flame gun is used. Secondly, if the flame torch contains ferromagnetic parts, then the

effect of the magnetic field upon the torch should be considered before use.

1. A manual pressure relief valve (as shown in Figure 2) is provided on each magnet half to

allow the gas pressure in the helium vessel to be vented. When the valve is opened it must

be connected to the quench duct (as shown in Figure 2) to prevent helium escaping into the

room.

2. The valve on the magnet half that is being refilled needs to be open during the refill process

to prevent pressure build-up during the helium transfer. The pressure should be slowly

released down to atmospheric pressure. The helium can pressure can be observed on the

E5083 cryo-monitor.

3. Position the NON-MAGNETIC transport vessel in a suitable place next to the magnet.

4. Vent the transport vessel so that it is close to atmospheric pressure.

5. Unscrew the nut holding the siphon entry bung into the cryostat that is located on the service

turret (shown in Figure 2).

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6. Fit the nut, washer and 'O' ring to the cryostat end of the helium siphon, but leave the bung

in place. It is advised that when the magnet is at field that a diffuser is used on the end of

the siphon so that cold gas is not blown onto the magnet.

7. Slowly lower the appropriate end of the transfer siphon into the transport vessel and close

off the pressure relief to force cold gas through the siphon (the warm siphon will create boil

off which will pressurise the vessel). When a dense plume (“flame”) of cold gas is seen to

issue from the siphon, it is ready to be inserted into the cryostat.

8. Open up the pressure relief valve on the end of the magnet that is being filled (the pressure

relief valve on the other half of the magnet will be too distant to provide adequate relief) and

then slowly insert the siphon into the cryostat. Continue to push the siphon into the cryostat

until it hits the cone, then lift the siphon up 20mm.

9. Pressurise the transport Dewar to a maximum of 5 psi to maintain the transfer rate.

10. Monitor the level on the helium monitor (switch the sample rate to FILL). Refer to the

Operating Data manual for a calibration of the level probe.

NOTE: most gas bottles are MAGNETIC, and so should be kept outside the 10 gauss line/magnetic

shield of the magnet.

NOTE: The Helium level probe has an uncalibrated length (as defined in the Operating Data

manual) above the maximum reading.

11. During the entire duration of the transfer of liquid helium, the quench ducting should be

seen to frost up. If this does not happen, or the helium level in the magnet does not increase,

then there may be a blockage preventing helium exhaust leading to a build up of pressure in

the magnet cryostat, or there may be insufficient pressure to maintain the transfer. Excessive

vibration of the quench ducting (which may also be audible), and a defrosting of the ducting

can indicate that liquid is no longer being transferred, only warm gas, which will also cause

excessive boil-off of liquid helium from the magnet cryostat.

12. The helium sensor should be monitored during the entire helium transfer. The helium

monitor should be switched into fill mode to enable a fast sampling rate during the fill. The

helium level should rise steadily throughout the duration of the helium transfer. If the

helium level becomes stuck at in intermediate value, or begins to fall, then there is a

problem with the transfer. For example, ice in the siphon preventing transfer of liquid, the

siphon arm in the transfer vessel not actually being in liquid or there being a restricted flow

of gas from the helium exhaust. The transfer should be stopped immediately, and the

problem identified and rectified.

13. When the cryostat is full, or the transport vessel is empty, vent the pressure in the transport

vessel. Remove the siphon from the magnet cryostat.

14. Replace the siphon entry bung (as shown in figure 7) and close the pressure relief valve as

quickly as possible, to prevent the possibility of air entering the helium can and causing ice

to form. Ensure that the O-ring is in good condition and defrosted before sealing the helium

can.

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Figure 7 – The correct fitting for a bung for sealing the entry port of a magnet as found on the

service turret.

15. Remove the transfer siphon from the transport vessel.

16. Change the sampling rate of the helium level meter to “Normal” once the transfer is

complete.

17. There is risk of pressure build-up after a helium refill particularly if the helium vessel is

very full. Monitor the pressure in the cryostat (using the E5083) over a two hour period

following the refill, if the pressure rises above set pressure (usually 1040 mbar) this could

indicate there is danger of pressure build up.

18. The pressure relief valve is set to open automatically at approximately 1100 mbar

(depending on atmospheric pressure), if the pressure rises above 1150 mbar then the manual

pressure relief valve should be opened to reduce the helium vessel pressure.

NOTE – Any transport vessel, whether empty or containing liquid helium, should not be left

blocking escape routes or regular hallways (control measure D).

3.5 Monitoring the Shield Temperatures

During a Level 1 service call, the resistance output from the temperature sensors should be

measured to ensure that are providing the correct temperatures. All of the thermometers on the

magnet should be checked. The locations of the sensor connections and wiring diagrams can be

found in the operating data. Measurements of the resistance of the thermometers should ideally be

taken each day. This will allow sufficient data to be collected to check that there are no

problems/excessive variations on the sensors. A typical log sheet for recording the resistance and

temperatures of the thermometers is shown in table 4.

Using a ten-pin wiring box, the resistance of the thermometer should be measured using a digital

multimeter. The lead resistance should be subtracted to calculate the actual resistance of the

thermometer (i.e. measure the resistance across V+ to V-, then subtract the resistance across V+ to

I+).

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Site data for the shield temperatures taken since the last service call should be reviewed to detect

any irregularities.

When performing these measurements, it may be necessary to move around the magnet whilst at

field. Care must be taken remove all magnetic objects from your person to minimise the risk of

injury.

3.6 Monitoring Cryogen Usage

As well as monitoring all of the thermometers, the cryogen usage should also be monitored during a

level 1 service call. In order to do this, spot readings of the helium level should be recorded as

directed in section 1.1 every 2-3 hours. Readings should be taken on the main helium level probes

to ensure the accuracy of the data. The data acquired manually may be compared with data from a

computerised data acquisition system to ensure that the data logged is correct. Otherwise daily and

weekly readings taken on site should be reviewed.

The alarm level on the helium level meter should also be checked to ensure that it is still set to the

helium. The alarm level should be set at the correct level to alert the user that the magnet needs

refilling with level shown in the operating manual.

3.7 Monitoring Room Temperature Shim Currents

Site data of the room temperature shim currents (if used) since the last service call should be

reviewed to detect long term drift of the superconducting shims.

3.8 Monitoring Cryocooler Run-Time and Pressures

The cryocooler run-time and operating pressure should be monitored as detailed in the Daily

Readings during a level 1 service call. The absolute run-time should also be noted, because the

lifetime for a cold-head is 10,000 hours and for the compressor absorber is 20,000 hours.

Comparisons with any data stored on a computer using acquisition software should be checked for

accuracy.

3.9 Checking for icing

3.9.1 Observing ice formation

The presence of ice on the wall of the magnet cryostat could be an indication of touch between

components of the magnet or a problem with the cryocooler that could compromise the performance

of the magnet. A manual inspection of all of the external surfaces of the magnet (including the

magnet bore) should be made to check for the formation of ice. Particular attention should be made

to entry points on the service turret (figure 2) and the joints on the top of the cold-head turrets

(figure 3), (i.e. places where there are O-ring seals and valves), which are more liable to be the

location of a leak.

Care should be taken when performing these checks as they require the engineer to work in close

proximity to the magnet, and therefore in high magnetic fields. Engineers should be aware of the

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dangers caused by ferromagnetic objects that will be attracted towards the magnet. These objects

will become projectiles and/or may trap the engineer against the side of the magnet. This will cause

personal injury and/or compromise the safety of the magnet.

When inspecting the bore of the magnet, particular care must be taken because of the magnitude of

the magnetic field as already detailed above.

3.9.2 Operation of the valves/Internal check for icing

In addition to checking the surface of the cryostat, a thorough check should be made of entry ports,

non-return valves and vent ports on the service turret for ice. If the whole of the neck on the service

turret becomes blocked so that there is no passage for the helium gas to escape then a dangerous

pressure build-up will occur in the vessel.

All of the following checks should be conducted if the burst disk is intact. To check if the burst

disk is intact, the procedure for changing the burst disk as detailed in section 4.2 can be followed.

Even if a Burst & Tel Burst Disk is used and the circuit on the disk itself is intact, it should not be

assumed that the burst disk is in a satisfactory state (i.e. there may be a hole/deformation causing a

leak). Testing the cryostat for leaks is detailed in section 3.9.3.

All of the entry ports on the service turret should be checked visually to ensure that they are

properly sealed. These ports use a plug and O-ring; and should be seated as shown in figure 7. The

O-rings should be checked to see that they are not frozen (as this could indicate a leak), and there is

not a build-up of ice in the shim port, current-lead port, or in either of the helium transfer ports.

When the cryostat is sealed, the non-return valve should be checked to ensure that it is not blocked

and is functioning correctly. To do this, the non-return valve can be checked by making it the only

exhaust for helium from the cryostat. This positive pressure should be detected by using a helium

flow gauge. The bypass valve can also be checked in this way by connecting a non-return valve to

the bypass valve and checking for flow of helium gas in the same way. Note that the cryocoolers

will have to be turned off for this operation to allow the magnet to resume normal boiloff mode.

To de-ice the neck take a thin walled stainless steel tube (e.g. nitrogen blow-out tube) and connect it

to a supply of pure helium gas. If a helium gas bottle is used then use a long connecting line,

approximately 15 metres. ON NO ACCOUNT BRING THE GAS BOTTLE WITHIN THE 10

GAUSS CONTOUR. Set a flow of helium gas through the tube and, when the air is judged to have

been displaced insert the tube in turn into the iced-up parts. The helium gas will evaporate solid

condensed air. This procedure should only be used when the magnet is not at field.

When replacing various parts, it may be difficult to fully tighten threads and make the ports air-tight

due to ice in the screw threads. Motor driven hot air guns do not function correctly in high

magnetic fields, so a suitable means of melting the ice must be employed. A small flame torch may

be used subject to obtaining permission to use it. Permission should be obtained both locally and

from the Agilent Technologies Service Manager pending the following safety concerns. There

should be no sources of fuel (e.g. flammable materials), and under no circumstances should a flame

torch be used during the transfer of liquid helium where it is possible that there will be liquid

oxygen condensed on the transfer lines. Secondly, if the flame torch contains ferromagnetic parts,

then the effect of the magnetic field upon the torch should be considered before use.

Care should be taken when performing all of the checks in this section. Every effort should be

made to keep the amount of time that the cryostat is open to air to a minimum so that there is little

chance of a build-up of ice. The engineer should be aware of cold helium gas exhausting from the

vents and ports during these checks and be aware of the dangers from asphyxiation and

17

hypothermia. The engineer should also be aware of safe working practices at heights due to the

location of the cryocooler service turret. The service engineer must verify the turret is leak-tight

before leaving the site.

18

3.9.3 Leak detecting on the turret

As mentioned above, on source of icing are leaks. Once inspected, the turret has therefore to be

sealed and checked for leaks. There are two methods of leak detecting that apply to the service

turret and cryocooler turrets separately, and these are detailed below.

When testing the service turret, there must be a positive pressure in the cryostat from the helium

gas. If the pressure gauge showing the operating pressure of the cryostat is in the vacuum or low

range, then a positive pressure should be produced in the magnet by switching the Helium Level

meter to “Fill” which will temporarily increase the boil-off of the system (this must be switched

back to “Normal” once the desired pressure has been achieved). With a positive pressure, soap

solution should be sprayed over all of the joints and seals. If a leak is present, then with a positive

internal pressure, the soap solution should bubble indicating the source. Note the cryocoolers must

be off for this operation.

The cryocooler turrets may be checked in the same way

3.10 Cleaning

The cryostat can be cleaned using a light water based detergent. If necessary, an isopropanol based

cleaner may be used to remove greasy stains.

19

4. Service Call – Level 2 – Performed by Agilent Technologies

4.1 Service Call Level 1

The main function of a level two service call is to fill the magnet with helium, check the burst disk

on the magnet, to change the cold heads on the magnet and to change the charcoal absorber in the

compressor (every second level 2 service call). Whilst this is taking place, then the checks that are

detailed in as part of a level 1 service call (as detailed in section 3) should be performed so that the

performance of the magnet can be evaluated.

The service engineer should be aware of the safety considerations detailed in section 3.1 and the

control measures detailed in appendix A. During all operations that are performed, the service

engineer should be wearing the correct protective clothing. In high magnetic fields, non-magnetic

safety shoes should be worn. When dealing with cryogenic liquids and gases, long sleeved shirt,

trousers and jackets, protective gloves and goggles should also be worn (control measure C).

4.2 Changing the burst disk (two year intervals)

There is no recommended lifetime of a burst disk or any known form of degradation. The bursting

disc should be checked every two years and replaced if necessary. Generally if the bursting disc is

leak-tight and shows no evidence of damage then it can remain in place.

The burst disk is a graphite burst disk that bursts at a pressure of 5p.s.i. The burst disk can be

accessed by removing the twelve M6 screws from the flange that secures it in place. The location of

the burst disk is shown in figure 2. The burst disk can then be replaced, taking care to ensure that

the O-ring underneath the burst disk is not damaged and remains in place during this operation.

The fittings on the quench port should be correctly replaced to make the helium exhaust of the

cryostat air tight.

Whilst performing this operation, the engineer should be aware of cold helium gases that will vent

through the quench port, and aware of the possible dangers of asphyxiation and of frostbite/cold

burns. A personal oxygen monitor should be kept with the engineer at all times during this

operation.

When replacing the 12 M6 screws, it may be difficult to fully tighten the screws and make the

exhaust air-tight due to ice in the screw threads. Motor driven hot air guns do not function correctly

in high magnetic fields, so a suitable means of melting the ice must be employed. A small flame

torch may be used subject to obtaining permission to use it. Permission should be obtained both

locally and from the Agilent Technologies Service Manager pending the following safety concerns.

There should be no sources of fuel (e.g. flammable materials or liquid oxygen), and under no

circumstances should a flame torch be used during the transfer of liquid helium where it is possible

that there will be liquid oxygen condensed on the transfer lines. Secondly, if the flame torch

contains ferromagnetic parts, then the effect of the magnetic field upon the torch should be

considered before use. Care must be taken when using the flame torch not to direct the flame on to

the burst disk, as this may lead to a compromise of the burst disk performance.

20

4.3 Changing the Cold Heads

The minimum of a cryocooler is 10,0000 hours. The following work instruction describes the safety

issues, tools required and procedure for replacing a cryocooler.

4.3.1 Safety Whilst Changing the Cold Head

There are several safety issues that need to be addressed when changing the cold-head. For further

information regarding installation, service and operational procedures, please consult the Sumitomo

Service and Installation manual, copies of which are included with the magnet Operations Manual.

General information on safety whilst working on magnet systems can be found in section 3.1.

Further information is detailed in this section. The actual procedure for changing a cold-head is

taken from the Agilent Technologies Work Instruction ITE10, and should be treated as confidential

information.

Non-magnetic tools should be used for any mechanical operations. Care should be taken when

moving magnetic fittings and other ferromagnetic objects (such as clamps) as they will be attracted

to a magnetic shield (if used). The cold head is also magnetic, and should be passed to an assistant

at the side of the magnet. No magnetic objects should be lowered over the end past the bore of the

magnet. Furthermore, compressed gas cylinders are usually magnetic, and therefore should be

located far enough away from the iron shield (outside the 10 gauss line) so that the cylinder is not

attracted towards the shield/magnet.

The cold head and flex-lines contain high pressure helium gas (approximately 1.62 MPa (16.2 bar,

235 p.s.i.) under static conditions and in the range 2.1 to 2.3 when operating. Hitting the equipment

with a sharp edge or touching it with a pointed object may cause an explosion or escape of gas.

Handle the equipment with extreme care. The cold head, compressor unit, compressor absorber and

flex lines are pressurized with helium gas. Purge the helium gas from all pressurized components

before disposing. Open the purging valve gradually or this may result in serious injury. The

minimum bending radius of the flex lines is 300 mm (11.81 inches). Bending the flex lines at a

smaller angle may cause explosion or escape of gas, and so this must be avoided.

Do not disassemble the equipment for purposes other than maintenance specified in the service

manual under any circumstances. Disassembling the equipment may result in electric shock,

explosion or escape of gas.

This cryocooler includes a high-voltage section which needs to be handled with extreme care.

Make sure no power is applied to the compressor unit before starting operation when connecting or

disconnecting the cold head power cable. Failing to observe these precautions may result in electric

shock.

When removing the cold-head, every effort should have been made to warm it to room temperature.

However, there may be cold surfaces in the cryocooler sock. Contact with these surfaces should be

avoided without protective clothing as detailed in section 3.1.

Finally, due to the location of the cold-heads, there may be hazards regarding working at heights

and in confined spaces due to the dimensions of the room. Safe working practices should be

employed regarding working at height and manual handling. See also control measures in place as

at the end of this manual.

21


All tools used near the magnet must be NON-MAGNETIC.

30V - 2 Amp power supply.

Helium gas and regulator.


liquid helium transport Dewar and the helium gas bottle that is placed outside the 10 gauss


Non magnetic metric hexagonal Allen Keys set

Cut down 4 mm Allen key (for removing M5 screws under the cold head).

2 × Non magnetic adjustable spanners

Various non magnetic tools

2 × Adjustable spanner, (All non-magnetic) for high pressure gas lines

Blanking flange and ‘O’-Ring

NW25 Flapper valve

4.3.3 Procedure

1. Switch off compressor.

2. Switch off the water chiller (to prevent a low temperature error on the compressor).

3. Disconnect power cable to cold head at compressor.

4. Disconnect the pressure heater cable from the E5083.

5. Connect p.s.u. to pins C-F on the ten pin plug at the cryocooler, switch on p.s.u. passing 1 to

1.5 Amps into heater. (Check that the heater connections for the cryocooler heater with the

magnet operation manual.)

6. Monitor the temperature sensors on the cryocooler 1st and 2nd stages; they should begin to

warm up.

7. Reduce the pressure in the cryostat while the cold head is warming up by opening the ball

valve on the service turret.

8. After approximately 30 minutes the cold head should be warm enough to allow removal.

Disconnect the two gas lines at the cold head.

9. Vent the pressure in the helium vessel to atmospheric pressure by opening the ball valve on

the service turret. The pressure in the helium vessel can be monitored on the E5083. Fitting

a flapper valve (if available) in series with the ball valve is a good way of venting the

cryostat without the risk of sucking air back into the system.

10. Remove the 8 × M5 screws from the top flange and try to remove the cryocooler by pulling

vertically. If it does not move wait for another 15 minutes and try again. The cold head

flange can be jacked up by screwing bolts into the tapped holes provided.

11. With the magnet at field, care should be taken as the cold head is magnetic. The cold head

should be passed to an assistant at the side of the cryostat. Do not lower the cold head over

the end past the bore of the magnet.

22

12. Once the cold head has been removed, quickly cover the turret with a blanking flange fully

seated on the 'O' ring to prevent air entering the turret.

13. Fully warm the cryocooler and ensure the copper flanges, have not been bent when the

cooler was removed.

14. Loosen the clamp that secures the cryocooler to the top flange.

15. Remove the copper flanges from the old cold head. Rebuild on to the new cold head.

16. Ensure that the internal links (inside the turret) are free from ice, then insert the new

cryocooler. You should be careful when handling the cryocooler as the lower portion is

made of thin tubes and may easily be damaged if dropped, etc. With the cryocooler inserted

and pushed down firmly, tighten all the screws. The turret should not be left open to

atmosphere for more than 30 seconds if possible.

17. Check for leaks around the 'O' ring seals.

18. Re-connect the gas lines and cryocooler power cable. Check the gas pressure on gauge on

front of the compressor. It should be 2.1-2.3 MPa when running and (1.6-1.7 Mpa static).

19. Switch on the cryocooler. Monitor the temperature on the cold head. The temperature of the

second stage should reduce to less than 4.3 K in approximately one hour.

20. Reconnect the pressure heater cable on the E5083.

4.4 Changing the compressor absorber (every 2nd Level 2 service call)

The lifetime of an oil mist absorber for a compressor is 20,000 hours. The operator must be in

possession of the relevant Sumitomo Technical Instruction and Operation manuals before

attempting to change an absorber. The procedures in these references must be followed.

4.4.1 Safety Considerations

The cold head and flex-lines contain high pressure helium gas (approximately 1.62 MPa (16.2 bar,

235 p.s.i.) under static conditions and in the range 2.1 to 2.43 MPa (20-24 bar or 290-350 psi) when

operating). Hitting the equipment with a sharp edge or touching it with a pointed object may cause

an explosion or escape of gas. Handle the equipment with extreme care.

Do not disassemble the equipment for purposes other than maintenance specified in this work

instruction under any circumstances. Disassembling the equipment may result in electric shock,

explosion or escape of gas.

The cold head, compressor unit, compressor absorber and flex lines are pressurized with helium

gas. Purge the helium gas from all pressurized components before disposing. The purging valve

must be opened gradually or serious injury may result.

The absorber weight is about 11.0kg. Safe working practices should be employed regarding manual

handling of such component to ensure that no personal injury is sustained, or any component of the

compressor is damaged.

23


All tools used to be NON-MAGNETIC near the magnet.

1″ open-end wrench

1⅛″ open-end wrench

1-3/16″ open-end wrench

Snoop liquid

Cotton wipers

13 mm open-end wrench

Cross head (Phillips) screw driver

4.4.3 Procedure

To replace the absorber, refer to the Sumitomo Technical Instruction Manual section 3-1-1.

24

5. Decommissioning the Magnet

Decommissioning a superconducting magnet is a highly specialised procedure that should only be

performed by personnel trained to do so. Decommissioning may only be performed by trained staff

members, as the system contains a vacuum vessel and cryogens. There is a risk of explosion if

decommissioning is performed incorrectly. In the event of requiring the magnet system to be de-

energised (and, if required, the helium reclaimed and magnet system warmed up), Agilent

Technologies should be contacted.

The sections that follow are the service instructions for Agilent Technologies personnel only, and

should be treated as confidential. The work instructions are based on the Agilent Technologies

Work Instructions ITE47 (Energisation Procedures), ITE 45 (Removal of Helium) and ITE39

(Warm Up Procedure). For further details, these work instructions should be consulted.

5.1 Safety when Decommissioning the Magnet

The service engineer should be aware of all of the safety concerns detailed in section 3.1 and the

control measures put in place in appendix A. During all operations that are performed, the service

engineer should be wearing the correct protective clothing. In high magnetic fields, non-magnetic

safety shoes should be worn. When dealing with cryogenic liquids and gases, long sleeved trousers

and jackets, protective gloves and goggles should also be worn (control measure C).

5.2 De-energising the magnet

This procedure assumes that the magnet is at field and the level of helium is above the minimum

operating level. The amount of helium should not fall below this level at any time during the de-

energisation process. This procedure is to be performed using a Xantrex power supply, the

installation manual will give special instructions for connecting up both magnet halves.


All tools used to be NON-MAGNETIC near the magnet.

De-mountable current lead

De-mountable shim lead

Main current cables

Shim current cable

Electrical service cable

Nut washer and 'O' ring for main current lead

Nut washer and 'O' ring for shim current lead

Xantrex power supply

Liquid helium (if required) contained in a NON-MAGNETIC container(s).

½" diameter Helium transfer siphon

Helium gas bottle with pressure regulator

Pressure gauge

NW25 fittings



line/magnetic shield of the magnet Diode box and cables for connecting to Xantrex PSU

25

5.2.2 Procedure

It is mandatory to decommission a magnet by de-energising and removing liquid helium to a

transport Dewar.

Note it is always advisable to de-energise a magnet using a Xantrex power supply. Figure 8 shows

the Xantrex power supply set-up and the electrical connections at the rear of the power supply unit.

The advantages are that the voltage can be controlled and there is no chance of the internal diodes

clipping and therefore getting the energy of the magnet dissipating in the helium can. Such action

can trigger a quench.

Figure 8 Front and rear views of Xantrex power supply.

During the de-energisation process it is also required that the axial shim switches are open to

reduce the chances of inducing a quench.

The following instructions detail the process of de-energising the magnet:

1. Prior to any connection taking place, ensure that the power supply is functional. Check that

the unit can be powered up, and a voltage is shown on the Xantrex units when the voltage

dial is turned clockwise.

2. Position the power supply at a safe distance from the magnet, in a field of less than 50

Gauss. Choose the position such that the power supply can be removed from the room

without it going too close to the magnet. The distance from the magnet should not be so far

that the cables will not reach.

3. Make sure that a NON MAGNETIC scaffolding is secured safely near to the cryostat, to be

able to reach the service entry ports comfortably.

4. Check that the electrical mains requirement for the power supply is set for the mains voltage

available. Make sure the power supply is turned off and the all of the superconducting

switch heaters are turned off. The voltage and current control dials should be set fully

anticlockwise for both the Master and Slave power supplies.

5. Check that the terminals on the main current lead are not shorting, either to each other or the

outer tube of the lead.

26

6. Connect the main current cables to the copper lugs, note that the cables have different sized

crimped ends to distinguish positive and negative. Ensure that the correct size crimped end

goes to the correct copper lug determined by the stud diameter.

7. Connect the copper lugs to the terminals on top of the current lead. Spring apart with a

screw-driver if necessary, and ensure that the locking screw is firmly tightened. Check that

the lug is firmly gripping the copper and brass terminal on the lead. Check that the studding

or cable ends are not touching thereby creating a short.

8. Connect the other end of the cables to the lugs on the rear power supply unit. Observe the

correct polarity determined by the stud diameter.

9. Connect the 9 way D type cable between the emergency discharge unit and the magnet

power supply unit. This lead will also ensure an indication of voltage across the magnet.

10. Switch on the protection unit whilst ensuring the Xantrex PSUs are turned off. (If the alarm

is activated, press the reset button and ensure the voltage indicator is not less than –1 Volts.

Note: if the voltage indicated is negative, then it means that the magnet is running down and

URGENT attention to catch the magnet will be required (need to switch the Xantrex and

ensure 0 volts across the magnet).

11. Open the by-pass valve to vent down the system.

12. Remove the bung from the main current entry port and move the nut washers and 'O' ring

from the bung to the main current lead. Do not leave the port open to atmosphere for more

than a few seconds as air will condense in the port and will lead to blockage. Push the lead

down gradually. Cold helium gas will emerge from the entry port. If the rush of gas

becomes too great then hold the lead steady, or retract it a few inches, until the rush of gas

decreases. Finally when contact can be felt with the male connector in the cryostat push the

lead home. The length of the male pin in the cryostat is 50mm. Ensure that the lead is

pushed fully home so that contact is made over the whole length. If the lead cannot be

engaged, remove and check for icing. If ice is present, the de-ice the port as described in

section 3.9.2.

13. Rotate the lead gently so that the cables and exhaust port will not touch the shim lead.

Tighten the nut so as to effect a gas tight seal. DO NOT USE FORCE TO ROTATE THE

LEAD.

14. Remove the bung from the shim lead entry port and move the nut washer and 'O' ring from

the bung onto the shim lead. Do not leave the port open to atmosphere for more than a few

seconds as air will condense in the port and will lead to a blockage. Push the lead in

gradually so as to avoid too large a rush of cold gas. When the lead is felt to touch the

connector at the bottom then gently rotate the lead until it is felt to engage in the keyways.

Do not apply downward pressure while rotating the lead to find the keyways. DO NOT

ROTATE THE LEAD ONCE ENGAGED. Push the lead home and tighten the nut to effect

a gas tight seal. If the lead cannot be engaged, remove and check for icing. If ice is present,

the de-ice the port as described in section 3.9.2.

Note: Ensure that the current lead should be inserted first to the magnet. If any evidence is seen that

the magnet has started to run down, then the first immediate action should be taken is to ensure that

the power supply voltage is increased so as to ensure the current flowing through the PSU is close

to the current in the Magnet. After the PSU is up and running THEN you try to insert the shim lead.

27

The current lead must not be removed from the magnet at any time whilst the magnet is being de-

energised.

15. Connect both the gas vents on both the shim lead and current leads are connected to a gas

vent. In order to stop the current lead over-heating, it is important that the cold gas exhausts

past the leads themselves, and so the ports should not be blanked, restricting gas flow.

16. Connect the shim lead to the Shim current controller (Digishim).

17. Switch ON the Xantrex PSUs and the Shim Current Controller (Digishim).

18. The current should be removed from both of the Z2 superconducting shims before

commencing de-energisation. This can be done by means of a “Digishim” unit. Select the

Z2 shims. Activate the Z2 shim heater so that the shim switch is opened. The current in the

shim should be then run down to 0 A using the “ramp to zero” function. Leave the Z2 shim

heater on during the de-energisation process.

19. Turn fully clockwise the current limit and the voltage setting of the Slave power supply

(ONLY THE SLAVE POWER SUPPLY).

20. Set the current limit of the master power supply so that the total current limit is about 5 A

greater than the current required in the magnet. Remember that because two power supply

units are being used in parallel, the actual current limit will be DOUBLE the limit set on the

master power supply.

21. Increase the voltage of the Master Xantrex slowly so as to achieve a current through the

power supply which is approximately 1 Amp lower than the current in the magnet. This is

that the magnet voltage will be –10 to –20 mV and therefore the magnet will be de-

energising rather than energising.

22. Open the SC magnet switch. As soon as the SC switch is opened, a small voltage across the

magnet will be generated. Note for every 1 Amp incorrect of current set in the SC switch

before its open, the maximum voltage across the switch after is opened is about 10-20

millivolts. This effect is only applicable because the Xantrex are voltage driven rather than

current driven.

23. De-energise the magnet for at the same rate as per the energisation instruction.

Note: Under no circumstances you should have more that –3.5 Volts across the magnet, so that you

do not activate the internal diodes. If the de-energisation voltage is less than –1.0 Volts, the alarm

will come on the protection unit. Press the button to acknowledge the alarm.

Warning: In the event that you realize that the voltage across the magnet is approximately -1.0

Volts, and by changing the Xantrex voltage, does not change the voltage across the magnet, you

may have clipped the internal diodes. It is ESSENTIAL that you increase the voltage on the

Xantrex until you get 0.0 Volts across the magnet. After couple of minutes, reduce the voltage of

the Xantrex slowly so that you can continue de-energising the magnet and ensuring you do not

decrease the voltage across the magnet to less than –3.5 Volts.

24. It is recommended that the magnet is parked in persistent mode during these helium fills.

During the helium fill, both of the current and shim leads should be left in place. In order to

persist the magnet, the following procedure should be followed:

28

a) The voltage dial on the master power supply should be slowly reduced to zero. The

value will not be exactly zero because of the interference of the voltage signal (you may

notice an offset of few millivolts).

b) Close the Main SC switch and observe the voltage across the magnet jumps a few

millivolts. This will indicate that the SC magnet switch is closed.

c) Wait for 60 Seconds after the above event to ensure that the switch is properly closed.

d) Reduce the voltage slightly of the Master Xantrex unit until you get a magnet current

reduction of about 1A. Ensure that the voltage across the magnet is not changing. If the

voltage across the magnet reduces, it means the switch is still open.

e) Reduce the voltage of the Master Xantrex slowly. Note the current must be reduced

linearly so as to prevent the switch from opening.

f) In the event that the SC switch opens, the Audible alarm will come on. It is important to

quickly wind the voltage on the Xantrex so that the voltage across the magnet is zero,

and also reset the audible alarm. Set the field to the required accuracy and repeat the

above process to close the switch.

g) When the main SC switch is closed and the current is out of the leads, switch off the

heaters for the axial shim.

25. After the helium fill is complete, then the de-energisation process can be resumed as

described from instructions 17 to 23.

26. The de-energisation process should be planned so that you can reduce the voltage of the

Xantrex to as close to 0.0 volts, but without decreasing the voltage across the magnet below

-3.5 V, before you leave the magnet unattended for overnight continuation of the de-

energisation. This means that the energy from the magnet will be dissipated in the current

leads and the diode in the protection unit. DO NOT switch off the power of the protection

unit during de-energisation of the magnet. The fans to extract the heat from the diodes in the

protection unit must be operational at all times to remove the heat from the unit. Also,

ensure that there is enough helium in the magnet so that the level will still be above

minimum operating level when the engineer resumes work the following morning.

27. When the current is such that the magnetic field produced by the magnet is below 3 T, the

diodes (if available) can be connected in series with the magnet so that the de-energisation

process is completed more quickly. This can be done as follows:

a. The magnet should be placed in persistent mode as described in instruction 24.

b. The diode box should be connected in series with the magnet and the power supply.

c. The de-energisation process should be re-commenced as described in instructions

17-22.

28. When the current in the main magnet reaches zero, then the current in the superconducting

shims should be reduced to zero. This can be done by means of a “Digishim” unit. Each of

the shims should be selected in turn. The heater for the shim switch should be activated so

29

that the shim switch is opened. The current in the shim should be then run down to 0 A

using the “ramp to zero” function. The main magnet switched should be then closed, and the

PSU unit and “Digishim” unit switched off.

29. The main current lead and s/c shim lead can now be withdrawn. Loosen the black nut (it

may be necessary to leave it to warm first if a plug of ice has formed around it). Pull

vertically upward on the lead and withdraw it fully. Quickly replace the nut and baffles so

that air does not condense in the entry port. Wear gloves during the operation as it will be

necessary to hold the body of the cold lead.

Note: Do not attempt to rotate any of the leads on removal from the cryostat as this may damage the

internal connections.

30. After removal of all the leads a final check should be made that the cryostat is properly

fitted with the non-return valve and that all exhaust ports are correctly sealed. Check the

following:

a. Main current baffles are in and sealed properly by tightening the black seal nut.

b. Shim baffles are in and sealed properly by tightening the black seal nut.

c. Syphon baffles are in and sealed properly by tightening the black seal nut.

d. The non-return valve (or helium recovery line) is connected to the main exhaust port.

e. The burst disc is correctly seated and making a gas tight seal.

31. If at any stage of the de-energisation process the magnet quenches, then it is important to

reduce the Master Xantrex voltage to 0.0 V.

5.3 Removing the helium

This section assumes that the magnet has been completely de-energised as described in section 5.2.


Helium transfer siphon

Enough empty (but cold) NON-MAGNETIC helium Dewar’s to capture all of the liquid in the

Helium can (up to a maximum of 500 litres).

Helium gas bottle(s) & regulator

Length of rubber tube

NW25 pressure gauge

NW25 'T' piece

NW25 “Christmas tree” fitting

NW25 Clamps and O-rings

5.3.2. Procedure

1) Turn off the cryocoolers and allow the liquid helium to evaporate through the check valve

2) Remove non-return valve from helium exhaust and replace it with the NW25 'T' piece,

complete with pressure gauge.

3) Remove siphon entry bung, fit nut, washer and 'O' ring onto siphon. Insert siphon, push

down slowly into the cone and rotate the siphon clockwise until it is screwed firmly in place,

forming a gas tight seal.

30

4) Begin to pressurise the helium can up to a maximum of 3 psi. Once the siphon is pre-cooled

and dense gas is seen to issue from the siphon. Push it slowly into the empty storage Dewar.

Monitor the helium level. A dense plume of cold gas/liquid will issue from the storage Dewar

gas vent if the storage Dewar is full. In this case, stop the transfer by closing the valve on the

transfer siphon (if applicable) or by disconnecting the helium gas bottle and de-pressurising the

helium can (if not). Change storage Dewar’s and restart transferring liquid helium as described

above.

5) When the level monitor is reading zero and the helium can is empty the pressure in the

helium can will fall to zero.

6) The transfer siphon should be removed from the magnet and replaced with the bung. The T-

piece and pressure gauge should be removed from the helium exhaust and replaced with the

non-return valve.

5.4 Warming the magnet

This procedure assumes that the magnet has been de-energised as described in section 5.2, and all of

the liquid helium removed from the helium can as described in section 5.3. This describes the work

instruction for bringing the outer vacuum can (OVC) up to atmospheric pressure and warming the

magnet to room temperature.


Fittings for Cryocooler sock let up

NW16 “Christmas Tree” fitting

Helium gas bottle(s) & regulator




Fittings for OVC let up

Bursting disc blank

NW25 non return valve

NW25 speedivalve

Dry nitrogen gas (liquid nitrogen)

Rubber tubing (as described above)

Pump out port adaptor & NW40-25 reducer

Small electric heater (optional for faster warm up)

Fittings for recirculation of cold gases.

Pump

Rubber tubing (as described above)

Siphon pick up tube

Copper Tubing and heater

Pressure gauge

Helium gas bottle and regulator.

An extra heater could be used to blow hot air at the bottom of the cryostat.

31

5.4.2 Procedure

1. Disconnect power cable to cold head at compressor.

2. Disconnect the two gas lines at the cold head.

3. Connect p.s.u. to pins C-F on the ten pin plug at the cryocooler, switch on p.s.u. passing 1 to

1.5 Amps into heater. (Check that the heater connections for the cryocooler heater with the

magnet operation manual.)

4. Monitor the sensors on the cryocooler 1st and 2nd stages; they should begin to warm up.

After approximately 30 minutes the temperature sensors should be approaching room

temperature.

Venting the Main Vacuum Space and Warming the Magnet

The following procedure needs to be followed in the full to ensure safe warm up:

5. The attachments required for venting the magnet vacuum space are shown in Figure 9. A

pump out port adaptor should be fitted to the pump out port. The actuator of the pump-out

port adaptor should be screwed into the bung, with the bung fully inside the pump-out port.

A check valve constructed using a stainless steel blanking plate and a Speedivalve should be

connected to the vacuum space in series with the pump-out port adaptor as shown in Figure

11. The blanking plate should be taped on one side only so that it can flap and act as a non-

return valve in the case of over-pressure in the main vacuum space.

Warning: No other non-return valves or other parts should be used, so as to prevent air leaking

back into the vacuum space. Ensure a “flapper” valve is fitted to the helium can exhaust, (or ensure

that the bypass valve is open to the vent line) as any remaining helium will evaporate very quickly

when the vacuum is lost, causing a high pressure in the helium can, which could damage the

internal parts of the cryostat. Preferably the blanking plate should be removed and a burst disk

should be used just in case the flow rate is too high from the LHe can.

6. On the other end of the Speedivalve (Figure 9) a “Xmas Tree” type of connection is made so

that the rubber tubing carrying the dry nitrogen gas can be connected to the system.

O-Ring &Carrier

Stainless SteelBlank PlateTape

Check Valve

Figure 9 – Schematic and actual views showing layout required for re-pressurising the

OVC space.

7. Ensure the drop off plate of the cryostat is free to move.

32

8. Add a heater close inside the bore tube as shown in Figure 10. The purpose of this heater is

to keep the bore tube warm and dry. This will ensure the helium can vacuum will not

collapse the tubing.

Figure 10 – The heater arrangement to warm up the bore tube of the magnet.

9. On the opposite side of the heater, add a plastic cover on the bore tube as shown in figure

11. The purpose of this plastic is to restrict the air flow through the bore tube and only to

allow a downward air flow so that the cryostat at the bottom can be warm. Preferably the

plastic should be extended to the bottom of the cryostat.

Figure 11 – Cover of the bore tube with a small downward aperture directing the hot air flow

downwards (generic view for 7 T magnets).

33

10. Ensure Speedivalve on the pump out port is closed.

11. Open the pump out port.

12. Slowly open the speedivalve and check that Nitrogen (it is imperative that the nitrogen gas

is dry) is flowing in the cryostat vacuum.

13. Ensure that amount of helium exiting the cryostat (due to the increase in temperature) is also

not excessive to create overpressure in the LHe can.

14. Continue to open the Speedivalve ensuring that there is still a flow of nitrogen from the gas

bottle until the speedivalve is fully open.

15. When venting the main OVC space on the magnet system a fixed volume of dry nitrogen

gas should be introduced so that the warming process is started, but that the pressure in the

OVC is approximately 0.5 bar when the magnet OVC reaches room temperature. The

amount of nitrogen required is 2.13 kg (1.8 m3 at 290 K and 1 atm) which corresponds to a

pressure drop of 40 bar from a standard 300 bar, 47.2 l gas cylinder. This means that the

drop-off and transit plates should not fall during the warm up process.

Note: The volume of gas introduced corresponds to a pressure of 7 mB at 4.2 K and 167 mB at

100 K.

16. Whilst Nitrogen is introduced to the vacuum space, there is a need to start getting all other

parts necessary to do the re-circulation of Helium in the helium can.

17. When the boil off from the LHe reduces significantly, then the re-circulation tubing can be

connected as shown on Figure 15 for the inlet gas to the LHe can. A non-return valve (not

shown) should be used to stop the LHe can from over pressurizing.

18. A siphon pickup tube should also be added with all the necessary fittings and to add a rubber

tubing to it.

19. Connect the rubber tubing to the siphon pickup tube and connect to the inlet of the pump.

The rubber tubing from the siphon to the inlet of the pump should be about 2 meters length

so as to allow sufficient distance for the helium gas to warm up. The length of the tubing

from the outlet to the Helium can must be kept as short as possible.

NOTE: The re-circulation Helium flow will reduce significantly if the length of the tubing is too

long. Try and maintain the length of the tubing as short as possible.

NOTE: If the frost extends close to the pump, then connect the rubber tubing to a length of coiled

copper tubing and re-route the tubing in front of the heater so as to stop very cold gas getting into

the pump, or if this is not possible, add another heater on the tubing for the first few hours of warm

up.

20. Connect rubber tubing from the pump outlet to the LHe can inlet connection (at the bypass

valve) as shown in figure 12.

34

Figure 12 – Inlet of Helium re-circulation showing the pressure relief valve, The tubing for re-circulation and also the tubing that finally will be connected to the He gas bottle

to ensure always positive pressure in the LHe can.

21. When the boil off from the LHe can is reduced sufficiently, close any exhaust valves from

the LHe can and switch on the re-circulating pump. You will quickly notice that the pickup

tube will freeze demonstrating that cold Helium gas taken from the bottom of the Helium

can is sucked out.

22. Continue to fill the vacuum with Nitrogen. Check the pressure of the vacuum. This can be

done by trying to lift with your finger the blanking plate. If the force needed is very little,

then it means that the pressure is very low and soon the pressure will be closed to

atmospheric.

23. As the vacuum is getting less and less you will notice that the cryostat temperature will start

to reduce dramatically.

24. When the pressure is close to atmospheric pressure, the magnet will require close

supervision so that when the drop off plate falls, it must be placed back in the right position

so as to prevent any air from entering the vacuum space.

25. Close the speedivalve when the drop off plate falls.

26. Ensure that the transit plates are also pushed back when the pressure in the vacuum space is

positive.

27. When the nitrogen gas flow from the vacuum space is reduced, start secure in close position

the drop off plate and also one of the transit plates.

28. Check that nitrogen gas can escape from the blank plate on the pump out port, and when the

flow of nitrogen gas is low, secure close the second transit plate.

Helium

from bottle

Helium

from pump

Helium

to pump

35

29. On occasion check that the blank plate can float and allow enough gas pressure to escape

from the vacuum space.

30. In addition, check that there is a pressure valve on the LHe can to ensure positive pressure in

the helium can.

31. Note: It has been observed when warming up large magnet system; a negative pressure is

generated in the LHe can when all the helium has boiled off. This negative pressure is

possibly due to the heat exchange between the magnet mass and the helium gas in the LHe

can. This heat exchange will reduce the volume of the helium gas and thus generate a

negative pressure. If the pressure is negative in the LHe can, then open the regulator from

the Helium gas bottle to introduce helium to the magnet.

32. Ensure that the helium re-circulation is proceeding. Frost should be formed on the rubber

tubing of the helium re-circulation tubing.

33. After about 3-4 days for large magnets, you will notice that the frost of the siphon pickup

tube is getting less and less. When the frost is no longer visible on the pickup tube, then the

temperature of the magnet should be high enough to transit the magnet.

34. Check the temperature of the magnet and if it is greater than 0 °C (and above the dew point

of water) then the magnet can be safely transited. If this is not the case, then permission

from the Installation Manager or Project Engineer is required.

36

6. Procedures Required Following a Quench

Following a quench it is mandatory to evacuate the magnet room as soon as it is safe to do so

having regard to the safety of all personnel in the magnet room. Before re- entering the room

oxygen levels (as shown on oxygen meters either fixed or personal) must have returned to

normal levels.

Following a quench or bursting disc rupture, the neck of the cryostat should be sealed as soon as

possible after quench gas has stopped evolving. The service turret (as shown in Figure 2) should be

defrosted so that the burst disk can be replaced. Following the quench, there is no magnetic field in

the magnet, so it is safe to use motor-driven heat guns to defrost parts of the service turret and vent

lines. The procedure for changing a burst disk is detailed in section 4.2 along with the associated

safety concerns. In the absence of a spare burst disk, seran wrap (cling-film) can be used as a

temporary measure to seal the cryostat. Ensure that the cryostat is air tight, and that any remaining

helium gas in the cryostat can still pass through the non-return valve so that there is no build up of

pressure. Agilent Technologies should be contacted immediately on the dedicated service

department hotline so that a more thorough check of the magnet system can take place.

37

Appendix A CONTROL MEASURES IN PLACE

Control

Measure

Description

A

Site Survey Questionnaires (Quality Form 632) are issued to new customers; completed

questionnaires are returned to Agilent Technologies for appraisal by the Customer

Service Organisation staff in order to determine the risks to installation engineers for

each installation.

B

A pre-installation meeting takes place prior to dispatching an installation engineer; this

meeting is documented on Quality Form 641 and confirms that the installation engineer

is fully briefed on the nature and hazards of the task, and has been trained appropriately.

C

The following Personal Protective Equipment is provided for Agilent Technologies installation engineers:

Anti-splash safety glasses.

Leather gloves designed to protect the hands from cryogenic

temperatures.

Overalls or long sleeved shirts and long trousers.

Safety footwear with non-ferrous impact-resistant toecaps.

Personal oxygen monitors featuring audible and visual alarms.

D

Cryogenic liquefied gases will be delivered to designated storage areas, or will be

cordoned off and marked with appropriate warning signs.

E

Any gases evolved within the magnet room will be conducted to the outside air via

Quench ducts

F

Customers are issued the leaflet “Safety Considerations for the Installation & Operation

of Magnet Systems” to prepare them for the hazards of magnet ownership. This contains

precautions, which should also benefit Agilent Technologies installation engineers.

G

Compressed gas cylinders are transported by cylinder trolleys wherever possible.

Ferromagnetic cylinders are not allowed in the magnet room (System Service Manual).

H

Access to the magnet room during installation will be kept to a minimum, and will be

controlled by the Agilent Technologies installation engineer. Once the magnet has

begun to be energised, entry to the magnet room will be restricted. (System service

Manual)

K

Agilent Technologies installation engineers who will carry out this task are formally

trained in the safe handling of cryogens and are experienced in the installation of

superconducting magnets.

L

All installation engineers have been formally trained in lifting operations. (Training

records)

38

Control

Measure

Description

M Agilent Technologies Quality Form 641 (pre-installation meeting) has been reviewed

and updated to include enhanced safety checks, at para 5, before installation engineers

are despatched.

N

The installation engineer’s standard installation equipment contains free standing

“Strong Magnetic Field” and “No Entry” warning signs suitable for cordoning off

magnet rooms, cryogenic vessels, or other hazardous areas.

P

Under the pressure equipment directive all vessels containing liquid cryogenic fluids or

vacuum are designed to PD 5500

Q

The customer interface drawing as in the technical specification TS 1554 XX where XX

is the current revision defines the physical parameters of the magnet.

This is issued to all interested parties who will handle the magnet.

R

The magnet will be rigged into place by a specialist rigging contractor (with agreement

by Agilent Technologies) before cooling and energisation.

S Working at height or being injured by objects falling from height.

All people required to work on the magnet installation or service are required to read

and comply with the following Agilent Technologies control documentation

RISK/PROD/010- General risk assessment

RISK/PROD/103- Use of mobile scaffold towers

RISK/PROD/014- Use of man carrying forklift cage attachment

RISK/MAINT/003.- Despatching and receiving large magnets requiring use of hired

mobile cranes.

In general it is expected that all installation and service work on the magnet will be

carried out using non ferromagnetic scaffold platforms or mobile scaffold towers

conforming to the local regulations. The use of ladders or stepladders is not approved,

39

Helium

Level(mm) Pressure Run-Time Pressure Run-Time

Min.= mm (bar) (hrs) (bar) (hrs)

SignPatient End Service End

Daily Checks

Date

Compressor Operation

Condensation

Cry

osta

t

Pre

ssure

Additional Comments

Table 1 – Typical log for recording daily readings.

40

Helium

Level Inner Outer

(mm) Shield Shield Pressure Time Pressure Time

Min.= mm (K) (K) (bar) (hrs) (bar) (hrs) RT

Sh

im D

ataPatient End Service End

Coolin

g

Wate

r

Pre

ssu

re Comments Sign

Con

de

nsa

tio

n

Cry

osta

t P

res.

ED

U B

atte

ry

ED

U H

ea

ter

Weekly Checks

Temperature Compressor Operation

Date

Table 2 – Typical log for recording weekly checks.

41

RT Shim Currents (Amps)

Date

Z2

Z3

Z4

ZX

ZY

Z2X

Z2Y

X2

-Y2

XY

Z(X

2-Y

2)

ZX

Y

X3

Y3

Comments Sign

Table 3 – Typical log for recording room temperature shim currents.

42

Ten Pin Seal No. - Resistance (Ohms) / Temperature (K) Temperature

Log

Vac. Port Service End Patient End

Date / Time

2

(HJ-K

L)

4

(HJ-K

L)

5

(AB

-DE

)

5

(HJ-K

L)

6

(AB

-DE

)

6

(HJ-K

L)

7

(AB

-DE

)

7

(HJ-K

L)

8

(AB

-DE

)

8

(HJ-K

L)

9

(AB

-DE

)

9

(HJ-K

L)

10

(AB

-DE

)

10

(HJ-K

L)

Comments

Resistance

Temperature

Resistance

Temperature

Resistance

Temperature

Resistance

Temperature

Resistance

Temperature

Resistance

Temperature

Resistance

Temperature

Resistance

Temperature

Resistance

Temperature

Table 4 – Typical log for recording shield and cryocooler temperatures. Refer toOperating data for actual pin connections.

43

Level 1 Service Call

Checks for the Formation of Ice in the Cryostat

Test Performed

Visual Checks Condensation

on Cryostat Surface Ice

Internal Checks Valves

On Service Turret Entry Ports

Leak Detection Service Turret

Cryocooler Turret

Burst Disk Intact

Leak-free

Test Performed

System Grounding Connected correctly?

Helium Level Meter Alarm Level Correct?

Test Performed

EDU Unit Battery (+ Installation date)

Heater

EDU Monitor Battery (+ Installation date)

Functioning Correctly?

To be checked daily

Compressor Please see additional sheets

Operation

RT Shim Please see additional sheets

Currents

Helium Level Please see additional sheets

To be checked several times per day

Temperatures Please see additional sheets

Table 5 – Typical log for checking tasks during a level 1 service calls

44

Revision History

Revision Date Comments

D 02-Apr-11 Updated for magnet #2.

E 27-Jun-12 Updated to Agilent format for magnet #3.

Compressor updated to F70. Cryocooler lifetime 10000 hours.

F 29-Apr-13 Additional details added to helium fill instructions to prevent

pressure build-up.

45

www.agilent.com

Agilent Technologies

In This Book

This book contains service details

for the 0.35 T MRI/Radiotherapy magnet system.

© Agilent Technologies, Inc.

2013 Printed in the UK

http://www.agilent.com/

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