Introduction toSlow Extraction to the North Targets

• Third order resonance & extraction principle• Extraction channel equipment• North transfer line TT20• Beam instrumentation• Extraction setup procedure• Interlocks

J. Wenninger

Acknowledgments to all colleagues that have provided figures, information etc… and to K. Cornelis and G. Arduini for reading the draft versions.

104/21/23

Third order resonance

• In a linear machine the phase space of a particle is a circle/ellipse.

• In the presence of sextupoles:

• The phase space is distorted for large amplitude particles.

• The region of stable motion is delimited by three lines, the separatrices. The orientation of the separatrices at any point in the ring depends on the location & strength of the sextupoles (in reality the separatices are not perfect straight lines).

• As the tune approaches the third order resonance (fractional tune 1/3 or 2/3), the stable area between the separatices shrinks, particles with smaller amplitudes become unstable.

• Once they are caught by the resonance, the particles move out along the separatrices.

x

x’

x’

x

2

|Q1-Qres | | Q2-Qres | | Q3-Qres |> >

Third order resonance extraction• To extract the beam using the third order resonance, a

septum is placed such as to intercept the particles that move out to large amplitudes along one separatrix.

• To ensure that the beam exits the machine at the septum which is placed outside the machine aperture (protection!), the closed orbit is displaced towards the septum such that the septum becomes the horizontal machine aperture during extraction.

• By pushing the horizontal tune towards the resonance (at the SPS QH = 26 2/3), it is possible to extract the beam slowly from the machine : SLOW EXTRACTION.

• At the SPS the chromaticity is set to H -1 (Q’H -30) to increase the tune spread and ease control of the extraction. This means that momentum of the beam varies during extraction. At the SPS a particle spends 2-3 milliseconds in the resonance before it enters the septum gap.

3

x’

x

septum

1

2

3

4

12 3 4

x

s (beam direction)In real space

(top view)

1,2,4… are turn numbers

Slow extraction to the North Area• SPS has 8 ~ 0.75 m long extraction sextupoles

(LSE, LSEN).

• 3 sextupoles are used for the present SE to produce the appropriate phase at the ZS:

• LSE.22402 K2 = -0.168605 (m-3)• LSE.40602 K2 = -0.168605 (m-3)• LSEN.42402 K2 = 0.168605 (m-3)

• This scheme is setup for a simultaneous extraction to the North and to the West, and has not been changed since extraction to West has stopped.

• No sextupoles are installed between LSS6 and LSS2 to have a linear machine between the 2 ZS’. There is no amplitude growth between LSS6 and LSS2 and the ZS was cutting out the largest amplitude particles.

• The phase advance between the West and North ZS must be 9x360°. This requires a change of the QF1/QF2 main quadrupole strengths, “Q-split”:

• Keep tune constant (26.62++).• Re-adjust phase advance West-North.• KQF1 = -1.1956 10-4 (m-2)• KQF2 = +1.1956 10-4 (m-2)

4

Beam preparation : RF Issues

• The Fixed Target beam that is used for slow extraction consists of two batches injected at an interval of 1.2 seconds from the PS.

• Each batch corresponds to 5 PS turns and fills 5/11 of the SPS ring (SPS is exactly 11 times larger than the PS):

• SPS average radius = 1100 m

• PS average radius = 100 m

• The injected beam is captured by the SPS 200 MHz RF system. Within the batches each 5 ns bucket is filled. At injection the bunch length is 5 ns (at the base of the bunch, i.e. ~ 4 length).

• For a total of 4620 RF buckets, about 4200 buckets are filled. • BUT :

• The experiments prefer a continuous (non-bunched) beam. • It is much cleaner to extract the beam with the RF OFF, because the synchrotron oscillations induce noise (by modulating the particle tune through chromaticity).

Slow extraction is (normally) with RF OFF !

5

RF GymnasticsTo increase momentum spread for faster de-bunching and produce a beam shape that is as rectangular as possible, we perform some RF gymnastics before extraction :

• RF Voltage ~ 7 MV at the start of the 400 GeV flat top.• Jump to unstable phase for ~ 0.5 ms and back to stable phase.• Wait until bunch length minimum (~ 2 ms) max energy spread.• Switch off RF.

This has effect of increasing the rms dp/p from ~ 0.02% to 0.04%.

Unstable phase

p/p max

RF OFF

RF voltagePeak ampl.

6

dp/p

Time/phase

LSS2 Extraction Channel

• 5 electrostatic septa (ZS) in ½ cell 216

• 3 thin magnetic septa (MST) in ½ cell 217

• 5 thick magnetic septa (MSE) in ½ cell 218

• TCE collimator to protect QDA217

• TPST absorber to protect MST septum• Dipoles MBB.21530, MBB.21550 and MBA.21570 and MBA.2159 (just upstream from the ZS) are displaced outwards by up to 1.1 cm to provide more aperture for the extracted beam

• The quadrupoles in ½ cells 216, 217 and 218 are enlarged (QDA, QFA) :• QD : diameter 83 mm.

• QF : H = 152 mm, V = 38 mm.

• QDA, QFA : H = 151 mm, V = 91 mm.

216 217 218

7

Slow Extraction

5xZS 3xMST

5xMSE

Extracted beam

Circulating beamEnvelope

ZS is oriented // to beam envelope

8

Extraction Bump• 5 horizontal extraction dipoles

(‘bumpers’) are used to move the beam locally towards the extraction channel.

• 4 vertical bumpers are used to flatten the orbit in the vertical plane at quadrupoles 217 and 219.

• The bump is normally reasonably well closed, leakage of the order of ~1 mm.

Longitudinal position (m)

(m)

216 218217 219

ZS

MST

MSE

9

Septa locationis approximate !

22 mm

Displaced MBB’s and MBA’s

Magnetic SeptaElement L

mag (m)

B (T) Kick/mag (mrad)

I nom (A) Septum Thickness (mm)

Gap Height (mm)

MST 2.25 ~0.26 0.367 ~3500 4.2 20

MSE 2.24 ~1.5 2.120 ~20300 17.2 20

MSEExtractionchannel

10

Electrostatic septum (ZS)

-220kV

-6kV

-3kV

AnodeCathode

Ion-traps

• Active length ~ 3 m (per septum).• Anode (grounded) :

• Equipped with 2000 W.75Re.25 septum wires.

• Wire diameter 50 m (first ZS) to 100 m.• Wire spacing 1.5 mm.

• Cathode at a voltage of -220 kV (110 kV/cm).• Anode-cathode gap : 20 mm.

11

• Ion traps (circulating beam side):• Voltage of -3 to -6.5 kV• ~ 2-3 kV diff. top/bottom• Sum of all voltage difference should be ~ 0 to avoid orbit distortions at low energy. Critical for leptons, usefulness less evident now…

Electrostatic septum

12

Sparky !!!

13

• Electrostatic devices like the ZS’s are prone to spark !!• Sparks are neither good for the beam, nor for the ZS’s themselves.• A fixed display with the spark rate is available. To be surveyed…

• We could interlock this with the new SW interlock system. To be seen…

• The ZS’s are very sensitive to electron clouds induced by high intensity LHC beams – can lead to very high spark rates !! For that reason, during LHC beam MDs :

• The girders are retracted.• The ZS voltage is set to 0. • The ion traps are left on to clean ions produced by the e-clouds.

Girders & Motors

14

• The septa are mounted on girders that can be retracted whenever the extraction is not used. The girder position may be adjusted to reduce losses.

• The (wire) anode planes can be adjusted (upstream and downstream ends) to optimize the alignment of the anodes and minimize the losses. Typical changes are of the order of few tenth’s of mms.

Element Upstream INBEAM

Downstream INBEAM

Upstream RETRACTED

Downstream RETRACTED

ZS 68 mm 40 mm 97 mm 78 mm

MST 34 mm 47 mm 79 mm 87 mm

MSE 40 mm 70 mm 92 mm 125 mm

Extraction instrumentation

• The slow extracted beam has no RF structure (de-bunched) which makes position measurements with ‘classical’ BPM electronics impossible.

• The slow extracted beam is therefore measured with Secondary Emission Monitors (SEMs) made of 20 m thick Al or Ti foils (conversion efficiency of ~ 4% (electrons per proton)).

• There are 5 basic monitor types :• BSG : grid with typically 16 strips (width 0.5-2.5 mm).

• BSP : 2 (1/2) foils to measure the position (compare Left/Right resp Top/Bottom foil signals).

• BSM : same as BSP, but on a step motor.

• BBS : thin movable strip (1 mm) to scan across the beam.

• BSI : a single foil to measure beam intensity.

• Steering is performed with BSPs and BSGs.• Profiles are obtained with BSGs and BBSs (extraction channel & targets).

15

Extraction Instruments : BBSHorizontal BBS

Vertical BBS

Circulating beam BBS : to check septa margins wrt circulating beam.

BBSV.21691

BBSH.21638 BBSH.21691 BBSH.21758

BBSH.21773

BBSV.21773

BBSH.21798

BBSH.21831

BBSV.21831

BBSH.21876

BBSV.21876

BBSH.21778 BBSH.21836

16

Horizontal Measurements (2003)

BBSH.21638 BBSH.21691 BBSH.21758

BBSH.21773

BBSH.21798

BBSH.21831

BBSH.21876

Vertical Measurements (2003)

BBSV.21691 BBSV.21773 BBSV.21831 BBSV.21876

Circulating Beam Measurements (2003)

Circulating beam BBS

BBSH.21778 BBSH.21836

19

Extraction Measurements (2003) : grids

Horizontal Grid

BSGH.21632BSGH.21856

Septum jump : 16 mm (adjusted with sextupole strength or bump amplitude)

This reference with WEST ON : large amplitudes cut by West ZS!

TT20 Transfer Line

11% slope

• The TT20 TL is used to transport the slow extraction beam to the North targets T2, T4 and T6.

• Extraction to T2 : ~880 m TT21 , TT22 and TT23• Splitter 1 to T6 : ~275 m TT25• Splitter 2 to T4 : ~170 m. TT24

• Two series of 3 splitter magnets (MSSB) are used to distributed the beam over the targets. The beams are cut VERTICALLY !

• Splitter 1 : T6 (deflected) and T2+T4 (straight)• Splitter 2 : T4 (deflected) and T2 (straight)

TTTT TT

TT

TT

TTTT

TT TT

Splitters

B

Field-free region,

height 90 mm

width 26 to 33 mm

Horizontally deflected by the B-field

Continues straight - no B-field

Losses

T2 T4 T6

• Loss optimization : large and parallel beam in the vertical plane (v ~ 24 km).

• The intensity sharing between the targets is adjusted by moving the beam vertically at the splitter.

22

75 mm

Splitters in TCC2 (MSSB)

Among the hottest objects in the SPS !

23

Targets and secondary beamsThe 3 proton beams are directed onto the primary targets:

T2 H2 and H4 beam lines

T4 H6, H8, and P0 beam lines

T6 M2 (muon) beam line

Experimental Areas:• ECN3: underground experimental hall that can receive the primary

proton beam with high intensity in ECN3• EHN1: surface experimental hall that can receive secondary beams

and/or attenuated primary proton beams • EHN2: surface experimental hall that receives the secondary beams or

intense muon beam.

24

TT20 Dumps

• There are 2 movable dumps blocks in TT20 :• A water cooled TED that can absorb the entire beam (for setup…). The TED is

located at position 2103 (~ 130 m from start of TT20).

• A un-cooled TBSE for personnel protection (access system !) in position 2106, ~90 m downstream from the TED.

• Both blocks are interlocked with access chain 9 (North Extraction).

25

TT20 BTVs

• There are 9 beam screens (BTVs) in TT20:• BTV.2100 and BTV.2102 just after the extraction channel.

• BTV.2103 in front of the TT20 TED.

• BTV.2116 ~ 20 m upstream, BTV.2117 downstream of the first splitter.

• BTV.2204 ~ 20 m upstream of the second splitter.

• BTV.2301 ~ 120 m upstream of T2.

• BTV.2309 ~ 15 m upstream of T2. Only for use when the wobbling magnets are at 0 ! Only in this case should the beam be centered on the screen.

• BTV.2401 ~ 120 m upstream of T4.

• BTV.2501 ~ 210 m upstream of T6.

26

Steering IssuesThere are special issues related to the split foils (BSPs) in TT20:

• The position must be reconstructed from the normalized signal difference of the two foils,

= (A-B)/(A+B)

• The conversion from to the real position depends of the profile of the beam, i.e. shape and emittance.

• For steering it is assumed that the profile is triangular, and the emittance is a good (typical) guess Note that the TRUE emittance may vary with intensity and target sharing.

• If the beam is completely on one side (A or B) the signal remains constant independently of the real position ‘saturation’ of the position.

In the steering program a monitor that is saturated is indicated in YELLOW (WARNING !).

Steering is delicate in TT20… more than in LHC ??

27

Beam position

Foil A Foil B

A-B

/A+

B

-1

+1

To ease life, a feedback (‘Autopilot’) can be run to automatically steer the beam on the target. But this works only to the last 2 monitors !

Target Monitors

Obsolete !28

• Intensities on page1 are based on the Ti BSIs installed upstream of the targets.• Intensity readings on page1 do not mean the beam reaches the target !• To be sure the beam is on target, one needs a good symmetry (see coming slides) and a

good profile from a BBS target scan.

Page1 intensity

Target Box Instrumentation

• Each target box is equipped with a number of SEM monitors.

• The details differ from one target to the other.

• Details are shown on the figure – for reference !

29

Target “symmetries”• The BSMs (H+V) in front of each target and the BSPs (H+V) in the TBIU are used to define the quality of the target steering in position and angle.

• For each monitor one defines (A and B are the signals of the 2 ½ foils) :

• The target symmetry ST is that is displayed on Page1:

BABA

BAS

if1||

1

VBSP

HBSP

VBSM

HBSMT SSSSS 100

• The BSMs are installed ~15 m upstream from the targets.

• The BSMs move horizontally according to the target Wobbling

ST should be > 75

30

Target Info for Steering

31

• The steering application provides for each target the intensity, symmetries and multiplicity.

• There are actually 2 SYMMETRIES :• Symmetry with the ‘little “a”’ : based on equation of the previous page.• Symmetry without the ‘little “a”’ : uses only the target BSP (not the BSM !). Angle

at the target may be (VERY) wrong !!• Only the symmetry with ‘a’ is good ! On page1 the a-symmetry is only displayed

when the BSM data is reasonable…a-symmetry Symmetry without ‘a’

Wobbling

Target

SPS protons

TAX B1

B1

• Target ‘Wobbling’ is made with 3 dipole magnets, 2 in front and one after the target. Its purpose is the change the beam angle at the target :

• Two (coupled) secondary beams from one target.• One secondary beam can be the primary beam.• Variable production angles.

• The currents of the wobbling magnets are surveyed by SIS (Software Interlock System) to avoid damage to the target box.

• When the wobbling is changed the horizontal BSM position must be adapted to be centered on the new reference trajectory !

Wobbling magnetsPart of the secondary beam line control system (CESAR)

32

Target scans (BBS)

T2

T4

T6

• Most target heads are made of Be :• Height 2 mm• Width 160 mm• Length 40-500 mm

• Beam sizes at the target ~ 2 mm (at the base of the profile – to fit the target size !). Depend on intensity/sharing !

Extraction Channel/TT20 BLMsThe beam loss profile for the extraction channel is optimized by :

• Adjustment of the septa girders and the ZS anode planes

• Adjustment of the vertical orbit in the SPS.

ZS

MST MSE

TCE/TPS

34

Good loss profile

Splitter BLMs

Splitter 1 Splitter 2

The beam loss profile for the splitters is optimized by :• Angles at the splitters (orthogonal steering).

•Horizontal position at the splitters (V position defined by intensity sharing !!)

35

Spill control : SERVO-SPILL

• Feedback loop from BSI in TT20 to LQE116 quadrupole string.• Controls the rate at which the beam is extracted by acting on Qh via LQE114.• Provides a smooth and steady extraction rate.

36

Spill ‘Harmonics’• 50 Hz noise and its harmonics that are present on the mains lead to small

modulation of the tune at those frequencies. This in turns leads to a modulation of the extracted intensities.

• The servo bandwidth is not sufficient to act on those perturbations.• But the perturbations can be corrected because this noise has a (rather) stable phase

and amplitude with respect to the 220 V mains feed-forward of signals at nx50 Hz that are added to the servo control signal for the LQEs.

Example from 2003

Before FF After FF

Harmonics Control

FF settings (2006 example)

Current to LQEs

Extraction Setup / 1Initial Conditions :

• Extraction sextupoles, bumpers, septa and servo OFF.• ZS, MST and MSE girders OUT.• TT20 TED (dump) IN. • Beam OFF.• Tunes at 400 GeV Qh = 26.62-26.63, Qv = 26.58, Qsplit on (1 Unit)• Octupoles off on the flat top.

1. Change Beam Mode for FTARGET beam to “TT20-TED”.

2.Mask appropriate interlocks in SIS (Software Interlock System).

39

Extraction Setup / 2

3. Beam OFF. 4. Check that bumper functions are at 0.5. Switch Bumpers ON.6. Beam ON.7. Flatten orbit in LSS2 using knobs (1 knob UNIT = 1 mm) :

• H position at 216 & 218, V position at 217 and 219 < 0.2-0.5 mm.• Safe a reference orbit (flat top !).

8. Beam OFF.

40

Extraction Setup / 3

9. Trim extraction knob : 1 UNIT.• Sets extraction bump, extraction sextupoles,

magnetic septa strengths to nominal.

10.Check that bumpers follow their function.11. Beam ON.12.Check closure of extraction bump (~1 mm leakage is ‘normal’).

• Compare with previous reference.

41

Difference orbit after-before extraction bump

(2006 setup)

Extraction Setup / 4

13.Enable RF gymnastics (2 LTIMs, RF5), followed by RF OFF (2 LTIMs, coarse & fine, RF3). You can no longer measure the orbit….

42

Extraction Setup / 5

14.Trim horizontal chromaticity QPH down by -1.

43

Extraction Setup / 615. Beam OFF.16. Switch ON extraction sextupoles and magnetic septa. Check

currents.17. ZS voltage ON to -220 kV. ZS must be ok.18. Move girders (ZS, MST, MSE) in beam. 19. Un-mask SIS interlocks that were masked at step No. 2.20. Beam ON. There should be no losses, no extraction.21.Put TT20 BTV2103 IN to observe extracted beam on TED.22. Slowly move QH up at the end of the FT. Observe intensity (page1

and BTV). Establish a natural extraction.

44

Extraction Setup / 7

22.Check beam position on BTV. Should be centered in H and V. If off in H, trim strength of MSE by some tens of microrads.

23.If you wish:• Switch ON servo quadrupole PC LQE1143M.• Switch ON servo-spill FB.

24.Check beam position in the extraction channel (SEMs).

Finally : Beam to target, extraction optimization, losses… Long hard work !

45

Extraction Setup / Variant…

In case the slow extracted beam is bunched (‘25 ns beam run’) the following points are different :

13.The RF remains of course ON. No gymnastics !!14.Trim horizontal chromaticity QPH up by +1.

Because the extraction losses are much larger, the extracted intensity is typically lower by a factor ~ 10.

46

Radiation /Beam Loss Issues

47

• The extraction channel in LSS2 is very HOT, with dose rate > 1 mSv/hour.• In 2006 we have observed even larger losses than in the past (see C. Arimatea’s presentation at the OP days 07).

• This year it is important to understand this issue:• Careful monitoring of MST/MSE temperatures and BLMs• Diagnostics of beam tails (V plane) ?• …

LSS2 21 Nov 2006 - LSS2 17 Nov 2004 - LSS2 17 Nov 2003

Activité rémanente 30 heures après l'arrêt des f aisceaux

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

213 214 215 216 217 218 219 220 221 222 223

Position

Déb

it de

dos

e en

µS

v/h

ZS MST MSE

MPLH

Interlocks for SE : hardware

48

Hardware Interlocks : • Connected to the new Beam Interlock System.• If triggered :

• Beam dump of the circulating SPS beam.

• Interlock channels :• TT20 vacuum.• BA80 (TCC2) vacuum.• LSS2/Extraction channel BLMs (any BLM channel above threshold).• TT20/Splitter BLMs (any BLM channel above threshold).• TT20 TED movement (interlock when moving OR not on end-switch [IN or OUT]).

• The beam is also stopped by timing (i.e. CPS beam goes to SPARE) through the Software Interlock System:

• After 3 consecutive faults on BLMs (configurable !).• As soon as a vacuum fault is detected.

This avoid dumping the beam on the ‘injection dump’.

Interlocks for SE : sofware

49

Software Interlocks (SIS, Software Interlock System) : • If triggered :

• Beam dump of the circulating SPS beam – WILL AFFECT ALL BEAMS !!!!• Beam stop through the timing system (beams with timing destination FTARGET).

• The interlocks are conditioned with the beam mode.• Setting the correct mode is VERY IMPORTANT !!!

• Signals :• All extraction element PC states (sextupoles, bumpers, septa).• All TT20 PC states.• The position of the septa girders (ZS, MST, MSE).• The ZS high voltage and the ion trap voltages.• Current of the wobbling magnets (~ 1 A tolerances) & TBIU/TBID position.• Target intensity (T6 < 1012 p/second, T2 & T4 < 5x1011 p/second).• Target fans ON.• Possibly more to come…

• Important: if the mode is NO-EXTRACTION, SIS requires as a consistency check that

• The girders are retracted.• ZS high voltage is 0.• All extraction element PCs are OFF (sextupoles, bumpers, septa).