MA-450 MARCO high strength magnetic quadrupole lenses · MA-450 MARCO High Strength Lenses The...

MA-450

MARCO high strength magnetic quadrupole lenses

Description

Microanalyical Research Centre

School of Physics

University of Melbourne

Parkville, 3010

AUSTRALIA

Ph: + 61 (0)3 8344 5376

Fax: + 61 (0)3 9347 4783

Email: [email protected]

Web: http://www.ph.unimelb.edu.au/marco

July 2005 v2

MA-450 MARCO High Strength Lenses

The Microanalytical Research Centre 2

Contents

1. INTRODUCTION .............................................................................................................................3

2. DESIGN CONSIDERATIONS ........................................................................................................4

3. LENS PARAMETERS......................................................................................................................5

4. LENS PERFORMANCE ..................................................................................................................6

5. LENS APPLICATIONS....................................................................................................................7

6. LENS MOUNTING STAGES..........................................................................................................9

REFERENCES.........................................................................................................................................12



1. Introduction The MA-450 lens was designed with the aim of forming the components of a high strength lens system for focusing more magnetically rigid ions than the typical beam of H+ ions up to 3 MeV (mE/q2=3 where m = mass in mass units, E is the energy in MeV and q is the ion charge)). Depending on the lens system configuration and the working distance, ions up to mE/q2=14 can be focused. For example, in a suitable lens system, it can be possible to focus 3.5 MeV He+ ions, or H+ up to 14 MeV.

The lens is cut from a single piece of thermal stress relieved iron using a multiple pass wire plasma cut process. This cutting process does not introduce any new stresses in the iron. By means of computer control of the wire, it is possible to cut the optimum hyperbolic pole tip profile with the termination angle selected to eliminate parasitic

Figure 1: The MA-450 lens. This version shown here is the Type 6 that has the yoke cut-outs for access to the specimen chamber by detectors. The Type 5 is identical

but without the yoke cut-outs.



duodecapole field components in the bore of the lens. Sub-5 micron precision is possible with this machining method.

The yoke is fitted with a set of 18 turns of copper laminations (see Figure 2) designed to carry currents of up to 80 Amperes to achieve the high-magnetic rigidity focusing action. A further optimisation of the design, clearly seen in Figure 1 and 2, is the pole tip extensions in the downstream side of the yoke which extends the lens length into the volume of drift space occupied by the winding laminations.

The MA-450 lens is available in two types:

Type 6: As shown in Figure 1, the yoke is machined, prior to wire cutting, with cut-outs designed to pass detectors and microscopes in to the specimen chamber. Type 6 lenses are designed to be the final lens of the multuplet system positioned closest to the specimen chamber. No compromise of lens quality is observed with the incorporation of the yoke cut-outs.

Type 5: Is identical to Type 6 but without the yoke cut-outs. Type 5 lenses are designed to be the upstream lenses in the multuplet lens system.

2. Design Considerations The minimisation of the working distance of the lens system is hampered by the need for target chamber ports at back angles for X-ray detectors (see discussion in Ref. [2]). For example, with x-ray detector ports at 135°, the only way to continue reducing working distance using conventional magnetic quadrupole singlets is to reduce the overall diameter of the quadrupole. But smaller yoke diameter, and the concomitant reduction in available coil volume, inevitably leads to reduction in lens bore diameter as well. All other lens system parameters being equal, this reduction in bore diameter produces a proportional increase in the sensitivity of the system to parasitic multipoles contributions, because a given tolerance on lens machining and pole tip finish produces larger parasitic multipoles in lenses with smaller bore diameter. Ideally, reduced working distance should be obtained without any reduction in lens bore and yoke diameter.

The solution to this problem is illustrated in Figure 1. The quadrupole yokes are shaped with pole-tip extensions, which bring the quadrupole pole tips 30 mm closer to the target. Also, to accommodate unrestricted detector access at 135° to the beam, the yokes have cut-outs between poles for symmetric detector ports. The cut-outs permit these ports to pass between coil windings, and four cut-outs are used to maintain four-

Figure 2: Lens under construction. Type 6 shown.



fold symmetry. The yokes have been thickened between the coils to maintain the cross-section despite the iron lost to the cut-outs. The result is an effective working distance of 80 mm despite a yoke outer diameter of 270 mm. This working distance allows the use of a cylindrical 150 mm diameter chamber and provides ample space for a microscope mirror for normal viewing of the sample under the beam [2]. A lens bore diameter of 14 mm was selected as a compromise between the opposing requirements of maximum mass-energy product and maximum bore diameter to minimise parasitic multipoles. This permits the use of a standard beam tube of 12.7 mm outer diameter with an inner diameter of 9.9 mm providing good angular acceptance and ease of alignment.

3. Lens Parameters Parameter Value Unit

Physical length (including pole tip extension), L 100 mm

Bore radius, a 14 mm

Number of turns 18 turns

Outer yoke diameter 270 mm

Winding overhang on upstream side 20 mm

Lens effective length, L+1.1a 107.7 mm

Figure 3: Lens schematic (dimensions in mm)



4. Lens Performance The MA-450 lenses are deigned to be powered by the MA-900 Hall Effect feedback power supply system (see separate manual). When powered by the MA-900 power supply, a lens system configured from MA-450 lenses will maintain a 1 micron beam spot focus indefinitely. The MA-900 can supply up to 90 Amperes of current.

Figure 4: Measured pole tip field as a function of lens current.

Measurement of the pole tip field with an uncorrected Hall probe shows that the pole tip field can be as high as 3.6 kG (see Figure 4). With a bore radius of 7 mm, this corresponds to over 50 T/m at maximum excitation.

It is possible to operate the MA-450 lens continuously at high currents without the need for special cooling systems as shown in Figure 5.

Current (A)



Figure 5: Lens temperature rise as a function of current.

The maximum recommended temperature for the lenses is 45oC without fan forced cooling. This corresponds to a maximum current of approximately 80 A. More typically, a current of 35 A corresponds to the lenses becoming warm to the touch (~30oC).

The optical quality of each MA-450 lens is evaluated by the grid shadow method. In this method the lens focuses a line onto a fine grid and the shadow pattern observed on a screen downstream. The pattern shows an array of straight lines confirming the quality of the lens. The straightness of the lines confirms that parasitic multipole components of the lens field are significantly below 0.3%.

Figure 6: Application of the grid shadow method to verify the lens quality.

5. Lens Applications The MA-450 lenses are designed to be operated in a CSIRO/MARCO high excitation quintuplet system. This system is optimised for high demagnification, short working distance and low aberrations. A typical system configuration is shown on the next page.



Figure 7: Typical application as a high excitation quintuplet.



Figure 8: The Bordeaux Quintuplet configured with MA-450 lenses.

6. Lens mounting stages

The lens mounting stages for the MA-450 lenses system consist of either one triplet stage and two singlet stages or individual singlet stages. These stages are designed to be installed at the designated positions on a steel table at the end of the nuclear microprobe girder. A milled notch on the bottom of the stages allow them to be keyed to the table. The stages are secured by a single bolt to the table. It is possible to re-deploy the stages at different positions on the table for experimentation if required. This will change the pseudo-orthomorphic properties of the probe forming lens system.

LH micrometer RH micrometer

Rotation adjust micrometerStage securing bolt hole

Spring-loaded pinLH lensbearing

RH lensbearing

Lens singlet stage (typical).



The function of the components of the lens stages are to bring the lens into accurate alignment with the axis of the system (the axis of the system is defined above). The method for doing this is beyond the scope of this manual. Full details of possible methods are presented in Materials Analysis with a Nuclear Microprobe, M.B.H. Breese, D.N. Jamieson and P.J.C. King, John Wiley and sons, NY, 1996, chapter 3.

The specifications of the lens mounting stages are as follows:

• X (horizontal) and Y (vertical) translation micrometers adjustment precision 10 μm.

• Rotational alignment micrometer adjustment precision 10 μm = 0.00036o rotational alignment precision.

The lens mounting stages should be used as follows:

• Mount the three stages on the girder and secure with the appropriate bolts. Note that the triplet stage is not symmetrical, be sure to observe the arrow marking the beam direction.

• Assemble the roller bearing supports into their respective housings. Engage the key on the roller bearing supports with the slot on the housing. CAUTION: Do not scratch the special finish on the housing or else the roller bearing support will not move smoothly in the housing.

• With the stages secured to the table, mount the lenses on the tables. To do this, have an assistant hold the spring-loaded pin back while the lens is lowered carefully onto the roller bearing supports.

• Engage the rotation alignment tang on the lens with the rotation alignment

micrometer shaft and the spring-loaded pin.

• Centre each lens on its respective stage ensuring the same overhang in both the upstream and downstream edges.

Rotation alignment tang

Lens mounted on singlet stage.



• Coarse alignment of each lens can be accomplished by use of a temporary centre target plug inserted into the bore of each lenes during installation. With reference to a graticule in the theodolite the lens can be brought into alignment by making appropriate adjustments to the LH and RH micrometers.

• CAUTION: Ensure the centre target plug is made from a soft material that will not scratch the lens pole tips.

• Following coarse alignment, install the beam tube.

• CAUTION: Take care not to scratch the lens pole tips while inserting the beam tube.

• CAUTION: The beam tube through the lenses MUST NOT be made from stainless steel otherwise the magnetic purity of the lens field will be degraded leading to the introduction of aberrations which will degrade the performance of the lens system.

• Final alignment should be done with reference to the focused beam spot in the specimen chamber. Take particular care to minimise rotational alignment errors as these have a severe effect on the size of the focused probe.



References 1. M.B.H Breese, D.N. Jamieson and P.J.C. King, “Materials analysis using a

nuclear microprobe”, John Wiley and Sons, New York, 1996.

2. RYAN, C.G., JAMIESON, C.G., GRIFFIN, W.L., SIE, S.H., CRIPPS, G. and SUTER, G.F. - The CSIRO/GEMOC nuclear microprobe: a high performance system based on a new highly integrated design. Nuclear Instruments and Methods in Physics Research Section B - Beam Interactions with Materials and Atoms, 158/1-4, pp:97-106 (1999)

3. RYAN, C.G., and JAMIESON, D.N. -A versatile quadrupole quintuplet lens system for the CSIRO/GEMOC nuclear microprobe. Nuclear Instruments and Methods in Physics Research Section B - Beam Interactions with Materials and Atoms, 158/1-4, pp:18-23 (1999)

4. RYAN, C.G., JAMIESON, D.N., GRIFFIN, W.L., CRIPPS, G. and SZYMANSKI, R. - The new CSIRO-GEMOC nuclear microprobe: First results, performance and recent applications. Nuclear Instruments and Methods in Physics Research Section B - Beam Interactions with Materials and Atoms, 181, pp 78-82 (2001)

5. JAMIESON D.N., SPIZZIRRI P., SZYMANSKI R., ROUT B., SAKELLARIOU A., BELCHER W., and RYAN C.G. - The New Melbourne Nuclear Microprobe System. Nuclear Instruments and Methods in Physics Research Section B - Beam Interactions with Materials and Atoms, 190, pp:54-59 (2002)

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