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Beam Shaping ControlsPhase-space acceptance is controlled by object and divergence limiting slits. The object slits are formed by 5 mm diameter polished W rods in a "V" configuration allowing the slit-gap to be continuously varied from 0 to 1200 µm by a simple linear translation. There are also fixed apertures of 1, 2 and 5 mm. X and Y slits are computer-controlled, providing an effective slit-gap resolution of 0.04 µm per step. The W rods are mounted in a massive copper assembly connected to an external air-cooled heat-sink. The divergence slits, positioned at 2.6 m following the object slits, are also computer-controlled, providing a resolution of 1.6 µm per step [1,3].The lens system has most aberration terms concentrated into terms of the form x|thetan [2]. This means that controlling the ratio of theta/phi, via the divergence slit gap ratio X/Y, can be used to control the degree of aberration in the focussed beam-spot, at constant beam current. To facilitate this adjustment, the computer control of each slit-group is done via a graphical user interface with slider controls to manipulate the X/Y ratio, or adjust X and Y slit gaps together at constant X/Y. The object and divergence slits are also linked. This enables the adjustment of the ratio of divergence to object slit gaps, in order to tune the relative contributions of geometric image and aberration in the focussed beam spot. A final control adjusts all slit gaps in concert to vary total beam current while maintaining fixed ratios of X/Y for object and divergence slits [1,2]. Once this beam control is tuned (or loaded from previous settings), the user simply needs to slide the control to the desired beam current. The result is very user-friendly and easy to use.
[1] C.G. Ryan, D.N. Jamieson, W.L. Griffin, G. Cripps and R. Szymanski, "The New
CSIRO-GEMOC Nuclear Microprobe: First Results, Performance and Recent Applications",
Nucl. Instr. Meth. B181 (2001) 12-19.
[2] C.G. Ryan and D.N. Jamieson, "A high performance quadrupole quintuplet lens system for the
CSIRO-GEMOC Nuclear Microprobe", Nucl. Instr. Meth. B158 (1999) 18-23.
[3] C.G. Ryan, D.N. Jamieson, W.L. Griffin and G. Cripps, "The CSIRO-GEMOC Nuclear Microprobe:
A high-performance system based on a new closely integrated design", Nucl. Instr. Meth. B158 (1999) 97-106. |
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