Beam Shaping Controls
Phasespace 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
slitgap 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
computercontrolled, providing an
effective slitgap resolution of 0.04 µm per step. The W rods are mounted in a massive
copper assembly connected to an external aircooled heatsink. The divergence slits, positioned at
2.6 m following the object slits, are also computercontrolled, providing a resolution of
1.6 µm per step [1,3].
The lens system has most aberration terms
concentrated into terms of the form xtheta^{n} [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 beamspot, at
constant beam current. To facilitate this adjustment, the computer control of each
slitgroup 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 userfriendly and easy
to use.
[1] C.G. Ryan, D.N. Jamieson, W.L. Griffin, G. Cripps and R. Szymanski, "The New
CSIROGEMOC Nuclear Microprobe: First Results, Performance and Recent Applications",
Nucl. Instr. Meth. B181 (2001) 1219.
[2] C.G. Ryan and D.N. Jamieson, "A high performance quadrupole quintuplet lens system for the
CSIROGEMOC Nuclear Microprobe", Nucl. Instr. Meth. B158 (1999) 1823.
[3] C.G. Ryan, D.N. Jamieson, W.L. Griffin and G. Cripps, "The CSIROGEMOC Nuclear Microprobe:
A highperformance system based on a new closely integrated design", Nucl. Instr. Meth. B158 (1999) 97106.
