GRID SHADOW PATTERN-ANALYSIS OF THE SHANGHAI NUCLEAR MICROPROBE

The ion optics of the Shanghai nuclear microprobe system have been investigated with the grid shadow method. The level of parasitic aberration found in the magnetic quadrupole lenses of the probe forming lens system was found to be very low and typical of levels seen in other high-quality systems, such as the Melbourne system. It was found that the parasitic field components were mainly sextupole with a strength of between 0.070% and 0.28% of the main quadrupole field. The small level of parasitic field is not a significant limit to the resolution achievable by the system. A complication in the analysis of the grid shadow patterns was that some of the parasitic field was due to skew sextupole components (non-zero phase angle); however, these could be identified by comparing with simulated shadow patterns. Further experiments with the grid shadow method revealed an improvement by a factor of 4-6 in focused probe intensity, and could be achieved by installation of a beam steerer between the accelerator condenser lens and the beam switcher dipole magnet. The measurements show that the lens system is potentially able to focus probes with 1 mu m resolution.