Comparison between simulations and calibrations of a high resolution electrostatic analyzer

The ion beam spectrometer (IBS) is one of the three spectrometers in the Ca ssini plasma spectrometer instrument aboard the Cassini/Huygens spacecraft. The IBS is a very high energy resolution hemispherical electrostatic analy zer. The design values of the IBS are analyzer gap 2.5 mm, and middle radiu s 100 mm. Because of the high energy resolution required, special care had to be used in the design and manufacturing of the instrument. A simulation was developed in order to aid the designing process. Here we show that the best fit to the laboratory calibration of the IBS flight model is obtained using the simulation model, where the inner hemisphere is misaligned by abo ut 20 mum, where a maximum asymmetry of 75 mum is included in the inner hem isphere, and where the analyzer gap is increased from the design value by a bout 0.1 mm. We show here that geometric factors postulated in the theoreti cal model resulted in a better agreement between calibrations and simulatio ns than the geometric factors calculated from calibration data alone. We fi nd a value of (2.5 +/-0.1) x 10(-4) cm(2) sr for the geometric factor of th e flight model. Finally, we also show that the energy and angle responses o f the IBS are somewhat lower than the values calculated directly from the c alibration data: (2.0 +/-0.1)% is the value of the full width at half maxim um (FWHM) of the energy distribution, and (3.1 +/-0.1)degrees is the value of the FWHM of the azimuth angle distribution. (C) 2001 American Institute of Physics.