Simulations of the Response Function of a Plasma Ion Beam Spectrometer for the Cassini Mission to Saturn spaceweb@oulu.fi - last update: 27 July 2001 (JV)

To obtain very high (~ 1 %) energy resolution with spherical-section electrostatic analyzers requires high precision in both fabrication and in the alignment process. In order to aid in the calibration of the instrument and to help minimize fabrication costs, we have applied simulation models to the Ion Beam Spectrometer (IBS) for the NASA/ESA Cassini mission to Saturn. In our previous paper we studied the effects of misalignment and simple irregularities of the hemispherical surfaces on the performance of an electrostatic analyzer. We have considered a hemispherical electrostatic analyzer equipped with an aperture plate to collimate the stray electric field at the entrance apertures. The influence of a curved entrance aperture has also been added to the simulation model, and it's effects have been studied in detail. A cylindrical (3D) simultaneous overrelaxation (SOR) algorithm has been introduced to solve for the stray electric field. The maximum loss of transmitted particles with respect to the transmission of an ideal instrument has been set at 10%. We demonstrate that the deviation in the distributions of the energies is less than 0.2% and that the deviation in the distributions of entrance angles of transmitted particles is less than 0.1° . It has been found that the energy resolution of an electrostatic analyzer can be improved from dE/E=(1.6±0.2)% to dE/E=(1.3±0.2)% by the introduction of front aperture plates. Through the introduction of curved entrance slits, the azimuthal angle resolution has changed from b=(1.4±0.1)° for the simplified geometry simulation results of our previous paper to b=(2.3±0.1)°. We have confirmed that an accuracy of 25µm in the alignment of the two hemispherical surfaces is sufficient to give the instrument the desired resolutions.