Three-dimensional analysis of ion optics with misalignments of apertures

• A three-dimensional computer model of ion optics is developed.
• The operating values of perveance with shifted holes are calculated.
• The beamlet deflection angles are calculated and compared with linear optics theory.
• The erosion pattern of the acceleration grid with shifted holes is simulated.
• An explanation of the erosion pattern asymmetry observed on the periphery of ion optics is proposed.

A three-dimensional computer model of ion optics is developed to analyze the focusing properties and lifetime characteristics of ion engines. The simulation model includes the following main parts: solution of the self-consistent problem with calculation of the potential and space charge distribution; calculation of rarefied gas flows in the free-molecule approximation; modeling of charge-exchange collisions; calculation of surface erosion rate; prediction of the ion optics lifetime. In the present study, manufacturing tolerances on the mutual alignment of the apertures are investigated. The modeling results are compared with predictions of the linear optical theory. It is shown that operating values of the perveance at an offset of the holes may be estimated using a two-dimensional axisymmetric model. According to the simulation results the beamlet deflection angle weakly depends on the ion beam current and agrees well with linear optical theory. The observed displacement of the erosion pattern on the acceleration grid can be a measure of the apertures misalignment in the hot operational state of the ion optics. The reasons of the observed asymmetry of the erosion pattern on the periphery of the ion-optical system, which may arise even at a perfect alignment of the apertures, are investigated.