Progress In NEXT Ion Optics Modeling

*† Results are presented from an ion optics simulation code applied to the NEXT ion thruster geometry. The error in the potential field solver of the code is characterized, and methods and requirements for reducing this error are given. Results from a study on electron backstreaming using the improved field solver are given and shown to compare much better to experimental results than previous studies. Results are also presented on a study of the beamlet behavior in the outer radial apertures of the NEXT thruster. The low beamlet currents in this region allow over-focusing of the beam, causing direct impingement of ions on the accelerator grid aperture wall. Different possibilities for reducing this direct impingement are analyzed, with the conclusion that, of the methods studied, decreasing the screen grid aperture diameter eliminates direct impingement most effectively. I. Introduction ASA’S Evolutionary Xenon Thruster (NEXT) 1 is currently under development to follow the success of the NSTAR ion thruster. 2 NEXT has a 40 cm beam extraction diameter – giving more than twice the area of the 30 cm NSTAR thruster. This allows a larger amount of thrust, and a redesigned discharge chamber gives a flatter beam current density profile, which reduces erosion of the ion optics. 3 The NEXT thruster also has an accelerator grid 50% thicker than the NSTAR thruster. This is to allow for a longer life, as the thicker grid can withstand more erosion on the downstream face, and also prevents electron backstreaming more effectively than a thinner grid. Ion optics modeling is being used to aid the development of NEXT, especially in the area of erosion and life prediction. This area is the most difficult to address experimentally, as it requires operating the thruster for a very long time in order to determine the erosion behavior and life-limiting mechanisms. In this paper the ability of the model to simulate optics erosion is improved and applied to a specific situation. The accuracy of the potential field solver used in the ion optics model is analyzed and improved, with results showing improvement compared to experimental data. The model is also applied to simulate erosion in the apertures at the outer edge of the extraction grids. In these apertures, extremely low beamlet currents cause crossover of the beam ions and direct impingement on the accelerator grid aperture wall. Methods for mitigating this erosion are explored. II. Model Operation The computational model simulates a single 2-D axisymmetric aperture in an ion thruster. A computational mesh composed of evenly spaced rectangular cells is used to track particles in the simulation. The optics of the thruster are simulated using boundary cells in the domain. These cells may be arranged in an irregular way, allowing the simulation of cusps on the barrels of the grids. A typical computational domain is shown in Figure 1. The code uses the Particle-In-Cell 4 (PIC) method to simulate xenon ions, xenon neutrals, and doubly charged xenon ions. Each computational particle has a numerical weight that indicates the actual number of atoms represented by the particle. Flow field quantities for each cell are obtained by averaging the properties of all the particles in the cell, taking into account the weight of each particle. The potential field accelerates ions selfconsistently and electrons are modeled as a fluid. The potential solver is described in further detail below. The direct simulation Monte Carlo (DSMC) method 5 is used for processing particle collisions. Both charge exchange (CEX) and momentum exchange collision types are simulated. For the simulations performed in this paper, neutrals are not included in order to increase the speed of the model. In the potential mesh refinement study, the model is modified to allow a finer potential mesh without refining the particle mesh. However, cases at a refined particle mesh are also simulated in order to verify the potential mesh refinement. Figure 2 shows both potential mesh refinement and particle mesh refinement. If the entire box shown