Wilhelmus M. Ruyten

Particle simulation of ion optics and grid erosion for two‐grid and three‐grid systemsa)

A particle simulation code has been developed to study ion optics and the effect of charge‐exchange‐induced grid erosion in electron bombardment ion thrusters. The code is based on the particle‐in‐cell method to simulate the beam extraction and ion optics and a Monte Carlo method to calculate the charge exchange collisions between fast beam ions and neutral particles. The space charge effects for both fast beam ions and slow charge exchange ions are included. Both two‐ and three‐dimensional versions of this code have been developed for both two‐grid and three‐grid systems. The two‐dimensional model provides a qualitative description of the details of the ion flow and the charge exchange collisions. The three‐dimensional version has successfully predicted the experimentally observed erosion pattern in which the maximum erosion occurs at the geometric centers of accelerator grid apertures.

Absolute Doppler shift calibration of laser induced fluorescence signals using optogalvanic measurements in a hollow cathode lamp

We have studied the use of optogalvanic measurements on the neutral 3P1 and 3P2 levels of argon in a hollow cathode lamp for the purpose of calibrating Doppler shifts of laser induced fluorescence signals from an arcjet plume. By using a narrowband, frequency stabilized dye laser, a 5 MHz absolute accuracy was achieved, even using lines with widths in excess of 1 GHz. However, with the laser beam aligned along the axis of the hollow cathode lamp, the optogalvanic signal was shifted 10–35 MHz from line center, due to an apparent drift of neutral atoms along the lamp axis.

Further study of the effect of the downstream plasma condition on accelerator grid erosion in an ion thruster

Further numerical results are presented of earlier particle-in-cell/Monte Carlo calculations of accelerator grid erosion in an ion thruster. A comparison between numerical and experimental results suggests that the accelerator grid impingement is primarily due to ions created far downstream from the accelerator grid. In particular, for the same experimental conditions as those of Monheiser and Wilbur at Colorado State University, it is found that a downstream plasma density of 2 x 10 exp 14/cu m is required to give the same ratio of accelerator grid impingement current to beam current (5 percent). For this condition, a potential hill is found in the downstream region of 2.5 V. 6 refs.

Monte Carlo simulation of ion-neutral charge exchange collisions and grid erosion in an ion thruster

A combined particle-in-cell (PIC)/Monte Carlo simulation model has been developed in which the PIC method is used to simulate the charge exchange collisions. It is noted that a number of features were reproduced correctly by this code, but that its assumption of two-dimensional axisymmetry for a single set of grid apertures precluded the reproduction of the most characteristic feature of actual test data; namely, the concentrated grid erosion at the geometric center of the hexagonal aperture array. The first results of a three-dimensional code, which takes into account the hexagonal symmetry of the grid, are presented. It is shown that, with this code, the experimentally observed erosion patterns are reproduced correctly, demonstrating explicitly the concentration of sputtering between apertures.

Some analytical results for the fluorescence spectrum of a two-level atom in a bichromatic field

Some analytical expressions are derived for the elastic fluorescence spectrum of a two-level atom in a bichromatic field whose two modes have equal intensities and have equal but opposite detunings from the resonance frequency of the two-level system. The approximations are shown to be in excellent agreement with numerical results that were presented recently in this journal by Agarwal et al. [ J. Opt. Soc. Am. B8, 1163 ( 1991)].

Particle simulation of ion optics and grid erosion for two‐grid and three‐grid systems (abstract)a)

A particle simulation code has been developed to study ion optics and the effect of charge‐exchange‐induced grid erosion in electron bombardment ion thrusters. The code is based on the particle‐in‐cell method to simulate the beam extraction and ion optics and a Monte Carlo method to calculate the charge exchange collisions between fast beam ions and neutral particles. The space charge effects for both fast beam ions and slow charge exchange ions are included. Both two‐ and three‐dimensional versions of this code have been developed for both two‐grid and three‐grid systems. The two‐dimensional model provides a qualitative description of the details of the ion flow and the charge exchange collisions. The three‐dimensional version has successfully predicted the experimentally observed erosion pattern in which the maximum erosion occurs at the geometric centers of accelerator grid apertures.

Calculation of the quasi-energies and resonance behavior of the hydrogen Lyman-α problem

Abstract Recently, Bakshi and Kalman presented numerical results for the quasi-energies of the n = 2 multiplet in the hydrogen Lyman-α transition for a plasma in which both strong static and oscillating electric fields are present. Here, we show how recent work on related magnetic and optical resonance problems provides a simplified mathematical treatment, as well as greater insight into the complex resonance behavior of this interaction.

Comparison of 2-D and 3-D models of grid erosion in an ion thruster

Numerical results of particle-in-cell/Monte Carlo calculations of accelerator grid erosion in an ion thruster are presented. Specifically, it is shown that a three-dimensional model is required to account for the experimentally observed pitting of the accelerator grid between grid apertures. Some comparisons with earlier two-dimensional, axisymmetric model are made, and it is shown that, for identical operating conditions of the thruster, the wear-through time in the three-dimensional model is about two to three times higher than that obtained previously with the two-dimensional model, namely on the order of 10,000 hours for sample calculation.