Optical Diagnostics Options for Bismuth Hall Thrusters

Abstract

A bismuth-fed Hall thruster has recently been selected for further development as a potential solution for faster missions to the outer planets. With higher efficiency and higher power handling, this two-stage Thruster with Anode Layer (TAL) should meet the necessary requirements for such missions. As the thruster is developed, there will be a need for nonintrusive diagnostic methods to optimize the geometry and operating conditions. Such optical diagnostic methods for analyzing the velocity, energy, and number densities of BiI and BiII are developed and discussed here. Candidate transitions are selected and their lineshapes modeled with respect to hyperfine splitting and broadening mechanisms. Suitable transitions are selected on the basis of relative strength, hyperfine data, and accessibility to tunable diode lasers; candidate transitions include the 306.86, 784.25, and 854.7nm lines of BiI and the 143.68, 796.7, and 854.3nm lines of BiII. Preliminary spectrum measurements using a microwave discharge are presented and future work is discussed. Nomenclature A= magnetic moment hyperfine splitting parameter a = Voigt "a" parameter B = quadrupole moment hyperfine splitting parameter E = energy shift due to hyperfine splitting by nuclear spin = shift in wavelength/frequency from linecenter v = integrated range of wavelengths/frequencies v x = lineshape FWHM due to broadening mechanism, x E = energy between two electronic states F = total atomic angular momentum quantum number, including nuclear spin f 12 = oscillator strength x = lineshape function for broadening mechanism, x I = nuclear spin quantum number I v /I 0,v = ratio of transmitted to incident light intensity at a given wavelength/frequency, v In = relative intensity of hyperfine split transition J = total atomic angular momentum quantum number, excluding nuclear spin j = single electron angular momentum quantum number k v = spectral absorption coefficient L = total orbital angular momentum quantum number L a = absorption path length ,v = wavelength or frequency of light m = atomic mass n = number density S = total electron spin angular momentum quantum number S 12 = linestrength T = kinetic temperature u = bulk velocity v 0 = linecenter frequency w = parameter indicating distance from linecenter

Spectroscopic Measurement of Bismuth for Optical Diagnostics

“…A combination of laser-induced fluorescence (LIF) and absorption spectroscopy will be used to perform a portion of the necessary diagnostics. This paper will summarize the previously-presented 12 analysis of the bismuth spectrum, and revise the previously discussed line selections. It is expected that LIF will be used to probe excited states in the BiI and BiII atomic and ionic particle stream to deduce the three-dimensional (3D) velocity (energy) distribution.…”

“…In the previous paper, 40 th Joint Propulsion Conference paper AIAA-2004-3946 12 , several candidate Bi transitions were selected for analysis. The transition selected for ground state BiI number density determination by atomic resonance absorption spectroscopy was the line at vacuum wavelength 306.86nm (6p 3 4 S 3/2 -7s 4 P 1/2 ).…”

“…When measured in the laboratory, transitions will typically be broadened. The previous paper 12 discussed Doppler (assuming a Gaussian velocity distribution) and Lorentzian broadening mechanisms, and provided plots of a Voigt-profile which combines the effects of each.…”

Measurements in Stationary Reference Bi Discharge Cell for Diagnostics of a Bismuth Hall Thruster

“…In order to measure these quantities via non-intrusive optical diagnostics, it is necessary first to select electronic transitions for measurement, and to analyze the structure of each transition so that data can be properly interpreted. The general spectroscopy necessary to begin this analysis for bismuth has been discussed in previous conference proceedings, 5,6,7 and will not be repeated here.…”

“…Previously, 5,6,7 several candidate Bi transitions were selected for analysis and discussed. The transition selected for ground state Bi I number density determination by atomic resonance absorption spectroscopy was the line at vacuum wavelength 306.86nm (6p The selection of the 680.9 nm transition provides several advantages.…”