Figure 1. One of the frequency doubling chamber in the cavity ring-down spectroscopy laser in operation. The input light is 500 nm while the output light is 250 nm.
Figure 2. Average velocity vector plot of singly-charged ion in a 6-kW laboratory Hall thruster
operating at 300 V discharge, 20 mg/s anode mass flow rate. (From ref. 2).
The erosion of the channel walls is a primary failure mechanism for Hall thrusters.
Hall thrusters typically need to demonstrate operations for over 10,000 hours during flight qualification.
As such the ceramic wall material is typically chosen to be boron nitride for its low sputter yield.
This, however, also means that traditional methods (mass-loss, profilometry, and more) for studying Hall thruster wall erosion are very time consuming, as hundreds of hours of operation is needed in order to obtain a few points of meaningful data.
The long test time needed meant that empirical studies of Hall thruster channel wall erosion have progressed slowly over the years.
We will utilize a two-prong approach to learn more about the physical processes behind Hall thruster channel wall erosion.
We have performed two-axis laser-induced fluorescence (LIF) studies on the xenon neutrals and singly-charged ions near the channel wall inside the Hall thruster.
This data will allow us to construct angular energy distribution functions to study what kind of particles are bombarding the channel walls.
We will also implement a technique called cavity ring-down spectroscopy (CRDS) to study the density of the boron erosion products that are coming out of the thruster.
Combine with boron LIF we can back out the flux of boron particles being ejected, and the rate of channel wall erosion.
Due to the extraordinary sensitivity of these advanced optical diagnostics, we can obtain these measurements in matters of tens of minutes to hours in a spatially resolved and non-intrusive manner.
Figure 2 shows one of the first vector plot of averaged velocity for singly-charged xenon bombarding the walls of a 6-kW laboratory Hall thruster operating at its nominal condition.
Since LIF data can be used to extract velocity distribution functions, further analysis can retrieve the angular energy distribution functions of the bombarding particles.
These data can then be used to extrapolate the kind of angle and energy that the channel walls are seeing from the bombarding particles.
Combined with ion gun bombardment studies currently being carried out at Colorado State University, directed by Prof. Azer Yalin, the behavior of the ejected particles can be predicted.
Combined with erosion rate data from CRDS, we hope to ultimately be able to carry out fast parametric studies of Hall thruster channel wall erosion.
The main application of these diagnostics will be to speed up flight qualification and re-qualification tests by providing near-real time feedback.
Selected Relevant Publications
Wensheng Huang, Brittany Drenkow, and Alec Gallimore, "Laser-Induced Fluorescence of Singly-Charged Xenon Inside a 6-kW Hall Thruster,"
AIAA-2009-5355, 45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Denver, CO, 2-5 August 2009.
Huang, W., Gallimore, A. D., and Smith, T. B., "Two-Axis Laser-Induced Fluorescence of Singly-Charged Xenon inside a 6-kW Hall Thruster,"
49th AIAA Aerospace Sciences Meeting, submitted, Orlando, FL, 4-7 Jan., 2011.
W. Huang, T. B. Smith, C. J. Durot, A. D. Gallimore, and A. P. Yalin, "Development of a Cavity Ring-Down Diagnostics for Studying Hall Thruster Channel Erosion,"
57th Joint Army Navy NASA Airforce (JANNAF) Propulsion Meeting, Colorado Springs, CO, May 3-7, 2010.