Initial Cavity Ring-Down Density Measurement on a 6-kW Hall Thruster

Wensheng Huang, Alec Galimore, Timothy Smith, Lei Tao, Azer Yalin
2011 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit   unpublished
We present the initial results from a cavity ring-down sensor for measuring the density of sputtered boron atoms originating from the discharge channel of a 6-kW Hall thruster. The sensor traps 250 nm light in a high-finesse cavity to greatly increase its sensitivity to boron atoms. Measurements were obtained with the thruster operating at seven conditions spanning discharge voltages from 150 to 600 V, and anode mass flow rates from 10 to 30 mg/s. Power level at these operating conditions
more » ... from 1.5 to 10 kW. Boron density was found to vary from <1×10 14 to ~1×10 15 m -3 with the higher density measurements being found near the two channel walls. We apply a simple two-dimensional velocity model to obtain boron flux. The boron flux is then correlated to the boron nitride sputter rate, which is found to vary from 4×10 -4 to 6×10 -3 mm 3 /s. A simple power law equation is formulated to correlate the relative sputter rate for the 6-kW Hall thruster as a function of the tested operating conditions. Although the uncertainty in this initial result is high, the result clearly demonstrates previously untapped potential for the use of cavity ring-down spectroscopy to study the problem of Hall thruster channel wall erosion. high because it requires accurate knowledge of the state of the plasma species in the region where the measurement is taken. Cavity ring-down spectroscopy (CRDS) provides a novel solution to the problem of obtaining parametric experimental sputter rate data. This technique is non-intrusive, spatially resolved in the radial direction, and capable of measuring absolute density with extreme sensitivity. The time scale for deploying CRDS diagnostic is on the order of hours to days. For this technique, laser light trapped in a high-finesse cavity is resonantly absorbed by erosion products. Since the trapped laser light makes many passes through the test sample, CRDS has an exceptionally high sensitivity to even minute amounts of erosion products. Since the measurement results yield lineintegrated product density, some form of inversion is needed to obtain radial density profiles. One can then combine the CRDS density results with velocity results from modeling or a velocimetry technique like laser-induced fluorescence (LIF) to obtain the sputtering flux. Alternately, one can perform control experiments to correlate product density to sputter yield given a priori knowledge about the bombarding particles. The main disadvantages of CRDS are the high initial cost of associated equipment relative to more traditional diagnostics, the complexity of the setup, and the associated implementation challenges. However, CRDS can yield tremendous long-term savings in resource by greatly reducing the thruster and facility run time needed to obtain data of fidelity comparable to traditional diagnostics. The history of the use of CRDS in Hall thruster-related research is fairly short. The first work investigating the possibility of the use of CRDS for studying erosion in electric propulsion devices was by Surla, et al., in 2004. 9 The initial focus was on anode layer Hall thrusters, 10 which have metallic wall. Metallic elements have transition lines that were easier to access than the ceramic elements found in the walls of magnetic layer thrusters. To study the application of CRDS for magnetic layer type thrusters, which are much more common in the U.S. than anode layer type, a collaborative program between the University of Michigan and Colorado State University was initiated. Initial work focused on the feasibility of generating, delivering, and trapping 250 nm light for probing boron, as well as the use of continuous-wave-CRDS over pulsed-CRDS to enhance sensitivity. 11, 12 At the same time, basic research work was carried out on the sputtering behavior of boron nitride when bombarded by low-energy xenon. 4 The said collaborative research program culminated in the first published demonstration of the use of CRDS to measure the channel wall erosion rate in a magnetic layer Hall thruster, which is described in this paper. II. Theories A. Principles of Cavity Ring-Down Spectroscopy Cavity ring-down spectroscopy is a type of absorption spectroscopy. A great review of the topic and recent development of CRDS as a diagnostic can be found in an article by Berden, et al. 13 CRDS is based on the principle that by sending a beam of photons through an absorber and measuring the change in intensity, one can obtain the absolute line-integrated density of the absorber. CRDS differs from traditional absorption spectroscopy in that a cavity, formed from a pair of highly reflective mirrors, is used to trap light. Instead of one pass, the trapped photons make many passes through the absorbing medium. Each time the trapped light strikes one of the mirrors, a small fraction of it leaks out. The intensity of the leaked light is measured by a detector. If the light source is a continuouswave laser, the laser beam is cut off rapidly (e.g. with an acousto-optical modulator), and then the light intensity within the cavity begin to decay. The characteristic time constant of the decay, extracted from the detector signal, can then be used to calculate the absolute line-integrated density of the absorber inside the cavity. Figure 1 illustrates the operational principles of CRDS. The grey arrow pointing from the detector to the acousto-optical modulator (AOM) indicates some form of feedback so the AOM knows when to cut off the laser beam. Equations (1) and (2) govern the ring-down behavior and time constant in a typical cavity,
doi:10.2514/6.2011-5994 fatcat:g6zvlvp6sncgbkpuyw6c4mbzbi