Continuous-wave cavity ring down spectroscopy sensor for Hall thruster erosion measurement

Tao, Lei, author
Yalin, Azer P., advisor
Marchese, Anthony J., committee member
Menoni, Carmen S., committee member
Williams, John D., committee member
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Hall thruster and other Electric propulsion (EP) devices have become appealing alternatives to traditional chemical propulsion thrusters for space applications due to this high specific impulse (Isp), which allows high fuel efficiency. However, the uncertainty of the lifetime for Hall thruster hinders its development in future applications requiring a long operational time (several thousands of hours). Sputter erosion of boron nitride (BN) acceleration channel wall is principal lifetime limitation for Hall thrusters. The sputtered particles can redeposit causing a critical contamination effect. There is an urgent need for improved experimental tools to understand the BN sputter erosion process and lifetime assessment for Hall thrusters. The present research applies continuous wave cavity ring down Spectroscopy (CW-CRDS) as a diagnostic tools to study the sputter erosion process for Hall thrusters. Two CW-CRDS erosion sensors have been developed for in situ monitoring of sputtered manganese (Mn) and BN. As a stepping stone towards BN detection, a Mn erosion sensor was first developed. This sensor is based upon detection of Mn atoms via an absorption line from ground state at a wavelength of 403.076 nm. Measurements of sputtered Mn atom number density and its hyperfine structure are presented. Additionally, end-point detection has been done for a multilayer target, which can be potentially applied to the industrial sputtering systems. The same system has also been applied for detecting eroded atoms from the acceleration channel wall in an anode layer type Hall thruster. The results show the validity of the CW-CRDS erosion sensor for Hall thruster lifetime estimation. A BN erosion sensor has also been developed for the detection of sputtered boron atoms from Hall thrusters by probing atomic absorption lines of boron (250 nm) with CW-CRDS. A photonic crystal fiber was used to couple the ultraviolet laser light to the cavity within the vacuum chamber. The experimental detection limits and signal-to-noise values show potential for Hall thruster BN erosion studies. Finally, the velocity distributions of sputtered boron atoms at different ion energies were measured with laser induced fluorescence (LIF). These velocity distribution are necessary for interpretation of signals from the BN erosion sensor.
2011 Spring.
Includes bibliographical references.
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boron nitride
Hall thruster
cavity ring down spectroscopy
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