TY - JOUR
T1 - Spatio-temporal plasma heating mechanisms in a radio-frequency electrothermal microthruster
AU - Doyle, Scott James
AU - Gibson, Andrew Robert
AU - Flatt, Jason
AU - Ho, Teck Seng
AU - Boswell, Rod
AU - Charles, Christine
AU - Tian, Peng
AU - Kushner, Mark J.
AU - Dedrick, James Peter
N1 - © 2018, IOP Publishing Ltd. This is an author-produced version of the published paper. Uploaded in accordance with the publisher’s self-archiving policy. Further copying may not be permitted; contact the publisher for details.
PY - 2018/8/30
Y1 - 2018/8/30
N2 - Low-power micro-propulsion sources are currently being developed for a variety of space missions. Electrothermal plasma thrusters are of specific interest since they enable spatial control of the power deposition to the propellant gas. Understanding the mechanisms whereby electrical power is coupled to the propellant will allow for optimization of the heating and fuel efficiencies of electrothermal sources. Previous studies of radio frequency (RF) plasmas have shown a dependence of the gas and electron heating mechanisms on the local collisionality. This is of particular importance to thrusters due to the large pressure gradients that exist between the inlet and outlet when expanding into vacuum. In this work, phase-resolved optical emission spectroscopy and numerical simulations were employed to study plasma heating in an asymmetric RF (13.56 MHz) electrothermal microthruster operating in argon between 186-226 Pa (1.4-1.7 Torr) plenum pressure, and between 130-450 V (0.2-5 W). Three distinct peaks in the phase-resolved Ar(2p
1) electron impact excitation rate were observed, arising from sheath collapse heating, sheath expansion heating, and heating via secondary electron collisions. These experimental findings were corroborated with the results of two-dimensional fluid/Monte Carlo simulations performed using the Hybrid Plasma Equipment Model (HPEM). The influence of each mechanism with respect to the position within the plasma source during an α-γ mode transition, where plasma heating is driven via bulk and sheath heating, respectively, was investigated. Sheath dynamics were found to dictate the electron heating at the inlet and outlet, this is distinct from the center of the thruster where interactions of secondary electrons were found to be the dominant electron heating mechanism. Optimization of the heating mechanisms that contribute to the effective exhaust temperature will directly benefit electrothermal thrusters used on miniaturized satellite platforms.
AB - Low-power micro-propulsion sources are currently being developed for a variety of space missions. Electrothermal plasma thrusters are of specific interest since they enable spatial control of the power deposition to the propellant gas. Understanding the mechanisms whereby electrical power is coupled to the propellant will allow for optimization of the heating and fuel efficiencies of electrothermal sources. Previous studies of radio frequency (RF) plasmas have shown a dependence of the gas and electron heating mechanisms on the local collisionality. This is of particular importance to thrusters due to the large pressure gradients that exist between the inlet and outlet when expanding into vacuum. In this work, phase-resolved optical emission spectroscopy and numerical simulations were employed to study plasma heating in an asymmetric RF (13.56 MHz) electrothermal microthruster operating in argon between 186-226 Pa (1.4-1.7 Torr) plenum pressure, and between 130-450 V (0.2-5 W). Three distinct peaks in the phase-resolved Ar(2p
1) electron impact excitation rate were observed, arising from sheath collapse heating, sheath expansion heating, and heating via secondary electron collisions. These experimental findings were corroborated with the results of two-dimensional fluid/Monte Carlo simulations performed using the Hybrid Plasma Equipment Model (HPEM). The influence of each mechanism with respect to the position within the plasma source during an α-γ mode transition, where plasma heating is driven via bulk and sheath heating, respectively, was investigated. Sheath dynamics were found to dictate the electron heating at the inlet and outlet, this is distinct from the center of the thruster where interactions of secondary electrons were found to be the dominant electron heating mechanism. Optimization of the heating mechanisms that contribute to the effective exhaust temperature will directly benefit electrothermal thrusters used on miniaturized satellite platforms.
U2 - 10.1088/1361-6595/aad79a
DO - 10.1088/1361-6595/aad79a
M3 - Article
SN - 0963-0252
VL - 27
JO - Plasma sources science & technology
JF - Plasma sources science & technology
IS - 8
M1 - 085011
ER -