Spatiotemporal study of gas heating mechanisms in a radio-frequency electrothermal plasma micro-thruster

Abstract

A spatiotemporal study of neutral gas temperature during the first 100 s of operation for a radio-frequency electrothermal plasma micro-thruster operating on nitrogen at 60 W and 1.5 Torr is performed to identify the heating mechanisms involved. Neutral gas temperature is estimated from rovibrational band fitting of the nitrogen second positive system. A set of baffles are used to restrict the optical image and separate the heating mechanisms occurring in the central bulk discharge region and near the thruster walls. For each spatial region there are three distinct gas heating mechanisms being fast heating from ion-neutral collisions with timescales of tens of milliseconds, intermediate heating with timescales of 10 s from ion bombardment on the inner thruster tube surface creating wall heating, and slow heating with timescales of 100 s from gradual warming of the entire thruster housing. The results are discussed in relation to optimizing the thermal properties of future thruster designs.

A Comprehensive Cold Gas Performance Study of the Pocket Rocket Radiofrequency Electrothermal Microthruster

“…The specifications of PR have been previously described in depth in Charles and Boswell [10] and Greig [28]. The detailed dimensions and geometry of PR are specified later in the description of the PR simulation mesh.…”

“…Depending on the amount of RF power supplied (0.1-50 W), the plasma heats the gas to temperatures in excess of 1000 K [30][31][32][33] by depositing power directly into the propellant, thereby enhancing thrust production over cold gas performance levels, with minimal increase in thruster complexity. By controlling the propellant flow rate and the RF power in continuous or pulsed operation, PR can produce precisely manipulable thrust on the milliNewton-scale.…”

“…At the maximum pumping rate, the ratio of the PR plenum pressure to the chamber pressure is about 4 : 1, primarily dependent on the diameter and length of the discharge volume and the expansion tube, and secondarily on the species of gas. In earlier experiments [30][31][32][33], the pressure ratio was maintained at 2 : 1 by limiting the pumping rate.…”

Transient propagation dynamics of flowing plasmas accelerated by radio-frequency electric fields

“…Non-invasive optical diagnostic techniques have previously proven useful to the investigation of electric propulsion sources, providing enhanced spatial resolution compared to electrostatic probes. For example, optical emission spectroscopy has been used to measure neutral-gas heating in electrothermal plasma thrusters [15], and imaging and laser induced fluorescence velocimetry are effective for the study of cross-field electron transport [16] and ion velocities [17] in Hall-effect thrusters.…”

Neutral gas temperature estimates and metastable resonance energy transfer for argon-nitrogen discharges

“…The temporal evolution of a 60 W, 1.5 Torr N 2 discharge over the first 300 s of operation has previously been analyzed and used to identify three distinct heating mechanisms in Pocket Rocket with time constants of 80 ls, 8 s, and 100 s. 55 The heating mechanisms were identified as fast heating from ion-neutral collisions, intermediate heating from ion bombardment of the thruster walls creating wall heating, and slow heating from the thruster housing increasing in temperature over time. 56 The same experiment is repeated here using a 60 W, 1.5 Torr total plenum pressure argon discharge with 10% N 2 added, as 1% N 2 does not give sufficient rovibrational signal for rovibrational band fitting. Temporal evolution of gas temperature spanning seven orders of magnitude from 70 ls to 300 s is captured through the use of a specially written Labview program to simultaneously control the RF power generator and CCD array and synchronize the initiation and duration of the discharge with the integration timing of the CCD array.…”