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The temporal evolution of neutral gas temperature over the first 5 min of operation for an electrothermal radio-frequency micro-thruster with nitrogen (N2) propellant was measured using rovibrational band matching of the second positive N2 system. Three distinct periods of gas heating were identified with time constants of τ1 = 8 × 10−5 s, τ2 = 8 s, and τ3 = 100 s. The fast heating (τ1) is attributed to volumetric heating processes within the discharge driven by ion-neutral collisions. The slow heating (τ3) is from ion neutralization and vibrational de-excitation on the walls creating wall heating. The intermediate heating mechanism (τ2) is yet to be fully identified although some theories are suggested.

Direct measurements and modelling of neutral gas heating in a radio-frequency (13.56 MHz) electrothermal collisional plasma micro-thruster have been performed using rovibrational band matching of the second positive system of molecular nitrogen (N2) for operating pressures of 4.5 Torr down to 0.5 Torr. The temperature measured with decreasing pressure for 10 W power input ranged from 395 K to 530 K in pure N2 and from 834 K to 1090 K in argon with 1% N2. A simple analytical model was developed which describes the difference in temperatures between the argon and nitrogen discharges.

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Computational fluid dynamics (CFD) simulations of a radio-frequency (13.56 MHz) electrothermal capacitively coupled plasma (CCP) micro-thruster have been performed using the commercial CFD-ACE+ package. Standard operating conditions of a 10 W, 1.5 Torr argon discharge were used to compare with previously obtained experimental results for validation. Results show that the driving force behind plasma production within the thruster is ion-induced secondary electrons ejected from the surface of the discharge tube, accelerated through the sheath to electron temperatures up to 33.5 eV. The secondary electron coefficient was varied to determine the effect on the discharge, with results showing that full breakdown of the discharge did not occur for coefficients less than or equal to 0.01.