Volume and surface propellant heating in an electrothermal radio-frequency plasma micro-thruster

Abstract

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.

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

“…This value is in excellent agreement with the volume averaged result of 450 K after 1 s of operation [13]. After 100 s, the temperature near the walls has risen to 570 K, while the temperature on the centreline has risen to 535 K. Both these results are slightly lower than the volume averaged result of 580 K but are still in reasonable agreement.…”

“…T r1 corresponds to the central discharge region (r 1 = 0 − 1.1 mm) and is overlayed on Figure 4 at r = 0 mm, T r2 corresponds to the broad annular wall region (r 2 = 0.8−2.1 mm) and is overlayed on Figure 4 at r = 1.45 mm, and T r3 corresponds to the narrow annular wall region (r 1 = 1.4 − 2.1 mm) and is overlayed on Figure 4 at r = 1.75 mm. The initial fast heating can not be resolved using the present experimental data as the first measurement occurs at 1 s, so the same values as previously measured in the volume averaged temporal evolution study (τ 1 = 80 µs and T = 140 K) are used [13]. …”

“…Here, a spatiotemporal study of neutral gas temperature (T g ) in Pocket Rocket is performed for a 1.5 Torr (∼110 sccm), 60 W, N 2 discharge to confirm the two heating mechanisms suggested in the volume average temporal study and identify the third currently unknown heating mechanism [13]. Volume heating effects from ions will occur uniformly throughout the discharge volume, whereas surface heating effects will be more pronounced in regions in close proximity to the heated surface.…”

“…The experimental setup used here to capture temporal evolution of neutral gas temperature is the same as previously described [6,13], with a 4 mm diameter optic fiber used in conjunction with a monochromator to direct discharge photons onto a charge coupled device (CCD) array. Spectral resolution is 0.02 nm, which is sufficient to resolve the band head and tail region of the N 2 second positive system bands.…”

“…Volume averaged, temporally resolved, neutral gas temperature measurements in Pocket Rocket for a 60 W, 1.5 Torr N 2 discharge, showed three different heating timescales [13]. Fast heating, with a time constant of 80 µs, was suggested to be occurring through ion-neutral charge-exchange collisions within the discharge volume.…”