Direct Measurement of Axial Momentum Imparted by an Electrothermal Radiofrequency Plasma Micro-Thruster

Abstract

Gas flow heating using radio frequency plasmas offers the possibility of depositing power in the center of the flow rather than on the outside, as is the case with electro-thermal systems where thermal wall losses lower efficiency. Improved systems for space propulsion are one possible application and we have tested a prototype micro-thruster on a thrust balance in vacuum. For these initial tests, a fixed component radio frequency matching network weighing 90 g was closely attached to the thruster in vacuum with the frequency agile radio frequency generator power being delivered via a 50 ohm cable. Without accounting for system losses (estimated at around 50%), for a few 10 s of Watts from the radio frequency generator the specific impulse was tripled to ∼48 s and the thrust tripled from 0.8 to 2.4 milli-Newtons.

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

“…The third set of simulations "PR vacuum condition" models the performance of PR in a vacuum environment, and provides thrust results that are otherwise currently unobtainable from experiment. Finally, the fourth set of simulations "MiniPR vacuum condition" is a direct representation of the MiniPR experiment performed in the Wombat space simulation chamber [27]. The simulated thrust is compared to the experimentally measured values and the theoretical expectation in order to verify the reliability of the results obtained by simulation.…”

Vacuum Testing of a Miniaturized Switch Mode Amplifier Powering an Electrothermal Plasma Micro-Thruster

“…Although it is beyond the scope of this study to fully quantify the thrust gain from such a pressure increase of Figure 6, a quick estimate similar to the calculations and computer simulations of Charles and Boswell [8], Fridman et al [12], and Ho et al = 322 m.s −1 (where T is the gas temperature, k B = 1.38 × 10 −23 J.K −1 is the Boltzmann constant, γ Ar = 1.667 is the specific heat capacity for argon and m Ar = 6.64x10 −26 kg is the atomic mass of argon), the corresponding thrust from the momentum term (neglecting the neutral gas pressure term [12,29]) would be F cold gas = c s dm dt ∼ 0.19 mN at T ∼ 300 K. Recent computer simulations [24] show that approximately one tenth of the power injected into the plasma is converted into gas heating: hence with 10 Watts into MiniPR, 1 Watt or 1 J/s of kinetic energy ǫ is effectively transferred into heating the gas: ǫ = . The thrust gain from the plasma would be F plasma ∼ 0.17 mN yielding a total thrust of F total ∼ 0.36 mN.…”

“…System (2) is the reference laboratory radiofrequency set-up consisting of a custom-made miniaturized variable frequency solid-state matching network fed by a commercial (30-1,500 W MKS Spectrum) frequency adjustable (13.28-13.83 MHz) radiofrequency generator [16]. This impedance matching network was previously designed and implemented with a solenoid shape inductor [12,16] with 12 loops. The solenoid inductor has magnetic field extending way beyond its physical volume, thus can induce high losses when placed near metal object as well as have electromagnetic interference with other components nearby: the power into MiniPR for system (2) is P MiniPR = γ match loss (2) P mks ∼ 0.5 P mks .…”

“…Vacuum testing is carried out in the 1 meter-diameter 2 meterslong WOMBAT vacuum chamber [12] recently upgraded with a new vacuum pumping system (scroll, turbomolecular and cryogenic pumps) and a suite of access and viewing ports (Figure 2). In this second configuration both MiniPR and the Converter are inserted in WOMBAT vacuum so as to provide three unobstructed views of the Converter, the plasma via MiniPR plenum window and the plasma exhaust plume, respectively.…”

“…Neutral gas heating in Pocket Rocket mostly results from ion-neutral collisions in the bulk plasma (charge-exchange and elastic) and from heating at the plasma cavity radial walls [11]. While propellant heating and thrust production has now been demonstrated [12] and complemented by computational studies (computer fluid dynamics [13] and particle in cell [14] codes) following earlier analytical studies [15], Pocket Rocket's viability for space use will only be achieved by refinements to the radiofrequency [16] and gas [17] delivery systems to improve overall efficiency and footprint. Optimum operation and viability in space would require achieving pulsed plasma operation combined with pulsed gas operation.…”