Simulation studies of RF excited micro-cavity discharges for micro-propulsion applications

Abstract

A detailed computational modelling study of the micro-cavity discharge (MCD) thruster is presented. The MCD thruster concept incorporates a microdischarge with dielectric covered electrodes operated using alternating current (ac) excitation. The thruster geometry comprises a constant area pipe section followed by a divergent micro-nozzle. Two ring electrodes are embedded in the wall of the pipe section with the downstream electrode close to the pipe-micronozzle intersection. A microdischarge plasma is generated in argon propellant gas flowing through the thruster. A detailed plasma dynamics model coupled with the compressible Navier-Stokes equations is used to study the flow and plasma phenomenon in the thruster. Results show a highly pulsed microdischarge with plasma densities of ∼10 19 m −3 and current densities ∼700 mA cm −2 for an ac excitation in the radio frequency (RF) regime of 10 and 20 MHz. The dominant gas heating mechanism in these discharges is through ion Joule heating. Higher electron densities and spatially dominant thermal source terms are observed for the 20 MHz excitation compared with 10 MHz excitation. The addition of 20% nitrogen to the flow resulted in much better performance compared with the pure argon cases. A peak gas temperature rise ∼200 K is seen for a cycle-averaged power deposition of 76 mW. For the conditions explored in this study, the overall specific impulse of the thruster operating with the microdischarge plasma is found to be about 25% higher than a corresponding cold gas case.

Microplasma thruster powered by X-band microwaves

“…A comparison of representative operating parameters/conditions and thrust performance is given in Table II, for several microplasma thrusters of electrothermal type (with power inputs on the order of 1-10 W) that have so far been reported in the literature. [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]…”

“…Briefly, compared with rare gas atoms, such molecules are characterized by various low energy rotational, vibrational, and electronic energy states that are excited by plasma electrons; then, the excited states of molecules are quenched by collisions with background gas atoms/molecules, thus providing a pathway for transfer of the electron energy to gases through a series of inelastic collision processes. 39 When N 2 is added to Ar, a cooling effect occurs on the plasma electrons, and the electron temperature T e tends to be reduced. [117][118][119] The EEDFs in pure Ar plasmas are nearly Maxwellian below the inelastic collision thresholds with a slight depletion of electrons for energies E e > 11.6 eV [the first excitation threshold for Ar or the excitation energy for Ar*(3p 5 4s 3 P 2 )]; 116 with increasing the fraction of N 2 , a reduction in the EEDF occurs in the range E e ∼ 1-5 eV, owing to some resonant electron-molecule vibrational excitation processes.…”

“…The electromagnetic thrusters rely on the plasma acceleration via interaction of the current and magnetic fields: micro pulsed plasma thrusters. 31,32 The electrothermal thrusters rely on the plasma/gas heating via plasma discharge followed by supersonic plasma/ gas expansion through de Laval nozzles: micro arcjet thrusters, 33,34 hollow cathode thrusters, 35 and plasma thrusters using dc microdischarges, 36,37 ac/rf microcavity discharges, 38,39 capacitively coupled rf discharges in a dielectric tube, [40][41][42][43][44][45][46][47] and microwave discharges. 48,49 We have developed a mm-scale microplasma thruster of the electrothermal type with azimuthally symmetric surface wave-excited plasmas (SWPs), [50][51][52][53][54][55][56][57][58] consisting of a microplasma source followed by a converging-diverging micronozzle as schematically shown in Fig.…”

Rarefied gas electro jet (RGEJ) micro-thruster for space propulsion

“…In a xenon discharge at 100 (Torr), it was assumed by Boeuf et al [22] that only 25% of the energy was deposited in the neutral gas and the maximum gas temperature (~460 K) of their simulation matched relatively well with the gas temperature (~500 K) obtained from experiments. In this case, the ion Joule heating rapidly equilibrates with the gas since the ion-neutral mean free path is orders of magnitude smaller than the device length scale, a conservative approach is taken to obtain the upper bound for the gas heating, and the thermalization factor is assumed to be equal to one [21].…”

Fabrication and operation of microplasma reactor array for maskless nanoscale material etching

“…Compared to conventional low-pressure large-scaled plasmas used for material processing in the semiconductor industry, microplasmas possess special characteristics such as small size, high concentration of active particles, high-pressure or atmospheric pressure operation, low temperature, non-equilibrium and convenient portability, microplasmas have potential applications in many fields, such as light sources [1][2], analytical chemistry [3], micro-propulsion [4], biomedical [5][6][7]and environmental applications [8], etc. In recent years, the applications of microplasmas in direct material etching have attracted more and more attention.…”