Control of electron, ion and neutral heating in a radio-frequency electrothermal microthruster via dual-frequency voltage waveforms

Abstract

The development of low power micro-propulsion sources is of recent interest for application on miniature satellite platforms. Radio-frequency (rf) plasma electrothermal microthrusters can operate without a space-charge neutralizer and provide increased control of spatiotemporal power deposition. Further understanding of how the phase-resolved rf plasma heating mechanisms affect the phase-averaged bulk plasma properties, e.g. neutral gas temperature, could allow for in-flight tailoring of plasma thrusters. In this work, experimentally validated two-dimensional fluid-kinetic simulations were employed to study the spatially resolved electron and ion power deposition and neutral gas heating in a dual-frequency rf electrothermal microthruster operating at 1.5 Torr plenum pressure in argon. Experimental validation was performed through a comparison of the measured and simulated phase resolved Ar(2p 1 ) excitation rates, showing close agreement. Two types of dual-frequency voltage waveforms were investigated, and comprise the combination of a 13.56 MHz voltage waveform with 27.12 MHz and 40.68 MHz waveforms, respectively. Varying the phase offset of the higher harmonic relative to the fundamental 13.56 MHz voltage waveform was found to modulate the dc self-bias voltage by 11% and 3% of the maximum applied peak-to-peak voltage, respectively. The 13.56 MHz, 27.12 MHz dual-frequency voltage waveform provided the highest Electric propulsion (EP) sources are an established alternative to traditional cold-gas and monopropellant thrusters for employment on satellites 1,2 . The miniaturization of EP sources for micro-satellites is an increasingly important area of research, resulting in the development of new compact, low-power and charge-neutral 3 , prototypes 4-6 . Electrothermal EP sources are particularly suited to miniaturization as the maximum gas temperature typically scales inversely with the thruster volume 1 .One such electrothermal thruster is the Pocket Rocket, a low power (≤ 50 W) asymmetric radiofrequency (rf) capacitively coupled micro-thruster 7-13 . The Pocket Rocket operates at a relatively high pressure (≥ 133 Pa (1 Torr)) as this represents the Paschen (pressure × distance) minimum for ignition for the dimensions of the source 14 . The thruster is typically powered by a 12.8 -13.8 MHz rf voltage 15 , where power is deposited into the propellant primarily through ion-neutral charge exchange collisions in the powered electrode sheath 9,16 . Thrust is produced as the hot (1000 K) neutrals exit the thruster in an exhaust plume. As the exhaust is neutral no external neutralizer is required 17 .The thruster and power supply have recently been demonstrated to fit within a standard 1U CubeSat frame, as described in Ref. 2 and 15.The physically asymmetric geometry of the Pocket Rocket thruster ensures that a substantial negative dc self-bias voltage forms on the radial dielectric wall adjacent to the powered electrode 9 . This dc self-bias voltage forms to balance the net positive and negative fluxes onto the radial ...

Inducing locally structured ion energy distributions in intermediate-pressure plasmas

“…A depth of field of 24 mm, corresponding to the axial length of the source region, was applied to the simulated PROES images through a top hat integration along the line of sight 47 . Previous work has demonstrated agreement between the measured and simulated phase-resolved, Ar(2p 1 ) electron impact excitation rates in this source 22,36 . Here, experimental comparisons are presented for 13.56 MHz, 27.12 MHz and 40.68 MHz applied voltage frequencies at an applied voltage amplitude of φ rf = 450 V. Additional simulations are employed to draw conclusions up to 108.48 MHz.…”

“…The sheath extension is calculated as in Ref. 36, where the radial sheath edge S R is determined as the radius R that satisfies the Brinkmann criterion 37 .…”

“…As noted previously, increasing the driving frequency leads to an increased Ar + density adjacent to the powered electrode, giving rise to a reduced sheath width and an increase in the sheath capacitance 31 . The negative dc self-bias voltage at the radial wall enforces an relatively phase-independent radial Ar + ion flux 31,36,55 . A reducing sheath extension, coupled with an increasing Ar + mean-free-path results in a reduced chance for ion-neutral collisions within the sheath volume as the driving frequency increases.…”

Decoupling ion energy and flux in intermediate pressure capacitively coupled plasmas via tailored voltage waveforms

“…The use of tailored voltage waveforms in low-pressure ( 67 Pa, 0.5 Torr) planar rf-CCP sources is already well established 21,[36][37][38][39][40][41][42] , where application of the EAE has enabled enhanced control of the ion energy while maintaining near-constant ion flux through a modulation of the dc self-bias voltage 15,17,28,43 . Recently, work has begun to focus application in intermediate pressure discharges (≈ 133 Pa, 1 Torr) 24,44,45 , atmospheric pressure discharges 26,[46][47][48][49][50] , and non-planar geometries 32,41,51 .…”

“…As such, variation of the dc self-bias voltage in a HC source cannot typically be achieved without a corresponding change in ion flux 51 . Similar behavior in the ion flux is observed when transitioning from low pressure discharges to intermediate pressure discharges 26,32 .…”