Radio‐Frequency and Microwave Thrusters

Abstract

Both radio‐frequency (RF) and microwave electromagnetic energy can be used to heat a propellant gas to plasma temperatures followed by a gasdynamic converging{‐‐}diverging nozzle expansion in an electrothermal rocket mode. RF energy can be coupled to a gas either capacitively or inductively. Capacitive coupling operates effectively only at low gas pressures that are not desired in a thruster concept due to frozen flow losses. Inductive coupling has been shown to effectively couple RF energy to gases at atmospheric and higher pressures. However, because inductively coupled plasmas fill the entire absorption chamber, heat losses to the chamber walls are severe. Microwave energy can be used in one of several modes including, resonant microwave cavity, surface wave or propagating plasma modes, to ignite and sustain plasmas that can be localized away from any walls, thus reducing heat losses from the thruster. State‐of‐the‐art microwave thrusters utilize the microwave resonant cavity mode, where a standing wave is created within a cavity with plasma formation only at regions of high electric field intensity. Microwave thrusters have been demonstrated at frequencies ranging from 915 MHz to 14.5 GHz at power levels from 10 to 11 000 W. High specific impulses have been measured for microwave thrusters for both helium and ammonia propellants.

Bidirectional vortex stabilization of a supersonic inductively coupled plasma torch

“…For example, RF ICPs have been proposed as novel electrothermal space propulsion systems. However, with conventional gas injection configurations, heat losses to the source walls are currently seen as too high and lead to a much reduced thruster performance [46,47,53]. Reverse vortex gas injection may offer an interesting solution to overcome this challenge.…”

“…Supersonic ICPs have a number of practical industrial applications however, such as plasma assisted supersonic jet deposition, where improved thermal efficiency when using reverse vortex injection offers a number of benefits in terms of reduced gas or input power consumption, or even the possibility of different torch construction materials [44,45]. Additionally, as reverse vortex injection is extremely promising for reducing heat losses to the torch walls, this may be an enabling technology for high-performance applications, such as electrothermal ICP plasma thrusters, where heat losses currently represent a strong technical barrier [28,46,47].…”

Characterization of a radio-frequency inductively coupled electrothermal plasma thruster

“…Since there are no immersed electrodes, no erosion occurs and significantly higher lifetimes are possible. While METs have been extensively studied in the literature, [6][7][8][9][10][11][12] they have only recently flown in space. METs require high frequencies and are resonant devices where the thruster size and operating frequency are strongly coupled.…”

“…[19][20][21] Such ICP systems are well studied and understood in these communities, and they routinely achieve maximum temperatures above 10 000 K. [22][23][24][25] The first proposal to use ICPs as electrothermal thrusters appears at least as early as 1963, 26 with serious experiments mainly performed in the late 1980s and early 1990s. 10,27 For example, the Aerospace Corporation tested a 100 W thruster at an RF frequency of 36 MHz, with different gas mixtures. 27 Time-of-flight measurements gave a specific impulse of 377 s for a mixture of 3% O 2 in helium, and an estimated thrust of 12.6 mN with a thruster efficiency of 20%.…”