Capacitively coupled radio-frequency thrusters (CCRFT) are an attractive propulsion option for nanosatellites due to their low operating power, relatively high thrust, and low erosion of the electrodes. Such thrusters can serve as a direct replacement to cold gas thrusters, achieving higher specific impulse. CCRFTs produce thrust by generating a gaseous electric discharge within a dielectric tube by means of RF power. The RF circuit couples to the plasma via a capacitor therefore blocking the dc current component. The discharge heats the gas inside the tube, and the heated gas is ejected from the tube tip or nozzle to produce thrust.
A capillary capacitively coupled radio-frequency thruster (C3RFT) is being developed at the Aerospace Plasma Laboratory, Technion. The small diameter of the dielectric tube allows operation of the device in very low flow rates 4-16 SCCM in Ar. The thruster utilizes a non-symmetric electrode arrangement and no nozzle. At constant input power, the thrust is shown to increase linearly with the flow rate. At an input power of 14 W and a flow rate of 8 SCCM, the thrust increases significantly by 60 % compared to the cold flow case, for a total thrust of ≈ 160 µN and specific impulse ≈ 70 s in Ar.
The C3RFT discharge is shown to be sensitive to electrode polarity. When the larger back electrode is powered and the front electrode grounded a mode transition of the discharge is observed. This phenomenon is marked by an appearance of bright plasma exhaust and reduction in input power from ≈ 15.17 W to ≈ 9 W. After the transition, the thrust to power ratio increases by 25 %. A sheath breakdown model is proposed to explain this transition. The thrust can be completely attributed to gas dynamic acceleration.
Computational fluid dynamics (CFD) plasma simulations were performed to analyze the spatial characteristics of the discharge in the C3RFT. The simulated thruster performance is found to be in good agreement with measurements. For a grounded back electrode case, the plasma potential is found to drop near the smaller front powered electrode creating a negative DC bias, in accordance with classical theory of asymmetric capacitive discharge. Electrons in sheath are heated to ~ 6 eV whereas in the rest of the capillary the electron temperature ~ 2 eV. The plasma density is 1019 m-3 whereas excited argon species are 1020 m-3.
