Mode Transition in Capillary Capacitively Coupled Radio Frequency Thruster

A miniature capacitively coupled radio frequency thruster with a capillary tube (C
³
RFT) is being developed and tested at the Technion. The thruster utilizes a nonsymmetric electrode arrangement and a dedicated radio frequency generator. The small diameter of the dielectric tube allows operation of the device in very low flow rates, < 8 SCCM in Ar. The measured thrust time response and wall temperature of 400 °C confirm an electrothermal mechanism of thrust generation. 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 compared to the cold flow case, for a total thrust of
$\approx 160~\mu \text{N}$
and specific impulse ≈69 s in Ar. When powering the larger inlet “back” electrode, discharge is observed to diverge from classical theory of asymmetric capacitive coupled plasma, leading to mode transition. This phenomenon is marked by the appearance of bright plasma exhaust and reduction in input power from ≈15.17 to ≈9 W. The thrust to power ratio increased by 25% after the transition. A simple sheath breakdown model is proposed to explain the transition.