Low-Weight Fixed Ceramic Capacitor Impedance Matching System for an Electrothermal Plasma Microthruster

Abstract

OVER the past few years, there has been increasing interest in the development of small-size radiofrequency (rf) plasma sources [1–5] for a variety of applications: portable sources for biological applications, process uniformity using hexagonal close packing of plasma sources, material milling with nonmetallic focused ion beam sources, and plasma thruster development for space use. There are a number of rf-driven thrusters currently being developed that span a large rf power range, i.e., over tens of kW in the VASIMR plasma rocket [6] and only a few tens of W in the Pocket Rocket electrothermal microthruster [5] or in the dielectric capillary rf thruster [7]. Although the use of rf power has some advantages (lower breakdown potential, higher plasma coupling efficiency, wider range of plasma parameters), its implementation on a spacecraft poses a number of challenges (electromagnetic radiation control, thermal control, weight/volume/power restrictions) [8]. Still, remarkable success was achieved with the rf inductive ion gridded thruster Rf Ion Thruster (RIT) flown on the Artemis mission [8,9]. With the emergence of CubeSat satellites and a variety of new remote sensing technologies providing cheaper access to space, there is increasing demand for low-weight, low-power, small-size, lowcost propulsion systems that can be operated with greener or cheaper propellants compared to the toxic propellants (i.e., hydrazine) used in chemical thrusters or to the expensive propellants (i.e., xenon) used in resistojets [8]. A small-size, low-power, rf-driven plasma device called Pocket Rocket has been proposed as a new electrothermal thruster [5,10,11]; its estimated performances are promising and in agreement with analytical studies on gas heating by ion-neutral charge exchange collisions [12] and other experimental charge exchange thruster studies using dc excitation [13]. Neutral gas heating in Pocket Rocket was recently measured by spectroscopy [14] for 10 W operation. However, its current heavy and bulky rf power impedance matching subsystem strongly limits its development as a low-power plasma thruster for space use on small satellites. Similarly, development studies of inductive or wave-heated rf thrusters in laboratories, whether immersed in avacuumor contiguously attached to large vacuum chambers, are usually carried out using fixed excitation frequency and variablevacuumcapacitors in an impedance matching network, where the rf antenna plays the role of the inductor in the resonant circuit [6,15–17]. It has recently been shown that efficient rf power transmission can be achieved at low power (a few hundred W) by varying the excitation frequency and using fixed ceramic capacitors directly mounted on the rf antenna [18]. Here, a low-weight (∼90 g) fixed ceramic capacitor and inductor impedance matching system mounted on a printed circuit board (PCB) is developed and used to couple rf power into the electrothermal Pocket Rocket plasma microthruster. The excitation frequency is swept to achieve maximum transmitted rf power to the plasma at the resonant frequency and the system is compared to a standard high-weight (∼ a few kg) variable capacitor impedance matching network pretuned at about 13.56 MHz.