Operating Radio Frequency Antennas Immersed in Vacuum: Implications for Ground-Testing Plasma Thrusters

O PERATING RF antennas in a vacuum presents several challenges that are relevant to the ground testing of plasma propulsion systems such as the Helicon Double Layer Thruster (HDLT) prototype. Antennas immersed in vacuum can come into contact with the plasma and, if one end of the antenna is grounded, direct currents flow from the plasma to ground. To the authors’ knowledge, these issues have not been investigated previously in the context of plasma propulsion. However, in asymmetric plasma discharges, which are used in materials processing, the RF biased substrate or electrodes are frequently immersed in the vacuum (and plasma) and biased negatively via a dc blocking capacitor. This enables the energies of the ions impinging the substrate to be controlled independently of the current to the substrate, which is important for various processing techniques [1]. Immersed RF antennas have also been employed in plasma immersion ion implantation (PI) devices [2]. In magnetically confined fusion experiments, the RF antennas are usually shielded with a Faraday cage to prevent the antenna from coming into contact with the plasma. This shielding reduces the power coupling of the plasma to the antenna and hence the efficiency of the system. The RF antenna in the H1 Heliac experiment [3], however, is in direct contact with the plasma and is used to increase the temperature of the plasma. The antenna, which is driven at 7 MHz, is shaped like a loop in the plane of the last closed flux surface, and one end of the antenna is grounded. This allows a direct current toflow from the plasmavia the antenna to ground.Currents of approximately 90 A and plasma potentials of 100 V have been measured during typical operation of the H1 Heliac. The HDLT is a new plasma propulsion concept based on the discovery by various researchers of energetic ion beams formed by a current-free electric double layer in low-pressure helicon plasmas [4,5]. The HDLT is electrodeless, does not require a neutralizer, can operate with a wide variety of propellants [6,7] and produces a verylow-divergence ion beam (less than 10 for argon and 6 for xenon) [8] that can be steered magnetically [9,10]. A prototype of the HDLT has been developed and an initial testing campaign was conducted in the CORONA space simulation chamber [11]. This has been followed by a more extensive testing program with the HDLT prototype immersed in a smaller vacuum chamber [12–14]. In this work, several deleterious phenomena are described that have been observed during this testing program. To alleviate these problems, modifications to the RF (13.56 MHz) matching box and antenna assembly have been implemented. These phenomena, their consequences, and the solutions implemented here are also relevant to ground testing other plasma thrusters such as the new VASIMR VX-200 prototype [15] and other concepts using RF or helicon antennas [16–24].