Operating radio frequency antennas immersed in vacuum: Implications for ground-testing plasma thrusters

Several of the challenges associated with operating the radio frequency (RF) antenna of the Helicon Double Layer Thruster (HDLT) prototype immersed in a vacuum have been discussed. 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. 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 HDLT source tube is surrounded by a double-saddle field helicon antenna, which is constructed from copper with a 25μm silver plating. To minimize the challenges associated with microarcing and parasitic plasma formation and enable the HDLT prototype to be operated at higher RF powers and flow rates, the current path on the antenna to ground must be removed or limited.