Design and preliminary characterization of a two-stage plasma cathode for space applications

Summary form only given: Electric propulsion (EP) systems and plasma contactors usually rely on thermionic emitters as primary electron sources. The most frequently used device is the hollow cathode assembly (HCA). The HCA employs a barium-impregnated tungsten insert to generate a plasma within an orifice, from which electron current is extracted. While HCAs are efficient electron sources, the presence of the thermionic emitter in contact with the plasma places a constraint on the device lifetime, due to barium depletion and ion bombardment of the insert. In many applications, the HCA lifetime is sufficient, but for long-term EP missions to the outer planets or permanent plasma contactor installations, a long-lived alternative would prove useful. A two-stage plasma cathode has been designed as an alternative to the HCA for these applications in space. In contrast with the HCA, this device is an electrodeless plasma source, which allows for longer operating lifetimes by eliminating the common HCA failure modes described above. The main electron source in the two-stage plasma cathode is a 2.45 GHz ECR microwave discharge, with the resonance heating zones set up by permanent magnets. In this microwave discharge, the amount of extractable electron current is limited by the amount of ion current incident on the surfaces of the discharge chamber. The relatively small amount of electron current that is extracted from this main discharge, through an extraction orifice, is used to form a beam of accelerated electrons, which is incident on a region of higher neutral gas pressure than exists in the microwave discharge chamber. In this region, the "ionization orifice," a dense plasma is formed, and the desired electron current is extracted from this higher-density plasma.The study presented here is divided into two parts. First, the main microwave discharge is characterized, and amount of electron current extracted from the main discharge is determined for various extraction orifice and electrode configurations. Plasma density and electron temperature within the microwave discharge are presented along with current extraction data. Next, the design of the acceleration stage and ionization orifice are presented, along with a first-order model, which is used to predict pressure variations, plasma properties, and extractable current from the two-stage source. Preliminary results from the two-stage plasma cathode experimental setup are presented alongside the expected results from the model.