Mercury hollow cathode plasma bridge neutralizers

Abstract

In all electric thruster systems \vhich eject positive ions, it is necessary to supply an equal number of electrons to maintain the space vehicle's electrical neutrality. It is imperative that the neutralizer be located outside the beam; it is also desirable that the voltage difference between the electron source and the ion beam be as small as possible to reduce the power input to the electron beam and to reduce the sputtering damage caused by charge exchange ions that strike the neutralizer after falling back through the beam-neutralizer potential. A unique solution to these apparently mutually exclusive conditions is to generate a "plasma bridge" to connect the cathode, located outside the beam, to the beam itself. This technique has been applied to neutralize the ion beam of a 20-cm mercury thruster. The electrons are emitted from a hollow cathode consisting of a cavity coated on the inside with barium carbonates, heated to emit electrons thermionically. Approximately 2% of the propellant vapor is diverted from the main propellant to sustain the hollow-cathode discharge. Probe measurements have indicated that the plasma inside the cathode is less than 15 v positive with respect to the emissive coating; therefore, damage from ion sputtering should be negligible over long operating periods.