Reduction of Energetic Ion Production in Hollow Cathodes by External Gas Injection

Studies of the hollow-cathode discharge have shown the existence of energetic ions at high-discharge currents that are likely responsible for the high erosion rates observed on the cathode keeper electrode. This work examines the effect of neutral gas injection in the discharge plume of a 250 A lanthanum hexaboride hollow cathode on the production of energetic ions to determine the conditions that yield cathode operation and life. Two different gas injector types are used to deliver neutral gas into the discharge plume and a retarding-potential analyzer is used for ion energy measurements. The flow splits between the cathode internal and external flows, and the number and locations of the external gas injection sites are examined as a function of the discharge current. It is found that increasing discharge current increases the energetic ion production at any given flow rate or injection location. External gas injection reduces energetic ion production for constant cathode flow, with collimated gas-jet injection performing better than distributed injection. Lifetime estimates of the keeper electrode surface due to sputter erosion by ion bombardment reveal that high-discharge current operation at low cathode gas flow produced very energetic ions and limited keeper lifetimes to less than 5000 h. Applying sufficient internal cathode gas flow and external gas injection can extend the keeper life to over 10,000 h at discharge currents of up to 200 A.