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External Mass Injection to Reduce Energetic Ion Production in the Discharge Plume of High Current Hollow Cathodes


Hollow cathode discharge studies have shown the existence of energetic ions at high discharge currents that are likely responsible for the high erosion rates and erosion patterns observed on the keeper electrode of the hollow cathode. This thesis uses experimental methods to study the effects of neutral gas injection in the xenon hollow cathode discharge plume on the production of energetic ions to determine the injection conditions that yield optimum hollow cathode operation and life. Parameters considered include the flow split between the cathode internal flow and the external gas injectors, the number of external gas injection sites, the locations of these injection sites, and the orifice size of the injectors, all as a function of the discharge current level. Two lanthanum hexaboride hollow cathodes of diameters 1.5 cm and 2 cm are studied for discharge currents of up to 100 A and 250 A, respectively.

Internal measurements of the 1.5-cm-dia. cathode with a Langmuir probe reveal plasma potential and electron temperature profiles that are characteristic of hollow cathodes and relative insensitivity to the discharge and flow conditions. Plasma density measurements inside the cathode show that the density is sufficiently high and in contact with the entire 2.5-cm insert length only at relatively low internal cathode flow rates so that space-charge limitations on the emitted electron current density are not an issue. At high internal cathode flows, the density peak is pushed toward the orifice plate and the plasma contact with the insert is reduced.

A retarding-potential analyzer is used to make ion energy measurements in the discharge plume of the 2-cm-dia. cathode at discharge currents of 25 to 250 A and at various gas-flow conditions. In general, increasing discharge current increases the energetic ion production at any given flow rate or injection location. External gas injection reduces energetic ion production with fewer energetic ions at higher external injected flow. Further, collimated gas-jet injection performs better than point-source (distributed) injection. Increasing cathode internal flow for constant external flow also reduces energetic ion production. The greatest reduction of energetic ion production occurs when both the internal and external flows were at their maximum values.

Lifetime estimates of the keeper electrode surface due to sputter-erosion by ion bombardment are calculated to determine the impact of the energetic ion generation on the cathode life. High discharge current operation at low cathode gas flow produced very energetic ions that limited keeper lifetimes to less than 5,000 hours. Applying sufficient internal cathode gas flow and external gas injection extended the keeper life to over 10,000 hours at discharge currents of up to 200 A. For higher discharge current operation, more cathode flow and/or injected flow will be required to increase keeper life, but this could not be explored in detail due to pumping speed limitations of the current facility.

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