Analytical and experimental studies of thermionically emitting electrodes in contact with dense, seeded plasmas

摘要

Interactions are considered between a moving,udalkali-metal seeded, dense plasma and audmetallic electrode whose surface properties areudinfluenced by the absorption of seed particles. Theudplasma behavior is governed by a set of differentialudequations, which are coupled to the surfaceudthrough the boundary conditions. These conditionsudare obtained by utilizing the particle desorptionudrate expressions of Levine and Gyftopoulos. Theudsolution of the problem yields the state of the surfaceudas well as the spatial distribution of plasmaudproperties. In particular, electrode voltage dropsudare predicted, which indicate whether the electrodeudoperates in a thermionic or arc mode. Theudmethod has been applied to a potassiwn-seededudargon plasma in contact with a tungsten electrodeudin a stagnation flow geometry. The results showudthat the plasma - surface interaction may lead toudlarge electrode currents at moderate voltage drops.udThese currents can be up to an order of magnitudeudgreater than what the random electron currentudwould be at the surface under conditions ofudperfect thermodynamic equilibrium at the surfaceudtemperature. R.esults of a comparable experimentudshow reasonably good agreement with the theory.