Barium Depletion in the NSTAR Discharge Cathode After 30,472 Hours of Operation

Abstract

Dispenser hollow cathodes rely on a consumable supply of barium released by impregnant materials in the pores of a tungsten matrix to maintain a low work function surface. Examinations of cathode inserts from long duration ion engine tests show deposits of tungsten at the downstream end that appear to block the ow of barium from the interior. A numerical model of barium transport in the insert plasma shows that barium generated upstream of the emission zone is ionized in the xenon plasma and forced back to the surface downstream. Barium resupply in the emission zone therefore occurs through the gas phase. In addition, the model indicates that the barium partial pressure in the insert may exceed the equilibrium vapor pressure of the dominant barium-producing reaction, and it was postulated previously that this would suppress barium loss in the upstream part of the insert. New measurements of the depth of barium depletion from a cathode insert operated for 30,472 hours reveal that barium loss is conned to a narrow region near the downstream end, conrming this picture of hollow cathode operation.