Effect of cesium seeding on hydrogen negative ion volume production

The effect of cesium vapor partial pressure on the plasma parameters has been studied in a dc hydrogen negative ion source. The cesium vapor pressure was varied up to 10−5 Torr and was determined by a surface ionization gauge in the absence of the discharge. Due to cesium seeding the negative ion relative density in the center of the plasma extraction region increases by a factor of 4, while the electron density decreases. The negative ion density increase is highest (a factor of 2) at the lowest hydrogen pressure. The velocity of the directed negative ion flow to the plasma electrode is also increased by the cesium seeding. Due to these factors the negative ion current is enhanced. A dramatic reduction of the electron component, which is consistent with the reduced electron density and temperature, is also observed. An enhancement of the volume production seems to occur in this type of plasma. Possible mechanisms for this type of volume process will be discussed.