Effect of cesium and xenon seeding in negative hydrogen ion sources

It is well known that cesium seeding in volume hydrogen negative ion source s leads to a large reduction of the extracted electron current and in some cases to the enhancement of the negative ion current. The cooling of the el ectrons due to the addition of this heavy impurity was proposed as a possib le cause of the mentioned observations. In order to verify this assumption, we seeded the hydrogen plasma with xenon, which has an atomic weight almos t equal to that of cesium. The plasma properties were studied in the extrac tion region of the negative ion source Camembert III using a cylindrical el ectrostatic probe while the negative ion relative density was studied using laser photodetachment. It is shown that the xenon mixing does not enhance the negative ion density and leads to the increase of the electron density, while the cesium seeding reduces the electron density. (C) 2000 American I nstitute of Physics. [S0034-6748(00)50502-4].