Increasing the space-charge limit and other effects of cesium seeding in hydrogen negative ion sources

The role of cesium seeding in increasing the negative ion current in volume sources is described. By a reduction in the local plasma potential the cur rent of extracted electrons is vastly reduced. As a result, cesium increase s the fraction of the transverse space-charge limit available to the ions b y as much as a factor of 3. In addition, cesium can increase the total spac e-charge limit by injection of Cs+ into the presheath-a newly recognized ph enomenon consistent with experimental measurements and determined from appl ication of a Double-Vlasov model for negative ion extraction. (C) 2000 Amer ican Institute of Physics. [S0034-6748(00)60002-3].