Review of negative ion source technology

H/sup -/ or D/sup -/ ions are required for generating high energy neutral beams in heating fusion plasmas. Two distinct types of H/sup -/ ion sources can be identified: (1) surface sources - in which the H/sup -/ ions are generated by particle collisions with low work function metal surfaces; and (2) volume sources - in which the H/sup -/ ions are produced by electron-molecules and electron-ion collision processes in the volume of a hydrogen discharge. Recent experiments demonstrate that reasonable H/sup -/ ion current density can be obtained from both types of sources. However, further technology must be developed on the control of cesium and the reduction of electron drain before these sources become practical units of a multi-ampere neutral beam injection system