Cesiation in Highly Efficient Surface Plasma Sources

Cesiation effect, a significant enhancement of negative ion emission from a gas discharge with decrease of coextracted electron current below negative ion current was observed for the first time on July 1, 1971 by location into discharge a compound with one milligram of cesium [1]. This observation became the basis for the development of Surface Plasma Sources (SPS) for highly efficient production of negative ions from the interaction of plasma particles with electrodes on which adsorbed cesium reduced the surface work-function [2-5]. The emission current density of negative ions was increased rapidly from j~ 0.01 A/cm to 3.7 A/cm with a flat cathode and up to 8 A/cm with an optimized geometrical focusing in the long pulse SPS, and up to 1 A/cm in direct current (DC) SPS. Discovery of charge-exchange cooling helped to increase the negative ion brightness by many orders to a level comparable with the best proton sources, B=j/T> 1 A/cm eV. Intensity of negative ion beams was increased by cesiation up to 10 times from mA to tens of Amperes. Now cesiation is routinely used in SPS for negative ions injection into accelerators and into large fusion devices.