30 years of high-intensity negative ion sources for accelerators

Thirty years ago, July 1, 1971, significant enhancement of negative ion emission from a gas discharge following an admixture of cesium was observed for the first time. This observation become the basis for the development of Surface Plasma Sources (SPS) for efficient production of negative ions from the interaction of plasma particles with electrodes on which adsorbed cesium reduced the surface work-function. The emission current density of negative ions increased rapidly from j~ 10 mA/cm² to 3.7 A/cm² with a flat cathode and up to 8 A/cm² with an optimized geometrical focusing in the long pulse SPS, and to 0.3 A/cm² for DC SPS, recently increased up to 0.7 A/cm². Discovery of charge-exchange cooling helped decrease the negative ion temperature T below 1 eV, and increase brightness by many orders to a level compatible with the best proton sources, B=j/T> 1 A/cm² eV. The combination of the SPS with charge-exchange injection improved large accelerator operation and has permitted beam accumulation up to the space-charge limit and overcome this limit several times. The early SPS for accelerators have been in operation without modification for ~25 years. An advanced version of the SPS for accelerators will be described. Features of negative ion beam formation, transportation, space-charge neutralization-overneutralization, and instability damping will be considered. Practical aspects of SPS operation and high brightness beam production will be discussed