Progress in the development of steady state negative ion sources for fusion applications

During the past few years or so, the development of steady state sources of negative hydrogen or deuterium ions has made a substantial progress. There are several approaches under investigation, some operating with long pulses and some in a true steady state, and delivering H/sup -/ or D/sup -/ beam currents of several hundred mA up to 1 A. The improvement in the gas efficiency from a few percent for pulsed sources to the range of 5% to 15% and possibly even higher for the new sources, is another advance that was not anticipated only 4 to 5 years ago. A higher gas efficiency has come as the result of using independent plasma generation and geometrical focusing of surface produced negative ions. The power efficiency has not been changed so much, but, in a high energy system, savings of a few kW/A are less important than the reduction of the gas load. It can be expected that within five or so years a small (