Design of Long Pulse/Steady State Negative Hydrogen Ion Sources for Fusion Applications

As part of the program to develop a high energy neutral beam injector based on neutralization of negative ions, several types of negative hydrogen (or deuterium) ion sources for long pulse or steady state operation are being studied at Brookhaven National Laboratory. By using parameters of ion sources when operating in a pulsed mode and without cooling (pulse length < 0.1 s), requirements have been determined for a long pulse (several seconds) or steady state operating mode and two sources have been designed and fabricated. First of the two is a Penning source, designed for a steady state operation with a cathode power density of 1 kW/cm2. For the range of cathode power densities between 0.2 kW/cm2 and 1 kW/cm2, nucleated boiling has to be used for heat removal; below 0.2 kW/cm2 water flow cooling suffices. Although this source should deliver 0.3-0.5 A of H- ions in a steady state operation and at full power, the other source, which has a magnetron geometry, is more promising. The latter incorporates twd new features compared to first designs, geometrical focusing of fast, primary negative hydrogen ions from the cathode into the extraction slit, and a wider discharge gap in the back of the source. These two changes have resulted in an improvement of the power and gas efficiencies by a factor of 3 to 4 and in a reduction of the cathode power density by an order of magnitude.