Development of large radio frequency negative-ion sources for nuclear fusion

The development of a large-area radio frequency (rf) source for the negative-ion beam production has been continued using a prototype source of 37 1 with a rf power up to 160 kW at 0.93 MHz. The design is based on a small rf driven cylindrical driver source on a large magnetically confined expansion volume (60 x 31 x 20 cm(3)) and is in principle scalable to any size. Although the H- yields reported earlier have not been exceeded, progress has been made in several areas. To protect the ceramic insulator of the driver from thermal stress an internal copper Faraday screen has been added. This has resulted in reliable long pulse operation with a duration up to 15 s. Various different grid materials have been investigated as potential options for cesium-free operation. No clear-cut conclusion can yet be drawn from those experiments due to unexpected copper sputtering from the Faraday shield onto the plasma grid. The dependence of the H- yield on gas pressure and filter strength have been studied. The effect of noble gases addition to the discharge has been investigated using a Langmuir probe. These measurements indicate that the enhancement of H- current may be caused by a change in plasma density brought about by the addition of a noble gas. (C) 2002 American Institute of Physics.