Low pressure and high power rf sources for negative hydrogen ions for fusion applications (ITER neutral beam injection).

The international fusion experiment ITER requires for the plasma heating and current drive a neutral beam injection system based on negative hydrogen ion sources at 0.3 Pa. The ion source must deliver a current of 40 A D(-) for up to 1 h with an accelerated current density of 200 Am/(2) and a ratio of coextracted electrons to ions below 1. The extraction area is 0.2 m(2) from an aperture array with an envelope of 1.5 x 0.6 m(2). A high power rf-driven negative ion source has been successfully developed at the Max-Planck Institute for Plasma Physics (IPP) at three test facilities in parallel. Current densities of 330 and 230 Am/(2) have been achieved for hydrogen and deuterium, respectively, at a pressure of 0.3 Pa and an electron/ion ratio below 1 for a small extraction area (0.007 m(2)) and short pulses (<4 s). In the long pulse experiment, equipped with an extraction area of 0.02 m(2), the pulse length has been extended to 3600 s. A large rf source, with the width and half the height of the ITER source but without extraction system, is intended to demonstrate the size scaling and plasma homogeneity of rf ion sources. The source operates routinely now. First results on plasma homogeneity obtained from optical emission spectroscopy and Langmuir probes are very promising. Based on the success of the IPP development program, the high power rf-driven negative ion source has been chosen recently for the ITER beam systems in the ITER design review process.

Long pulse large area beam extraction with a rf driven H(-)/D(-) source.

IPP Garching is heavily involved in the development of the rf driven H(-)/D(-) ion source for the ITER NBI. After the successful demonstration of the required physical parameters, the experimental conditions have been extended to long pulses and large area beam extraction. This paper contains descriptions of the source and power supply modifications necessitated for long pulses as well as the latest results including the first 1 h pulse. Suppression of the coextracted electron current is a key issue. Experiments with potential control, different magnetic filter fields, and cesium handling to suppress the electrons and stabilize the currents are also reported.