Improvement of the large current negative hydrogen ion source for neutral injection in the large helical device

A large negative hydrogen ion source with a rod-type magnetic filter was investigated for the construction of a neutral beam injection (NBI) system for the large helical device (LHD). A 1 3 - scale device was constructed and investigated. Three main phenomena are observed on seeding a small amount of cesium into the ion source. These are enhancement of the hydrogen negative ion (H−) current, a reduction in the electron beam extracted with the H− beam, and a decrease in the operating gas pressure. The enhancement factor of the H− current is 3–4, and a total H− current of 8.5 A is obtained from 20 × 40 extraction apertures 9 mm in diameter. This current corresponds to a current density of 17 mA cm−2. A multi-wall structure forming a lattice, called an egg-box cell, is installed between the magnetic filter rod and the plasma grid. The electron current with this cell is half as low as the electron current without the cell, but the H− current is hardly affected. This phenomenon is observed when cesium is seeded in the source chamber. The electron temperature and its density increase by a factor of 2 on strengthening the cusp field. Experiments to obtain a higher H− current density were carried out using the improved are plasma. A hydrogen negative ion current of 54 mA cm−2 was extracted from a restricted aperture of 36 extraction holes. The H− current was extracted from 20 × 40 full apertures of the ion source with strong cusp field. A total H− current of 10.4 A was obtained from these apertures. The optimum filling pressure is less than 1.0 Pa.