Negative ion sources for neutral beam injection into fusion machines

Plasma heating and current drive in future machines for magnetically confined thermonuclear fusion will require neutral beam injectors based upon negative ion sources. Although the technology of filament arc ion sources is considered adequately mature for large scale injectors, such as for ITER, ion source efficiency and reliability need to be improved. The power loading of the source and accelerator are especially problematic. This is particularly important for long pulse (1000 s) operation. Investigations with the Kamaboko source, a small scale model of the ITER arc source, illustrate the importance of the conditions of the plasma grid for reducing electron loading of the accelerator.