Development of a high current supersonic ion source for electromagnetic accelerators

The first report is presented on a supersonic ion source for electromagnetic (EM) accelerators, which in principle affords a much higher ion beam power than electrostatic accelerators because there is no space charge limiting effect. None of the present EM systems, however, operate as expected. The reason for this failure has been analyzed (K. Hirano, J. Plasma Fusion Res., 69 (1993) 684, 806) and attributed mainly to the fact that the EM force does not accelerate but even chokes the subsonic plasma flow which appears during the ionization phase of the inlet gas. Since this difficulty may be overcome by an ion source supplying a supersonic plasma flow to the EM system, development of such an ion source has started based on a thermal arc jet with the aim of yielding an ion current density of about 200 A cm−2. The advantages of a structure with a very narrow arc constrictor hole are known, so a hole size as large as 1 mm in diameter was tested to begin with. Favorable results of the experiments were that the arc jet can produce an arc current of 170 A and ejects a smooth symmetric plasma column from the expanding exit nozzle without producing any notable damage on both the cathode and anode surfaces.