Physics basis and future trends for negative ion sources.
The explanation of hydrogen negative ion formation in the hydrogen plasma volume is based on the hypothesis that the precursors of H(-) ions are rovibrationally excited molecules. It will be shown that this hypothesis is confirmed by the agreement between predicted and measured H(-) densities. The reason for the existence of an optimum pressure for the negative ion density in volume sources is discussed. The role of the magnetic filter in contemporary negative ion sources is analyzed. Experiments indicating the effect of the ion source surfaces (plasma electrode, collar, walls) will be described. It is concluded that vibrationally excited molecules, produced by recombinative desorption from surfaces, play a significant role. It is shown that a high H(-) ion emission from a high work function surface is very doubtful at the present state of knowledge. Therefore it is considered that the success of the cesium-free accelerator ion sources operating in the 100 mA/cm(2) range is due to production of vibrationally excited molecules on the plasma electrode or collar surface. In the collar case, the negative ion production is enhanced due to the absence of plasma in the production region; thus mutual neutralization loss is canceled. The physics of surface production of H(-) ions and some unexplained features related to it will be discussed.
Bacal
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M
Laboratoire de Physique et Technologie des Plasmas, Ecole Polytechnique, UMR 7648 du CNRS, Palaiseau, France. marthe.bacal@polytechnique.edu