The extraction of H− ions from a volume source is greatly improved by the presence of a weak magnetic field parallel to the plasma electrode that magnetically insulates this electrode from the bulk plasma. When biased positive, the plasma electrode depletes the electron population in the neighboring region. To maintain plasma neutrality, negative ions from the main plasma replace the electrons in this region. Thus a high fraction of negative ions builds up in front of the plasma electrode. The implications of these phenomena for negative ion beam formation are discussed.
This article reports work in progress on laser diagnostics of negative-ion transport velocity in H−-ion volume sources. The plasma dynamics after the laser shot is discussed in detail, and the effect of the potential perturbation on the H− velocities is evaluated. A method of evaluation of the H− transport velocity from single-laser-beam photodetachment experiments is proposed. To substantiate this method, two-laser-beam photodetachment experiments have been effected. The velocities thus determined are pressure dependent; they correspond to H− energies in the range 0.23–0.08 eV.
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