Dependence of Beam Direct Energy Conversion on Beam Energy and Current in Neutral Beam Injection Systems

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Abstract
Experiments and simulations were performed for helium ion beams to confirm the general validity of the two-dimensional beam direct energy conversion simulation code KUAD (Kyoto University Advanced Dart) for a wide range of beam parameters and to better understand how the performance of beam direct energy recovery is dependent on beam parameters. The experiments compared currents in the 60- to 140-mA range for 15-keV beam energy and from 130 to 250 mA for 20-keV beam energy. Beam behaviors numerically predicted for different currents and collector potentials were verified. Numerically obtained performance characteristics of the beam direct energy recovery generally show excellent agreement with experimental results within experimental errors. The only discrepancy occurs in the vicinity of the collector potential corresponding to the maximum energy recovery due to possible deviation from the axisymmetry of electrodes and to their small misalignment with respect to the beam axis. Beam perveance rather than beam energy or current is a good parameter for the evaluation of the performance of beam direct energy recovery. Maximum energy recovery efficiencies of 87 ± 4% for 15-keV and 85 ± 4% for 20-keV beams have been achieved.