High power negative ion beam development for heating and current drive in fusion plasmas

Part A. Negative ion source technologies for high energy and high power neutral beam injectors have made great progress during the last few years. After the development of a large negative ion source producing an H- beam of 50 keV, 10 A (with a current density of 37 mA/cm2) during 0.1 s, continuous production of a negative ion beam in a small scale H- source was demonstrated for a pulse length of 24 h at a beam of 50 keV and 0.3 A. Furthermore, the H- beam was accelerated up to a high energy of 300 keV, at 0.1 A and 0.5 s, in a caesium seeded ion source with a multi-aperture, two-stage accelerator. The beamlet divergence was measured with a single-aperture accelerator and found to be 5.5 mrad at 300 keV, 17 mA(11 mA/cm2). The current, current density, pulse length and beam divergence are close to the specifications required for the neutral beam injectors for JT-60U and ITER. Part B. Intense D- ion beams of 2.2 A, transmitted on a calorimetric target at 6.4 m from the source and with energies up to 100 keV in multisecond pulses, have been obtained at Cadarache in a joint Euratom-CEA-JAERI experiment. The plasma generator was operated with caesium. At the exit of the beam neutralizing gas cell, the neutral, positive and negative beam components have been separated electrostatically and measured calorimetrically for different gas target thicknesses. Energy recovery experiments have been conducted, for the first time with negative ions. The residual negative ion power has been recovered with an efficiency as good as 81% with 102 keV, 1.2 A D- beams. (author). 16 refs, 6 figs