Improvement of JT-60U negative ion source performance

The several hundred keV neutral beamline which is now operating on JT-60U m arks the first usage of negative ion sources to produce high power neutral beams. Because this was such a large advance in the state of the art with r espect to all system parameters, many new physical processes appeared durin g the earlier phases of the beam injection experiments. The physical mechan isms responsible fur these processes have been explored, and solutions have been implemented for some of them, in particular excessive beam stripping, the secular dependence of the are and beam parameters. and the non-uniform ity of the plasma illuminating the beam extraction grid. This has reduced t he percentage of beam heat loading on the downstream grids by roughly a thi rd and has permitted longer beam pulses at higher powers. Progress is being made in improving the negative ion current density and in coping with the sensitivity of the caesium in the ion sources to oxidation by tiny air or w ater leaks, and the cathode operation is being altered.