Improving Performance of Negative-ion Based Neutral Beam Injector for JT-60U
YAMAMOTO Takumi, KURIYAMA Masaaki, UMEDA Naotaka, LEI Guangjiu1), GRISHIAM Lary2), KAWAI Mikito, OHGA Tokumichi and JT-60NBI-Team
Japan Atomic Energy Research Institute, Naka Establishment, Naka, Ibaragi 319-1101, Japan
1) Southwestern Institute of Physics, China
2) Princeton Plasma Physics Laboratory, USA
Abstract.
The 500 keV negative-ion based neutral beam injector now operating on JT-60U is the first application of negative ion technology to the production of high current and power neutral beam for plasma heating and current drive. However, the beam power, pulse length, and operating acceleration voltage are all somewhat less than the design capability so far. It is apparent that this condition arises mainly from excessive beam heating of the accelerator grids, and divergent beams and less withstanding voltage at the accelerator. Short-scale-length peaks in the beam current density profile measured from the infrared image on a beam target were also observed. It was found that the peaks result from deflection of the outermost-row beamlets in grid segments caused by non-uniform electric field at downstream side of the extraction grid. It also affected the heat loading on a beam limiter seriously. To make the electric field uniform, a piece of Cu plate was installed and hence the deflection of the beamlet was corrected. As a result, the heat load on the beam limiter reduced than half, and 10sec pulse length operation was achieved at 2.6 MW of the injection power. Measurement of the Doppler shifted spectrum of D from the injection neutral beam has indicated that the stripping of negative ions occurs mainly in the first acceleration gap and inside the extraction grid and dominates a half fraction of the heat load on the grounded grid at a typical source operating pressure.
Keywords: negative-ion, NBI, JT-60U, heat load of grids, divergence of beam