Recent progress of negative ion based neutral beam injector for JT-60U

In the negative ion based neutral beam injector (N-NBI) for JT-60U, an extension of the pulse duration up to 30 s has been intended to study quasi-steady state plasmas, where the NBI pulse duration is significantly longer than the plasma current diffusion time. Achieving long pulse injection requires a reduction of the heat load on the grids of the ion source. The ion source was modified to reduce the pressure within the grid structure by increasing the vacuum conductance of the acceleration grids. The acceleration efficiency was improved from 0.74 to 0.79. The heat load of the grounded grid, which was the main heat load of the acceleration grids, decreased from 9.2 to 7.0% at a constant arc chamber pressure of 0.3 Pa. These improvements seemed to be caused by the reduction of beam ion stripping loss. Up to now, a long pulse injection of 17 s at the power of 1.6 MW with beam energy of 366 keV was obtained by using one ion source. The temperature rise of the cooling water from the grounded grid saturated at less than 35 °C during operation, so the heat load of the grid is significantly reduced for steady state operation at this power level.