Development of long-pulse high-power-density negative ion beams with a multi-aperture multi-grid accelerator

A long pulse acceleration of high-power-density hydrogen negative ion beams of 184 MW/m2 (0.97 MeV, 190 A/m2) has been successfully achieved up to a facility limit of 60 s by using a 5 stage multi-aperture multi-grid accelerator, which is the first demonstration of the ITER relevant beam with a pulse length over the time constant of cooling capability of the water-cooled acceleration grids. In order to extend the pulse length of such high power density beams, the extraction and acceleration geometries were tuned to match the beam optics to high power density by increasing the transmission at the extractor and the acceleration electric field at the first acceleration gap. After the modifications, the available current density at the same acceleration voltage was increased by 36%, and grid heat loads at each acceleration grid were reduced to less than 3% of the electric input power, which satisfied an allowable level for long pulse acceleration. Furthermore, since the conditioning procedure has been established for long pulse acceleration, a rapid conditioning of total 16 operational days up to 1MeV, 60 s class has been achieved, which was about half of previous results. Because JT-60SA and ITER NBIs are designed by the same concept, these results are applicable to the modifications and operations toward these NBIs directly.