Characteristics of long-pulse negative-ion source in the neutral beam injector of Large Helical Device

The injection duration has been extended beyond 100 s with a high-power hydrogen negative-ion source in a negative-ion-based neutral beam injector of the Large Helical Device superconducting fusion machine. The ion source is a cesium-seeded source with a thermally insulated plasma grid (PG), and optimized for a short-pulse operation of 2?3 s. The negative-ion production efficiency is strongly dependent on the PG temperature, and in the long-pulse operation it exceeds an appropriate temperature range of 200?300 °C, at which the optimum cesium coverage is formed on the PG surface. By making the PG temperature rise slower with a reduced arc power, the injection duration was extended to 110 s with an injection power of 110 kW. To extend the injection duration further with a higher injection power, stainless-steel cooling tubes have been mechanically attached to the PG for suppression of the PG temperature rise in the long-pulse operation. As a result, a long-pulse injection with an injection power of 200 kW was extended to 128 s until it was manually stopped due to the plasma collapse. However, the beam duration could be limited to around 3 min because the PG temperature rise was not saturated due to a low thermal conductivity with the thickness of the stainless-steel tube determined so that the short-pulse operation is also possible. On the other hand, the longitudinal beam distribution in a grid area of 25×125 cm^2 is observed to be more uniform than that with the uncooled PG. The temperature distribution of the individual grid parts becomes more uniform with the cooled PG, which should contribute to the improvement of the beam uniformity