High power density of hydrogen negative ion beams with over 70 MW/m2 at the energy of 500 keV has been demonstrated stably over 100 s by using a multi-aperture and three-stage accelerator. Such continuous negative ion beam accelerations over 100 s with high power density has never been achieved before in the world. This result fulfills the requirement of the negative ion source for the neutral beam injector (NBI) of JT-60SA (500 keV, 130A/m2 for 100 s) and also contributes to the 1 MeV negative ion accelerator for the ITER NBI.
In this negative ion source, Cesium (Cs) is seeded to enhance the negative ion production near the plasma grid. However, anomalous discharge, so-called arcing, which causes damage on filament, was observed when the Cs is seeded. This has limited the arc power and pulse length. In addition, negative ion current was gradually decreasing after 50 s because Cs evaporated from the chamber wall excessively deposits on the plasma grid when the chamber wall temperature is increased over 60 degree Celsius [1, 2]. For high energy beam acceleration, it has been concerned that the voltage holding capability is degraded when Cs leaks to the accelerator.
In this test, a small KAMABOKO ion source is attached on the top of the three-stage accelerator, whose aperture arrangement and the gap lengths are the same ones as the negative ion source of JT-60SA. To inject the necessary arc power with less filament damage, the input energy to the filament was successfully reduced from 10.6 J to 0.4 J by installing fast cutoff system of the arc power supply at 500 kV stage. To maintain the stable negative ion production, excess Cs from the chamber wall to the plasma grid could be suppressed by maintaining the wall temperature around 50 degree Celsius. As the result, 500 keV and 130 A/m2 beams have been achieved. The power loads on each acceleration grid were lower than the allowable value of 5 % of the total beam power. Then, the pulse length was gradually extended.
Even though the power loads on the grid was sufficiently low for the long pulse operation, the pulse length was limited up to 60 s initially due to breakdowns. This result indicates that cause of the breakdown is not only the thermal loads on the acceleration grids but also particle incident to these grids by beams. To extend the pulse length, the long pulse beam acceleration has been tried at slightly low energy of 400 keV. Consequently, the pulse length has been gradually extended over 60 s, and finally reached to 175 s. After that, pulse length of 500 keV beam was gradually extended, and reached over 100 s. Finally, acceleration of 500 keV, 154 A/m2 beams for 118 s has been achieved. No degradation of voltage holding capability for extraction and acceleration was observed even after the total amount of Cs injection corresponded to the same level of JT-60U by extrapolation of the chamber size. These operational technique for long pulse operation can be directly applied to JT-60SA NBI, and contribute to the ITER accelerator.
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