Achievement of stable negative ion production with Cs-seeded for long pulse beam operation in the prototype of Cs-seeded negative ion source for JT-60SA.

Long pulse negative ion beams of 500 keV and 154 A/m2 for 118 s have been achieved for the first time, which exceeds the requirement of the JT-60SA negative ion source. In order to solve the issues of such long pulse beams, the fast cutoff system of the power supply aims to reduce the surge current and to extend the lifetime of filaments and the suppression method of excess cesium (Cs) accumulation on the plasma grid (PG) to achieve stable negative ion production. By developing the fast cutoff system using a field programmable gate array unit for the arc power supply, the cutoff time has been reduced to 1/10 that of the original system and the lifetime of the filament was extended by three times. In order to achieve stable negative ion production, incoming Cs on the PG surface has been reduced by keeping the chamber wall temperature below 40 °C. As a result, a stable beam current drift of <6% has been achieved for the 118 s duration.

Experimental investigation of the Cs behavior in the cesiated H- ion source during high power long beam operation.

The behavior of the Cesium (Cs) in Cs-seeded negative ion sources has been investigated experimentally under the beam accelerations of up to 0.5 MeV. The pulse length was extended to 100 s to catch the precise variations in the Cs D2 emission, discharge power, negative ion current, and temperatures in the ion source. The variations of the negative ions were estimated by the beam current and the heat loads in the accelerator. This experiment shows that the buildup of temperature in the chamber walls lead to the evaporation of deposited Cs to enter the plasma region and influenced H- ion production. The H- ion beams were stably sustained by reducing the temperature rise of the chamber wall below 50 °C. A stable long pulse beam could be achieved through the temperature control of the surfaces inside the source chamber walls.

Measurement of heat load density profile on acceleration grid in MeV-class negative ion accelerator.

To understand the physics of the negative ion extraction/acceleration, the heat load density profile on the acceleration grid has been firstly measured in the ITER prototype accelerator where the negative ions are accelerated to 1 MeV with five acceleration stages. In order to clarify the profile, the peripheries around the apertures on the acceleration grid were separated into thermally insulated 34 blocks with thermocouples. The spatial resolution is as low as 3 mm and small enough to measure the tail of the beam profile with a beam diameter of ∼16 mm. It was found that there were two peaks of heat load density around the aperture. These two peaks were also clarified to be caused by the intercepted negative ions and secondary electrons from detailed investigation by changing the beam optics and gas density profile. This is the first experimental result, which is useful to understand the trajectories of these particles.