Abstract
Second deuterium operation of the negative ion based neutral beam injector was performed in 2018 in the large helical device. The electron and ion current ratio improves to Ie/Iacc(D) = 0.31 using the short extraction gap distance of 7 mm between the plasma grid (PG) and the extraction grid (EG). The strength of the magnetic field by the electron deflection magnet installed in the EG increases by 17% at the PG ingress surface, which effectively reduces the electron component in the negative ion rich plasma in the vicinity of PG apertures. The reduction of the electron current made it possible to operate at a high power arc discharge and beam extraction. Then, the deuterium negative ion current increases to 55.4 A with the averaged current density of 233 A/m2. The thermal load on the EG using 7 mm gap distance is 0.6 times smaller than the thermal load using a 8 mm gap caused by the reduction of coextracted electron current. The injection beam power increases to 2.9 MW in the beam line BL3, and the total beam injection power increases to 7 MW by three beam lines in the second deuterium campaign.
Highlights
A negative ion based neutral beam injector (N-NBI) which enables high energy beam injection provides the heating power for fusion required for central heating and current drive in high density torus plasma
The thermal load of grounded grid (GG) depends on the Iacc(D), and does not depend on the co-extracted electron current
The relationship between the Iacc(D) and the thermal load on the GG is not changed by difference of the extraction gap distance for 1 mm
Summary
Second deuterium operation of the negative ion based neutral beam injector (N-NBI) was performed in 2018 in the Large. The electron and the ion current ratio improves to Ie /Iacc(D) = 0.31 using the short extraction gap distance of 7 mm between the plasma grid (PG) and the extraction grid (EG). The reduction of the electron current made it possible to operate a high power arc discharge and beam extraction. The deuterium negative ion current increases to 55.4 A with the averaged current density of 233 A/m2. The thermal load on the extraction grid using. 7 mm gap distance is 0.6 times smaller than the thermal load using 8 mm gap caused by the reduction of co-extracted electron current.
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