Improvement of beam performance in the negative-ion based NBI system for JT-60U

Abstract

The injection performance of the negative-ion based NBI (N-NBI) system for JT-60U has been improved by correcting beamlet deflection and improving spatial uniformity of negative ion production. Beamlet deflection at the peripheral region of the grid segment due to the distorted electric field at the bottom of the extractor has been observed. This was corrected by modifying the surface geometry at the extractor to form a flat electric field. Moreover, beamlet deflection due to beamlet–beamlet repulsion caused by space charge was also compensated for by extruding the edge of the bottom extractor. This resulted in a reduction of the heat loading on the NBI port limiter. As a result of the improvement above, continuous injection of a 2.6 MW H0 beam at 355 keV has been achieved for 10 s. Thus, long pulse injection up to the nominal pulse duration of JT-60U was demonstrated. This has opened up the prospect of long pulse operation of the negative-ion based NBI system for a steady-state tokamak reactor. So far, a maximum injection power of 5.8 MW at 400 keV, with a deuterium beam, and 6.2 MW at 381 keV, with a hydrogen beam, have been achieved in the JT-60U N-NBI. Uniformity of negative ion production was improved by tuning the filament emission current so as to direct more arc power into the region where less negative ion current was extracted.

Progress of the development of the IPP RF negative ion source for the ITER neutral beam system

“…In addition there may be an advantage with respect to cesium consumption, which is needed to enhance the source performance (see below), as in the RF source there is no filament material that is evaporated and buries the Cs layer on the walls [7]. Present large filamented negative ion sources suffer from significant plasma non-uniformities [8], [9] which are assumed to be caused by the arc current and the magnetic filter field.…”

A Review of Fusion and Tokamak Research Towards Steady-State Operation: A JAEA Contribution

“…Table 1 shows the major specifications of the N-NBI system in JT-60U [118]. Since then, progress has been made to increase beam voltage by conditioning [119], to improve ion source uniformity [120,121] and to reduce heat load to the electrodes [122,123]. Table 1.…”

RADI—A RF source size-scaling experiment towards the ITER neutral beam negative ion source

“…This is achieved with narrow slits in the dummy grid. In order to have some control of the conduc- (4), feedthroughs for the starter filaments (5) and gas (6), three vertical (7), five horizontal (8) and three axial (9) diagnostic ports.…”

“…The maximum (routinely) achieved current densities -both for Table 1). However, present large filamented negative ion sources suffer from significant plasma non-uniformities [7], [8] which are assumed to be caused by the arc current and the magnetic filter field.…”