Achievement of 500 keV negative ion beam acceleration on JT-60U negative-ion-based neutral beam injector

Abstract

Hydrogen negative ion beams of 507 keV, 1 A and 486 keV, 2.8 A have been successfully produced in the JT-60U negative ion source with a three-stage accelerator by overcoming a poor voltage holding of the accelerator with large-size grids of ∼2 m2. This is the first result of H− beam acceleration up to 500 keV at a high current of over 1 A. In order to improve the voltage holding capability, the breakdown voltages of the large-size grids and small-size electrodes with uniform and locally strong electric fields were examined by changing the gap length. It was found that the voltage holding of the large-size grids was below half of that of the small-size electrodes with a uniform electric field which was used in the design of the accelerator. This degradation was found to be caused by the local electric field concentrations in addition to the size. Based on the results of the voltage holding tests and beam optics calculations, the gap lengths of the large-size grids were tuned to have a capability to sustain 600 kV. As a result, the gap tuning realized stable voltage holding during beam accelerations without significant degradations of the beam optics and stripping loss. These results indicated that stable 500 keV beam accelerations required for JT-60SA are feasible and this gap tuning is also applicable for the design of ITER accelerator.

Study of back streaming ion using a slot-type grounded grid in hydrogen negative-ion source

“…High-energy neutral beam injection (NBI) plays an important role for a magnetically confined fusion device for heating and current drive [1,2,3,4,5,6]. We have achieved 16 MW beam injection using 3 beam lines with 6 large scaled hydrogen negative ion (H − ) sources in the Large Helical Device (LHD) [3].…”

Improvements of the magnetic field design for SPIDER and MITICA negative ion beam sources

“…As this field marginally penetrates in the plasma source upstream of the PG, it also enhances the extraction of negative ions and reduces the co-extracted electrons. As in most of the existing experiments, the Suppression Field is generated by arrays of permanent magnets embedded in the EG and has two opposite peaks of about 50 -60 mT along each beamlet, in order to reduce the resulting deflection of the ions [7]. Nevertheless, numerical simulations and experiments on the MeV Test Facility at JAEA Naka (Japan) have shown that this field can degrade the final beam optics by introducing a zig-zag deflection of some mrad on the ion beamlets.…”

“…For multi-stage accelerators operating at higher acceleration voltage, the CESM magnets on the EG are not sufficient to prevent a large number of stripped electrons from being accelerated and reaching the accelerator exit. To this purpose an additional transverse magnetic field produced by a current flowing in the PG (long-range electron Suppression Field) has been successfully used for reducing the average number and energy of the electrons at the accelerator exit in the JT-60U injector at JAEA (Naka, Japan) [7].…”

Evaluation of negative ion distribution changes by image processing diagnostic

“…Large size and high current NBI systems used arc discharge type H -sources have been operated successfully in the Large Helical Device (LHD) [1] and JT-60U [2] in Japan. A larger H -source used RF discharge was developed at Max-Plank Institute fur Plasma Physics in Garching [3].…”