A Large Negative Ion Source Immersed in the Vacuum Vessel

To develop the powerful negative‐ion‐based neutral beam injection system for the large helical device, a 1/3 scaled H− source has been tested, which is immersed in the vacuum vessel. Intrinsic features of the vacuum‐immersed large H− source are described. Plasma density of higher than about 1012 e/cm3 is produced. During the beam extraction, it was found that a beam‐induced thin plasma exists in the vacuum vessel. In order to get a long pulse beam, this plasma was shielded and then the beam conditioning was completed up to the total beam energy of 70 keV for 100 ms, with the arc power up to about 50 kW. It is confirmed that H− current scaling as well as the plasma parameters would be similar to the scaling which is achieved by the previous 1/3 source.