Development of a variable proton ratio ion source

Extension of the heating experiment region of JT-60 with neutral beam untill the territory of low plasma density is important in order to facilitate the H-mode transition of the beam heated plasma through the heating profile variation. One method of the profile change is reduction in the beam energy so as to keep the shine through beam power under a permissible value while keeping the injection power constant. One of methods is to lower the proton ratio, to increase the molecular ion yield which results in the increase in low energy neutral particles. The injection power also increases by this method. The source chamber of the JT-60 spare ion source was so modified that it can change the plasma volume. The lowest proton ratio of 62 % which is compared with the value of 91 % of JT-60 ion source, was obtained by decreasing chamber depth, decreasing magnetic field strength at the back stream electron beam dump, keeping the filaments near the plasma grid and connection of plasma grid and cathode. Therefore, there is a fair prospect of posibility to expand the heating experiment region. We measured injection power at the proto-type NBI unit. The results indicate that the total injection power of 24 MW which is 20 % larger than the present power is expected to obtain if such method is applied to JT-60 NBI. (author)