Development of negative ion based NBI system for JT-60U

Development for enhancing the beam characteristics in the N-NBI system for JT-60U has been continued since 1996. In order to increase beam power and d uration time, a few countermeasures for improvement of source plasma non-un iformity have been tried. The first is changing are current limiting resist ors for changing are current distribution, the second is regulating filamen t temperature for altering an: discharge mode, the third is optimizing a ma gnetic barrier in an: chamber, and the fourth is blocking the beam accelera tion at a strongly non-uniform source plasma area by masking both upper and lower edge of plasma grid AU of these countermeasures have been confirmed to be effective.