Progress in Power and Pulse Width of N-NBI
Negative ion-based neutral beam injection (N-NBI) experiments have been carried out to demonstrate the feasibility of current drive and heating by N-NBI.
Since 1996, conditioning of the N-NBI system has been made in progress in JT-60U so that the injection power recently reached 5.1 MW for 0.67 s at 350 keV (Fig. 1). A new method termed 'pre-arcing', in which arc plasma is produced for 2 s in the ion sources prior to the beam extraction, allows the beam pulse width to be extended up to 1.9 s at 3 MW without breakdowns in electrodes (Fig.2); the longest pulse was limited up to 0.7 s before applying this method. Beam-driven currents by N-NBI are increased up to 0.37 MA from 0.28 MA previously obtained mainly due to increase in electron temperature of the target plasmas.
Optimization of the target plasma for current drive experiments are being addressed as well as further extension of the beam pulse and injection power to exhibit the current drive capability of N-NBI.
