Dynamics of Plasma during the Formation of a Weak Negative Central Magnetic Shear Configuration Using Counter Neutral Beam Injection in the JFT-2M Tokamak.

In the JFT-2M tokamak, discharges exhibiting good performance have been produced by counter-NBI after boronization. An improved core confinement mode under an H-mode edge condition is sustained for ˜5 τE with H89P˜1.5 and β N˜2.0 at a line-averaged electron density of around 70% - 80% of the Greenwald density limit. It is found that the q-profile changes from a monotonic one having q0˜1 to a zero or weak negative central magnetic shear configuration having 1 < qmin ≤ q0 < 3 during improved core confinement mode using counter-NBI under an H-mode edge condition. The central electron temperature very clearly increases when the sawtooth activity disappears, while the previous result without boronization exhibited a significant degradation of the energy confinement due to the accumulation of impurities. The mechanism governing the improved core confinement mode including the density peaking and/or ITB formation due to negative shear is presently unclear.