Effect of the q = 1 surface and sawtooth activity on pressure profiles and energy confinement in pellet fuelled JT-60 limiter plasmas

Improved energy confinement for pellet fuelled plasmas on JT-60 is mainly due to the peaked density and pressure profiles inside the q = 1 rational surface, where the confinement characteristics seem to be better than those in the outer (q > 1) region. In centrally fuelled pellet injection discharges, the sawtooth activity can be completely suppressed during 0.4−1 s, or the frequency of the sawteeth can be reduced by up to one order of magnitude during 0.5−1.5 s after pellet injection. For high current, low-q (Ip = 2.5−3.1 MA, q(a) < 3) discharges, reduction of the sawtooth frequency is strongly related with the enhanced confinement and the peakedness of the electron density profile. The contribution of the sawtooth activity to the global energy confinement increases systematically with decreasing safety factor at the plasma surface.