Spontaneous current relaxation in NBI heated plasmas on EAST

We report on current profile evolution in EAST NBI driven plasmas where two neutral beams are injected, one during the current ramp phase and the second during flattop. At the end of the current ramp phase, it is found that a flat q profile with q ₀ ∼ 1 is achieved with low magnetic shear in the core. It is observed that plasma current and density both relax much faster than resistive time, even in the absence of sawtooth activity when H–L transition occurs. Density fluctuations associated with magnetic perturbations (3/2) as a precursor to the H–L transition are observed. It is likely that these modes play a role in fast current transport. (paper)