Progress on physics understanding of improved confinement with fishbone instability at low q (95) < 3.5 operation regime in EAST

Improved confinement at the low q (95) < 3.5 operation regime with fishbone instability compared to sawtooth oscillation has been observed and investigated on the EAST under the dominant electron heating condition with a tungsten divertor. The formation of an internal transport barrier in the ion thermal channel strongly correlates to the excitation of the fishbone, accompanied by reduced particle outward transport in the core region identified by a central peaked density profile. Current density distribution is found to change from a monotonic shape with q (0) < 1 during sawtooth oscillation to a central flat structure, magnetic shear s similar to 0 at rho < 0.4, with the fishbone instability at where the higher off-axis bootstrap current fraction might play a critical role. Linear gyrokinetic simulation by NLT code was carried out to investigate the turbulent transport characteristics, which is qualitatively in good agreement with experimental measurements from CO2 laser collective scattering diagnostics. The electron-scale trapped electron mode that dominates the turbulent transport during sawtooth is found to be stabilized with the fishbone at identical heating power and plasma configurations.