(EX/P8-13) Observation of m/n=1/1 Mode Behaviors during Molecular Beam Fuelling and ECRH Discharges in HL-2A

Yi Liu1), Y.B. Dong1), W. Deng1)
1) Southwestern Institute of Physics, Chengdu, China

Abstract.  On the HL-2A tokamaks, series of experiments have been conducted to shed light on the persistent m/n=1/1 mode topology and its influence on transport properties in the plasma centre. Several new characteristics of central MHD activities during auxiliary fualling and heating are discussed, and some important information about the changes in local plasma parameters and how the plasma responds to the perturbation caused by molecular beam injection (MBI) or laser blow-off are provided. Several important central MHD activities, for example, sawtooth suppression, monster and compound sawtooth, and persistent m/n=1/1 oscillations, have been observed. A large, long persistent m/n=1/1 perturbation has newly been observed in the core region after molecular beam injection, a detailed study on the influence of MBI on central pressure gradient and the stability of central plasma has been made. Especially, possible mechanisms for the formation of the continuous mode or snake-like perturbation during MBI are discussed. In laser blow-off experiments, persistence of the m=1 oscillation after the internal disruption of inverted sawtooth is found. With the illumination effect of the impurity radiation, the island structure after sawtooth crash can be studied. Such a delicate study provides much information about the evolution of the central q profile and the reconnection process. During electron cyclotron heating (ECRH), a strong m=1/n=1 mode is excited when the heating power is high enough and the resonance position is located just around the core of plasma. With the strong m=1 oscillation driven by ECRH, a sawtooth tends to saturate or decrease in its ramp phase and the shape of sawtooth is usually changed, leading to formation of a saturated sawtooth, a hill, or a compound sawtooth. Experimental results indicate that there is evidence for an internal steep electron temperature profile associated with the q=1 rational surface before the compound crash. The effect of ECRH on plasma transport properties in the vicinity of the q=1 surface are investigated.

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