(TH/P3-6) MHD simulation on ablation cloud in tokamak and heliotron

Abstract.  It is well known that an ablation cloud drifts to the lower field side in tokamak plasmas, which leads to a good performance on fueling in tokamak. Such a good performance, however, has not been obtained yet in the planar axis heliotron; Large Helical Device (LHD) experiments, even if a pellet has been injected from the high field side. The purpose of the study is to clarify the difference on the cloud motion between tokamak and LHD plasmas by using the MHD simulation including ablation processes. It is found in tokamaks that the drifting motion is induced by a tire tube force, and that the pressure and density of the plasmoid have oscillation due to fast compressional Alfvén wave. The first trial simulations on the motion of the plasmoid with helical symmetry in a straight helical plasma also show that the plasmoid drifts to the lower field side similarly to tokamaks. However, an actual plasmoid drifts inward or outward of the torus depending on the location, since the plasmoid expands along the magnetic field and the rotational transform is greater than that in tokamaks in the plasma periphery. Thus, it is suggested that the difference of the pellet injections between the high field and the low field sides is reduced in helical plasmas.

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