Mitigation of large amplitude edge-localized modes by resonant magnetic perturbations onLHD

摘要

In the Large Helical Device (LHD), H-mode plasmas are produced, having large amplitude edge-localized modes (ELMs) which are induced by resistive interchange modes (RICs) at the l/2π = 1 rational surface near the foot of the edge transport barrier (ETB). These large ELMs expel a large fraction of plasma stored energy, up to 20% of the stored energy (Wp). The ETB and the l/2π = 1 surface are thought to be in the stochastic field region (SFR) generated intrinsically in three-dimensional magnetic configuration on LHD, because plasma shielding effects for such stochastic fields are not significant due to low electrical conductivity σ and moderate angular frequency of plasma rotation ω in the ETB and the field penetration depth is estimated to be comparable to the ETB width. Even if the ETB is in such intrinsic SFR where electron mean free path is much shorter than the connection length, large amplitude ELMs are excited. Such ELMs were mitigated by externally applied m = 1 = 1 resonant magnetic perturbations (RMPs), for the first time, in a stellarator/helical plasma, where m and n are the poloidal and toroidal mode numbers, respectively. The RMPs reduce the amplitude, enhancing the repetition frequency significantly. The energy loss fraction ΔW_P/W_P is reduced less than ～5% by the mitigation. This mitigation is realized without terminating the H-mode and large penalty to global energy confinement. The RMPs preferentially decrease electron density in ETB indicating enhanced particle transport, while electron and ion temperature profiles are nearly unchanged. Plasma shielding effects for the applied RMPs is also not significant because of low σ and moderate ω in ETB. Noticeable decrease in the gradients of electron density and pressure in the ETB and also at the l/2π = 1 surface is induced by the penetration of the applied RMPs. Enhanced ELM frequency indicates that MHD stability of ETB for RICs is degraded by RMPs. Potential mechanisms of ELM mitigation on LHD are discussed.