Rapid growth of resistive interchange modes and excitation of edge localized modes (ELMs) in an H-mode plasma of LHD

Rapid growth of low frequency magnetic fluctuations are observed about 1 ms before an edge localized mode (ELM) event in H-mode plasmas in an outward-shifted configuration on LHD. Long ELM-free H-phases more than ∼5 times of global energy confinement time are interrupted by the large amplitude ELMs with a repetition frequency typically less than 20 Hz. Magnetic fluctuations suppressed down to very low level during the ELM-free H-phase are strongly enhanced by an order of magnitude during each ELM event. The magnetic fluctuations are dominated by m=1/n=1 modes, where m and n are the poloidal and toroidal mode numbers, respectively. In the L-phase, the m=1/n=1 magnetic fluctuations are enhanced by a factor of 3 of the level of the ELM-free H-phase. In order to understand the magnetic fluctuation behaviors in the ELM free H-phase, at the ELM onset and in H-phase, the linear stability analysis of the plasmas for these three time slices has been performed against resistive interchange mode (RIC). The rapid growth of m=1/n=1 RIC can be qualitatively explained by a slight radial expansion of the edge transport barrier (ETB), of which expansion leads to an increase of the pressure gradient at the mode rational surface ι=1 in the magnetic hill region. The calculated eigenfunction of the radial displacement for the profiles simulating the ELM onset have a tearing mode character as well as interchange one, while those in the ELM-free H-phase and L-phase have typical interchange mode character. (authors)