Experimental Study on Nonlocality of Heat Transport in LHD

Abstract

The observation of a significant rise of the core electron temperature Te in response to edge cooling in a helical plasma was first made on the Large Helical Device (LHD). When the phenomenon takes place, the core electron heat flux is reduced abruptly without changing the thermodynamic values in the region of interest (core). Thus, the phenomenon observed in LHD can be equated to a “nonlocal transport phenomenon,” observed so far only in tokamaks. The nonlocal transport phenomenon in LHD takes place in almost the same parametric domain (i.e., in a high-temperature and low-density regime) as in tokamaks. Meanwhile, various new aspects of the nonlocal transport phenomenon have been revealed by the LHD experiments; for example, (1) in LHD, the nonlocal transport phenomenon has been observed in net current-free plasmas sustained only by electron cyclotron heating. This experimental result can completely rule out the contribution of the toroidal plasma current as a reason for the nonlocal transport phenomenon. (2) It has been found that during the nonlocal transport phenomenon, there appears a strong correlation between core electron heat flux and edge Te gradient on a timescale shorter than the diffusion time and a spatial scale longer than the microturbulence correlation length. At that time, it was also found that an envelope of density fluctuations is modulated with a low frequency (≤2 kHz), which suggests the existence of a long-ranged turbulent structure in the plasma, where the nonlocal transport phenomenon can appear.