Transport Dynamics and Multi-Scale Coupling of Turbulence in LHD

Abstract

Spatiotemporal correlation of heat transport and micro-to meso-scale or macro-scale coupling of plasma turbulence are investigated in LHD plasmas, where fast propagation of a cold pulse and a non-local temperature rise are observed. Evidence of a timescale shorter than the diffusion time and a spatial scale longer than the micro-turbulence correlation length between core heat fluxes and edge temperature gradients is found. At the same time, an envelope of turbulent density fluctuations is also found to be modulated. Observation of low frequency (≤ 2 kHz) amplitude modulation of density fluctuation suggests the existence of meso-to macro-scale turbulent structures in the plasma, in which a non-local temperature rise takes place. Relationships between the turbulence with long radial correlation length and the edge-core coupling of heat transport are discussed.

How is turbulence intensity determined by macroscopic variables in a toroidal plasma?

“…The hysteresis is also observed in the relation between turbulence intensity and temperature gradient. This is a more prominent proof for the violation of local closure compared with previous work [2][3][4][5][6][7][8][9][10][11]. In this paper, we present power modulation experiments in a magnetically confined plasma that shows rapid rises/falls of turbulence and turbulence-driven flux immediately following the rise/fall of heating power at distant locations, without waiting for the local change in gradients of mean variables.…”

“…Future magnetic fusion reactors are designed based on this formalism (local closure for gradient-flux relation). However, many observations of dynamic behavior contradict this traditional view [2][3][4][5][6][7]. In addition, the violation of such a local closure for momentum transport has also been confirmed (see an assessment [8]).…”

Observations of abrupt changes in the fluctuation spectrum on LMD-U

“…Turbulent plasmas often show fast time scale phenomena (e.g. L-H transitions [7][8][9][10][11] and transit transport [12][13][14][15]). However, temporal sequences in the evolution of fluctuations and its time scales at the transition have not been reported in a systematic manner.…”