The role of magnetic islands in modifying long range temporal correlations of density fluctuations and local heat transport

Abstract

This work explores the relation between magnetic islands, long range temporal correlations and heat transport. A low order rational surface (ῑ = 3/2) was purposely scanned outward through an Electron Cyclotron Resonance Heated (ECRH) plasma in the TJ-II stellarator. Density turbulence and the poloidal flow velocity (or radial electric field) were characterized using a two channel Doppler Reflectometer. Simultaneously, the ECRH power was modulated to characterize heat transport, using measurements from a 12 channel Electron Cyclotron Emission diagnostic. A systematic variation of the poloidal velocity was found to be associated with the stationaryῑ = 3/2 magnetic island. Inside from the rational surface, the Hurst coefficient, quantifying the nature of long-range correlations, was found to be significantly enhanced. Simultaneously, heat transport was enhanced as well, establishing a clear link between density fluctuations and anomalous heat transport. The variation of the Hurst coefficient was consistent with a magnetohydrodynamic turbulence simulation.

Causal impact of magnetic fluctuations in slow and fast L–H transitions at TJ-II

“…It should be noted that the radii in the L phase (∆t < 0) are somewhat smaller than indicated on the ordinate axes of the graphs (valid for the I phase, ∆t > 0), as mentioned above: ρ L −0.656 + 1.77ρ I , so the actual radii are, respectively, ρ L 0.69 and 0.76, bracketing the theoretical vacuum position of the rational surface (0.73). This is consistent with results from resistive MHD turbulence simulations, performed for similar parameters [55]. In these simulations, the mean poloidal flow was typically found to be symmetric around a rational surface, showing a double peak inside and outside from the rational surface, at distances compatible with the two radial zones of interaction found here.…”

The impact of rational surfaces on radial heat transport in TJ-II

“…[54], but we note that the other barriers in the range 0.2 < |ρ| < 0.55 are more likely of the type described in Ref. [42]: subdiffusive regions and flow shear layers associated with rational surfaces. It is interesting to note that this would mean that both types of transport barriers can be studied in significant detail in TJ-II plasmas.…”

“…The observed 'stepwise' propagation of the temperature perturbations (associated with points 1 and 2) can be understood from the impact of flow shear regions, associated with rational surfaces, on transport, as discussed in earlier work [42]. As a consequence, radial propagation may be slowed down near rational surfaces (as also observed on Alcator C-Mod [53]), leading to 'stick-slip' behavior and a characteristic 'staircase' shape of the propagation front and 'tails' developing in the Transfer Entropy plots at long time lags.…”

“…ECRH power modulation experiments have also been performed, with the goal of analyzing heat transport using standard Fourier techniques [27,42], although no full iota scan was made. In these experiments, one of the two ECRH systems was operating continuously at ∼ 200 kW, while the other ECRH system was modulated at low power, with a square waveform.…”

Observations of the effects of magnetic topology on the SOL characteristics of an electromagnetic coherent mode in the first experimental campaign of W7-X

“…The other bifurcation is the transport in the magnetic island with high thermal diffusivity and smaller thermal diffusivity [15,16]. The enhanced Hurst exponent and heat flux were located at a smaller radial position than the rational surface, indicating the dependence of turbulence on the magnetic topology in TJ-II stellarator [17,18]. The large-scale and smallscale edge turbulence in Wendelstein 7-AS were studied in detail, exhibiting an intense relationship with plasma confinement conditions [19][20][21].…”