Mesoscopic transport in KSTAR plasmas: avalanches and the E×BE \times B staircase

The self-organization is one of the most interesting phenomena in the non-equilibrium complex system, generating ordered structures of different sizes and durations. In tokamak plasmas, various self-organized phenomena have been reported, and two of them, coexisting in the near-marginal (interaction dominant) regime, are avalanches and the $E \times B$ staircase. Avalanches mean the ballistic flux propagation event through successive interactions as it propagates, and the $E \times B$ staircase means a globally ordered pattern of self-organized zonal flow layers. Various models have been suggested to understand their characteristics and relation, but experimental researches have been mostly limited to the demonstration of their existence. Here we report detailed analyses of their dynamics and statistics and explain their relation. Avalanches influence the formation and the width distribution of the $E \times B$ staircase, while the $E \times B$ staircase confines avalanches within its mesoscopic width until dissipated or penetrated. Our perspective to consider them the self-organization phenomena enhances our fundamental understanding of them as well as links our findings with the self-organization of mesoscopic structures in various complex systems

Turbulence induced particle flux measurements by microwave imaging reflectometry

In this paper we present a spectral analysis method [1] to measure the frequency spectrum of particle flux induced by fluctuations across confining magnetic fields using microwave imaging reflectometry (MIR) diagnostic [2, 3] in toroidal devices. In this technique density fluctuations measured by two correlated MIR radial channels are used to calculate radial particle flux spectrum over a finite frequency range. Analysis shows that during n=2 (with 0 phasing) non-axisymmetric magnetic perturbation (MP) the particle flux in the outward direction enhances and energy confinement degrades in a neutral beam heated discharge. Degradation of particle confinement is confirmed by reduction in line averaged density and stored energy signals. Confinement regains its value due to decrease in the outward particle flux after the MP field is switched off

First evidence of Alfvén wave activity in KSTAR plasmas

We report on first evidence of wave activity during neutral beam heating in KSTAR plasmas: 40 kHz magnetic fluctuations with a toroidal mode number of n = 1. Our analysis suggests this a beta-induced Alfvén eigenmode (BAE) resonant with the q = 1 surfac

SOL width and intermittent fluctuations in KSTAR

Radial profiles of the ion saturation current and its fluctuation statistics are presented from probe measurements in L-mode, neutral beam heated plasmas at the outboard mid-plane region of KSTAR. The results are consistent with the familiar two-layer structure, seen elsewhere in tokamak L-mode discharges, with a steep near-SOL profile and a broad far-SOL profile. The profile scale length in the far-SOL increases drastically with line-averaged density, thereby enhancing plasma interactions with the main chamber walls. Time series from the far-SOL region are characterised by large-amplitude bursts attributed to the radial motion of blob-like plasma filaments. Analysis of a data time series of several seconds duration under stationary plasma conditions reveals the statistical properties of these fluctuations, including the rate of level crossings and the average duration of periods spent above a given threshold level. This is shown to be in excellent agreement with predictions of a stochastic model, giving novel predictions of plasma–wall interactions due to transient transport events