SOL width and intermittent fluctuations in KSTAR

TL;DR

This study analyzes the radial profiles and fluctuation statistics of plasma in KSTAR, revealing a two-layer structure and blob-like filament bursts that influence plasma-wall interactions, supported by stochastic modeling.

Contribution

It provides detailed measurements of SOL width and fluctuation statistics in KSTAR, and validates a stochastic model predicting plasma-wall interactions from transient events.

Findings

Two-layer SOL profile structure observed

Far-SOL profile scale length increases with density

Blob-like filament bursts dominate fluctuation dynamics

Abstract

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 familiar two-layer structure, seen elsewhere in tokamak L-mode discharges, with a steep near-SOL profile and a broad far-SOL profile, is observed. 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.