Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the BES system on MAST

TL;DR

This paper measures the mean motion of turbulent density patterns in a tokamak using BES diagnostics and a cross-correlation method, revealing that observed poloidal motions are mainly due to magnetic field geometry rather than plasma flows.

Contribution

It introduces a cross-correlation time delay method for analyzing turbulent pattern motion and clarifies the physical origin of observed poloidal velocities in MAST plasma.

Findings

Mean plasma motion is predominantly toroidal.

Apparent poloidal motion arises from magnetic field geometry.

High-poloidal-beta discharges with magnetic islands show negligible pattern motion.

Abstract

The mean motion of turbulent patterns detected by a two-dimensional (2D) beam emission spectroscopy (BES) diagnostic on the Mega Amp Spherical Tokamak (MAST) is determined using a cross-correlation time delay (CCTD) method. Statistical reliability of the method is studied by means of synthetic data analysis. The experimental measurements on MAST indicate that the apparent mean poloidal motion of the turbulent density patterns in the lab frame arises because the longest correlation direction of the patterns (parallel to the local background magnetic fields) is not parallel to the direction of the fastest mean plasma flows (usually toroidal when strong neutral beam injection is present). The experimental measurements are consistent with the mean motion of plasma being toroidal. The sum of all other contributions (mean poloidal plasma flow, phase velocity of the density patterns in the plasma frame, non-linear effects, etc.) to the apparent mean poloidal velocity of the density patterns is found to be negligible. These results hold in all investigated L-mode, H-mode and internal transport barrier (ITB) discharges. The one exception is a high-poloidal-beta (the ratio of the plasma pressure to the poloidal magnetic field energy density) discharge, where a large magnetic island exists. In this case BES detects very little motion. This effect is currently theoretically unexplained.