Experimental determination of the correlation properties of plasma turbulence using 2D BES systems

TL;DR

This paper develops a method to relate plasma turbulence correlation parameters to measurements from 2D BES systems, enabling systematic comparison between theory and experiment, validated with simulations and applied to MAST data.

Contribution

It introduces a systematic procedure to map and invert correlation parameters between plasma turbulence and BES measurements, including resolution criteria based on PSFs.

Findings

Validated the correlation mapping with gyrokinetic simulations.

Demonstrated the sensitivity of the CCTD method to velocity and PSF variations.

Provided a practical framework for comparing turbulence models with experimental data.

Abstract

A procedure is presented to map from the spatial correlation parameters of a turbulent density field (the radial and binormal correlation lengths and wavenumbers, and the fluctuation amplitude) to correlation parameters that would be measured by a Beam Emission Spectroscopy (BES) diagnostic. The inverse mapping is also derived, which results in resolution criteria for recovering correct correlation parameters, depending on the spatial response of the instrument quantified in terms of Point-Spread Functions (PSFs). Thus, a procedure is presented that allows for a systematic comparison between theoretical predictions and experimental observations. This procedure is illustrated using the MAST BES system and the validity of the underlying assumptions is tested on fluctuating density fields generated by direct numerical simulations using the gyrokinetic code GS2. The measurement of the correlation time, by means of the cross-correlation time-delay (CCTD) method, is also investigated and is shown to be sensitive to the fluctuating radial component of velocity, as well as to small variations in the spatial properties of the PSFs.