# Characterization of edge and scrape-off layer fluctuations using the fast Li-BES system on COMPASS

## TL;DR

This study evaluates the capabilities of the Li-BES system on COMPASS for characterizing edge and scrape-off layer plasma fluctuations, highlighting its advantages, limitations, and the effectiveness of statistical detection methods.

## Contribution

It provides a detailed analysis of the Li-BES system's performance in fluctuation characterization and introduces the use of the generalized sequential probability ratio test for automatic event detection.

## Key findings

- Li-BES system effectively characterizes edge fluctuations
- Atomic physics influences measurement interpretation
- Validation of the Garcia-model for plasma filaments

## Abstract

Recently the Li-Beam Emission Spectroscopy (Li-BES) system on COMPASS has reached its full diagnostic power in terms of routine automatic operation in any kind of plasma scenarios and it is normally used as a standard tool for reconstruction of ultra fast density profiles in the edge region of COMPASS plasmas. The purpose of this study is to investigate the advantages and limitations of the COMPASS Li-BES system in characterizing plasma electron density fluctuations. We show how the atomic physics of plasma-beam interactions can affect the interpretation of the measurement at different radial positions and for different electron density profiles. We also demonstrate the usability of generalized sequential probability ratio test for automatic event detection. Using non-perturbative diagnostic, we verify the validity of the stochastic Garcia-model for scrape-off layer filaments and accompanying holes (density deficits).