Sensitivity of Microwave Interferometer in the Limiter Shadow to filaments in ASDEX Upgrade

TL;DR

This paper investigates how a microwave interferometer diagnostic on ASDEX Upgrade responds to filamentary density perturbations in the far Scrape-Off Layer, combining experimental data and forward modeling to determine sensitivity boundaries.

Contribution

It introduces a new diagnostic method and models its response to plasma filaments, establishing the sensitivity limits of MILS to density perturbations in the far SOL.

Findings

MILS detects transient filament events with high temporal resolution.

Sensitivity boundaries of MILS to filament size and density perturbation are quantified.

Forward modeling aligns well with experimental observations.

Abstract

Microwave interferometer in the Limiter Shadow (MILS) is a new diagnostic, installed on ASDEX Upgrade for electron density measurements in the far Scrape-Off Layer (SOL). At the chosen frequency of 47 GHz the region of measurements varies within several centimeters before and after the limiter, depending on the density. 200 kHz data acquisition allows resolving transient events such as edge localised modes (ELMs) filaments and turbulence filaments. The measured quantities, phase shift and power decay of the microwave beam, which crosses the plasma, are directly connected to the density and do not depend on any other plasma quantity. In this work, we analyse the influence of a filamentary perturbation on MILS signals. Simple representation of a filament is adopted, with parameters relevant to experimental filament properties, reported for ASDEX Upgrade. Forward modelling is done in COMSOL software by using RAPLICASOL, to study the response of the MILS synthetic diagnostic to the presence of a filament. Qualitative and quantitative dependencies are obtained and the boundaries of MILS sensitivity to filaments, or to the density perturbation in far SOL in general, are outlined.