SOFT: A synthetic synchrotron diagnostic for runaway electrons

TL;DR

This paper introduces SOFT, a synthetic diagnostic tool that simulates synchrotron radiation from runaway electrons, highlighting how geometric effects impact measurements and interpretation in fusion devices.

Contribution

The paper presents SOFT, a new simulation tool that models synchrotron emission considering geometric effects, improving interpretation of runaway electron diagnostics.

Findings

Geometric effects significantly influence synchrotron spectra.

SOFT accurately reproduces experimental synchrotron images.

Inaccurate emission models can lead to misinterpretation of data.

Abstract

Improved understanding of the dynamics of runaway electrons can be obtained by measurement and interpretation of their synchrotron radiation emission. Models for synchrotron radiation emitted by relativistic electrons are well established, but the question of how various geometric effects -- such as magnetic field inhomogeneity and camera placement -- influence the synchrotron measurements and their interpretation remains open. In this paper we address this issue by simulating synchrotron images and spectra using the new synthetic synchrotron diagnostic tool SOFT (Synchrotron-detecting Orbit Following Toolkit). We identify the key parameters influencing the synchrotron radiation spot and present scans in those parameters. Using a runaway electron distribution function obtained by Fokker-Planck simulations for parameters from an Alcator C-Mod discharge, we demonstrate that the corresponding synchrotron image is well-reproduced by SOFT simulations, and we explain how it can be understood in terms of the parameter scans. Geometric effects are shown to significantly influence the synchrotron spectrum, and we show that inherent inconsistencies in a simple emission model (i.e. not modeling detection) can lead to incorrect interpretation of the images.