Synthetic measurements of runaway electron synchrotron emission in the SPARC tokamak

TL;DR

This paper models synchrotron emission from runaway electrons in the SPARC tokamak using synthetic diagnostics, aiding in detection and mitigation strategies for high-energy REs during different plasma scenarios.

Contribution

It introduces a synthetic diagnostic approach with SOFT to simulate RE synchrotron emission in SPARC, considering various plasma conditions and magnetic configurations.

Findings

Synchrotron emission peaks in visible-IR range at 12.2 T magnetic field.

Detection strategies for REs are proposed using matched views.

Time resolution and spectral requirements for RE detection are assessed.

Abstract

With plasma currents up to 8.7 MA, the SPARC tokamak runs the risk of forming multi-MA beams of relativistic "runaway" electrons (REs) which could damage plasma facing components if unmitigated. The infrared (IR) and visible imaging and visible spectroscopy systems in SPARC are designed with measurements of synchrotron emission from REs in mind. Synchrotron radiation is emitted by REs along their direction of motion, opposite the plasma current. Matched clockwise and counterclockwise wide views are proposed to detect synchrotron and background radiation, allowing observation of RE synchrotron emission in both plasma current configurations. Due to SPARC's high toroidal magnetic field strength, 12.2 T on axis, the synchrotron light spectrum is expected to peak in the visible-IR wavelength range. The synthetic diagnostic tool SOFT (Synchrotron Orbit-Following Toolkit) is used to model synchrotron images and spectra for three scenarios, with appropriate magnetic equilibria for each: REs generated during plasma current ramp-up, steady-state flattop (although unlikely, but serving as a reference), and disruptions. Required time resolutions, achievable spatial coverage, and appropriate spectral ranges for various RE energies are assessed.