Measurements of runaway electron synchrotron spectra at high magnetic fields in Alcator C-Mod

TL;DR

This study measures runaway electron synchrotron spectra at high magnetic fields in Alcator C-Mod, revealing how RE energies decrease with increasing magnetic field and comparing experimental thresholds with theory.

Contribution

It introduces a combined experimental and modeling approach to analyze runaway electron spectra at high magnetic fields in a tokamak, including the use of a synthetic diagnostic for spectral reproduction.

Findings

Synchrotron radiation dominates RE power loss at high fields.

RE energies decrease as magnetic field increases.

Experimental thresholds align with theoretical predictions.

Abstract

In the Alcator C-Mod tokamak, runaway electron (RE) experiments have been performed during low density, flattop plasma discharges at three magnetic fields: 2.7, 5.4, and 7.8 T, the last being the highest field to-date at which REs have been generated and measured in a tokamak. Time-evolving synchrotron radiation spectra were measured in the visible wavelength range (~300-1000 nm) by two absolutely-calibrated spectrometers viewing co- and counter-plasma current directions. In this paper, a test particle model is implemented to predict momentum-space and density evolutions of REs on the magnetic axis and q = 1, 3/2, and 2 surfaces. Drift orbits and subsequent loss of confinement are also incorporated into the evolution. These spatiotemporal results are input into the new synthetic diagnostic SOFT [M. Hoppe, et al., Nucl. Fusion 58(2), 026032 (2018)] which reproduces experimentally-measured spectra. For these discharges, it is inferred that synchrotron radiation dominates collisional friction as a power loss mechanism and that RE energies decrease as magnetic field is increased. Additionally, the threshold electric field for RE generation, as determined by hard X-ray and photo-neutron measurements, is compared to current theoretical predictions.