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

In the Alcator C-Mod tokamak, runaway electron (RE) experiments have been performed during low density, ?attop plasma discharges at three magnetic felds: 2.7, 5.4, and 7.8 T, the last being the highest feld 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 (λ∼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 confnement are also incorporated into the evolution. These spatiotemporal results are input into the new synthetic diagnostic SOFT (Hoppe et al 2018 Nucl. Fusion 58 026032) 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 feld is increased. Additionally, the threshold electric feld for RE generation, as determined by hard x-ray and photo-neutron measurements, is compared to current theoretical predictions.