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

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 (λ ≈ 300-1000 nm) by two absolutelycalibrated 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.

Physics of runaway electrons in tokamaks

“…The exact source of this discrepancy is not certain yet. In C-MOD and FTU, comparison with modeling has indicated that synchrotron damping is sufficient to explain the anomalously high measured damping [226][227][228].…”

Spatiotemporal evolution of runaway electrons from synchrotron images in Alcator C-Mod

“…The full spectral and angular calculation of synchrotron emission is available in SOFT. However, because the angular spread of synchrotron emission is quite small (∼1/γ, where γ is the relativistic factor), a "cone" model -where radiation is only emitted along the RE direction of motion -serves as an adequate approximation of the full angular formulation, as discussed in [5,10,32]. Additionally, the cone model significantly reduces computation time, which for all SOFT simulations used in this work was ∼3700 CPU hours in total.…”

Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption

“…Most recently, in Tinguely et al. (2018 a , b , 2019), synchrotron spectra, images and polarization data were analysed with the help of the synthetic diagnostic Soft (Hoppe et al. 2018 b ) in a series of Alcator C-Mod discharges, providing valuable constraints on runaway energy, pitch angle and radial density.…”

“…Recent developments of synthetic synchrotron diagnostics have allowed detailed analysis of experimental synchrotron data. Most recently, in Tinguely et al (2018aTinguely et al ( ,b, 2019, synchrotron spectra, images and polarization data were analysed with the help of the synthetic diagnostic SOFT (Hoppe et al 2018b) in a series of Alcator C-Mod discharges, providing valuable constraints on runaway energy, pitch angle and radial density. Full-orbit simulations have also recently provided deeper insight into observations of synchrotron radiation in three-dimensional magnetic fields (Carbajal & del Castillo-Negrete 2017;del Castillo-Negrete et al 2018).…”