On the synchrotron emission in kinetic simulations of runaway electrons in magnetic confinement fusion plasmas

Abstract

Developing avoidance or mitigation strategies of runaway electrons (RE) in magnetic confinement fusion (MCF) plasmas is of crucial importance for the safe operation of ITER. In order to develop these strategies, an accurate diagnostic capability that allows good estimates of the RE distribution function in these plasmas is needed. Synchrotron radiation (SR) of RE in MCF, besides of being one of the main damping mechanisms for RE in the high energy relativistic regime, is routinely used in current MCF experiments to infer the parameters of RE energy and pitch angle distribution functions. In the present paper we address the long standing question about what are the relationships between different runaway electrons distribution functions and their corresponding synchrotron emission simultaneously including: full-orbit effects, information of the spectral and angular distribution of SR of each electron, and basic geometric optics of a camera. We study the spatial distribution of the SR on the poloidal plane, and the statistical properties of the expected value of the synchrotron spectra of runaway electrons. We observe a strong dependence of the synchrotron emission measured by the camera on the pitch angle distribution of runaways, namely we find that crescent shapes of the spatial distribution of the SR as measured by the camera relate to RE distributions with small pitch angles, while ellipse shapes relate to distributions of runaways with larger the pitch angles. A weak dependence of the synchrotron emission measured by the camera with the RE energy, value of the q-profile at the edge, and the chosen range of wavelengths is observed. Furthermore, we find that oversimplifying the angular dependence of the SR changes the shape of the synchrotron spectra, and overestimates its amplitude by approximately 20 times for avalanching runaways and by approximately 60 times for mono-energetic distributions of runaways.

Recent DIII-D advances in runaway electron measurement and model validation

“…The under-prediction of B T effects may be due to the neglect of spatial gradients in the 0-D 2-V FP modeling, as the next section will show the synchrotron emission is preferentially found inboard of the magnetic axis (where B T is considerably larger). Additionally, a recent full-orbit treatment of RE dynamics finds the synchrotron power substantially increases as compared to the guiding center approximations used in 0-D 2-V FP modeling [38].…”

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

“…Using TPM data on the magnetic axis and brightness formula B (4) produces spectra completely unlike experiment, as seen in figure 15b. One explanation for this is that an asymptotic expansion of P(λ, p, t), given by equation (21) in [20], is used to compute the spectra, as the full calculation becomes increasingly computationally-intensive for such low energies. More importantly, as seen in figure 5, synchrotron emission from REs on the magnetic axis is never directly viewed by the spectrometer, so emission from REs on other flux surfaces must be considered.…”

“…This estimation assumes that the spectrometer observes only a small toroidal slice of REs since emission is approximately tangent to their toroidal trajectories. Recently, more advanced synthetic diagnostics [11,21] have been developed and incorporate the magnetic field topology, detector geometry and spectral response function, and both momentum and spatial distributions of REs. The synthetic diagnostic SOFT has been shown to reproduce the general features of synchrotron camera images from C-Mod [11] and DIII-D [22]; it is used in this study for its synthetic spectrometer capabilities.…”

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

“…2018 b ) 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).…”

“…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).…”