Role of bremsstrahlung radiation in limiting the energy of runaway electrons in tokamaks

Suppression of bremsstrahlung losses from relativistic plasma with energy cutoff

We study the effects of redistributing superthermal electrons on bremsstrahlung radiation from hot relativistic plasma. We consider thermal and nonthermal distribution of electrons with an energy cutoff in the phase space and explore the impact of the energy cutoff on bremsstrahlung losses. We discover that the redistribution of the superthermal electrons into lower energies reduces radiative losses, which is in contrast to nonrelativistic plasma. Finally, we discuss the possible relevance of our results for open magnetic field line configurations and prospects of the aneutronic fusion based on proton-boron-11 (p-B11) fuel.

Energy distribution of lost high-energy runaway electrons based on their bremsstrahlung emission in the EAST tokamak

We report in this paper the study towards revealing the energy distribution of lost high-energy runaway electrons based on their bremsstrahlung emission. The high-energy hard x-rays are originated from the bremsstrahlung emission of lost runaway electrons in the experimental advanced superconducting tokamak (EAST) tokamak, and the energy spectra are measured using a gamma spectrometer. The energy distribution of the runaway electrons is reconstructed from this hard x-ray energy spectrum using a deconvolution algorithm. The results indicate that the energy distribution of the lost high-energy runaway electrons can be obtained with the deconvolution approach. In the specific case in this paper, the runaway electron energy was peaked around 8 MeV, covering from 6 MeV to 14 MeV.

Role of Kinetic Instability in Runaway-Electron Avalanches and Elevated Critical Electric Fields

The effects of kinetic whistler wave instabilities on the runaway-electron (RE) avalanche is investigated. With parameters from experiments at the DIII-D National Fusion Facility, we show that RE scattering from excited whistler waves can explain several poorly understood experimental results. We find an enhancement of the RE avalanche for low density and high electric field, but for high density and low electric field the scattering can suppress the avalanche and raise the threshold electric field, bringing the present model much closer to observations. The excitation of kinetic instabilities and the scattering of resonant electrons are calculated self-consistently using a quasilinear model and local approximation. We also explain the observed fast growth of electron cyclotron emission signals and excitation of very low-frequency whistler modes observed in the quiescent RE experiments at DIII-D tokamak. Simulations using ITER parameters show that by controlling the background thermal plasma density and temperature, the plasma waves can also be excited spontaneously in tokamak disruptions and the avalanche generation of runaway electrons may be suppressed.

Effect of Partially Screened Nuclei on Fast-Electron Dynamics

We analyze the dynamics of fast electrons in plasmas containing partially ionized impurity atoms, where the screening effect of bound electrons must be included. We derive analytical expressions for the deflection and slowing-down frequencies, and show that they are increased significantly compared to the results obtained with complete screening, already at subrelativistic electron energies. Furthermore, we show that the modifications to the deflection and slowing down frequencies are of equal importance in describing the runaway current evolution. Our results greatly affect fast-electron dynamics and have important implications, e.g., for the efficacy of mitigation strategies for runaway electrons in tokamak devices, and energy loss during relativistic breakdown in atmospheric discharges.

Applying the new gamma ray imager diagnostic to measurements of runaway electron Bremsstrahlung radiation in the DIII-D Tokamak (invited)

A new gamma ray imager (GRI) is developed to probe the electron distribution function with 2D spatial resolution during runaway electron (RE) experiments at the DIII-D tokamak. The diagnostic is sensitive to 0.5-100 MeV gamma rays, allowing characterization of the RE distribution function evolution during RE growth and dissipation. The GRI consists of a lead "pinhole camera" mounted on the DIII-D midplane with 123 honeycombed tangential chords 20 cm wide that span the vessel interior. Up to 30 bismuth germanate (BGO) scintillation detectors capture RE bremsstrahlung radiation for Pulse Height Analysis (PHA) capable of discriminating up to 20 000 pulses per second. Digital signal processing routines combining shaping filters are performed during PHA to reject noise and record gamma ray energy. The GRI setup and PHA algorithms will be described and initial data from experiments will be presented. A synthetic diagnostic is developed to generate the gamma ray spectrum of a GRI channel given the plasma information and a prescribed distribution function. Magnetic reconstructions of the plasma are used to calculate the angle between every GRI sightline and orient and discriminate gamma rays emitted by a field-aligned RE distribution function.

Toroidal runaway beams

We investigate the dynamics of a special group of runaway electrons which possibly play an important role in toroidal fusion devices. Starting from the torus center they are accelerated by a toroidal electric field and are hence forced to move across the toroidal magnetic field into regions with rising poloidal field in order to compensate for the centrifugal forces. Can such particles finally form a tight beam of relativistic runaways in the outboard region or is this prevented due to the perpendicular momentum they gain by passing the toroidal field? Since neither the energy nor the magnetic momentum of the particles is conserved this question has been treated by invoking the relativistic equations of motion. It turns out, however, that the problem can be essentially simplified since, apart from the centrifugal forces associated with the toroidal motion, the inertia forces are negligible. The resulting first order equation can be solved analytically. From the solution it is concluded that the formation of narrow runaway beams with diameters in the range of micrometers and very small pitch angles (v(perpendicular)/v(||)<10(-6)) appears feasible. Such electrons would perform low-frequency oscillations about three to four orders of magnitude lower than the gyrofrequency in the toroidal field. When passing the maximum poloidal magnetic field strength they are suddenly lost from the plasma region.