Study of runaway electrons in TUMAN-3M tokamak plasmas

Abstract

Studies of runaway electrons in present day tokamaks are essential to improve theoretical models and to support possible avoidance or suppression mechanisms in future large-scale plasma devices. Some of the phenomena associated with the runaway electrons take place at faster time scales, and thus it is essential to probe the runaway electrons to investigate underlying physics. The present article reports a few experimental observations of runaway electron associated events, at fast time scales, using a state-of-the-art multi-detector system developed at the Ioffe Institute and recently deployed on the TUMAN-3M tokamak. The system is based on the highperformance scintillation gamma-ray spectrometers for measurements of bremsstrahlung generated during the interaction of accelerated electrons with plasma and materials of the tokamak chamber. It includes a total three detectors configured in the spectroscopic mode having different lines of sight. Along with this hardware, dedicated algorithms were developed and validated that enables the separation of piled-up pulses, maximize the dynamic range of the detector and provides a counting rate as high as 10 7 counts per second. The inversion code, DeGaSum, has been used for the reconstruction of a runaway electron energy distribution function from the measured gamma-ray spectra. Using this tool, experimental analysis of the runaway electron beam generation and evolution of their energy distribution in the TUMAN-3M representative plasma discharges is performed. The effect on gamma-ray count rate during the magnetohydrodynamic activities and possible changes in the runaway electron energy distribution function during sawtooth oscillations is discussed in detail. Possible maximum limit of the runaway electron energy in TUMAN-3M is investigated and compared with the numerical analysis. In addition, the probability of the runaway electron generation throughout the plasma discharge is estimated analytically and compared with the experimental observation that suggests a balance between production and loss of the runaway electrons.

Study of runaway electron dynamics at the ASDEX Upgrade tokamak during impurity injection using fast hard x-ray spectrometry

“…Hard x-ray spectroscopy of hot plasma is one of the few methods that allow one to estimate the energy distribution of REs. The possibilities of RE diagnostics by the bremsstrahlung registration were demonstrated in measurements at small and medium size tokamaks Globus-M [7], HT-7 [8][9][10], FT-2 [11,12], J-TEXT [13,14], COMPASS [15] and TUMAN-3M [16]. In these experiments, the spectra of bremsstrahlung generated by the interaction of REs with the materials of the tokamak plasma facing components were measured.…”

Runaway electron mitigation with pulsed localized vertical magnetic field perturbation in ADITYA tokamak

“…The lead (Pb) shielded scintillator is positioned at the machine mid-plane (Z = 0) and collimated to view the HXR radiation from a cone of plasma and limiter. The thick target bremsstrahlung radiation coming from the limiter is assumed to be dominant [36][37][38].…”

Tokamak research at the Ioffe Institute

“…To provide energy distributions of fast electrons and ions in tokamak plasmas based on gamma-ray measurements, a spectrum deconvolution code DeGaSum has been developed at the Ioffe Institute [67]. The code was applied to get the energy distributions of runaway electrons in experiments at FT-2 [68,69], TUMAN-3M [70], JET [71] and ASDEX-Upgrade [72]. Examples of a hard x-ray spectrum recorded during an Ohmic TUMAN-3M discharge and the runaway electron distribution function reconstructed with the DeGaSum code are shown in figure 22.…”