A study of runaway electron confinement in the ASDEX tokamak

Abstract

The results of runaway electron confinement experiments from ASDEX are analysed to elucidate the structure of electromagnetic turbulence which may cause anomalous electron heat transport in the L-mode confinement regime. From a simple model, the radial correlation length (W) of the magnetic turbulence is determined to be about 1 mm. Using this value and that of the experimentally deduced electron thermal diffusivity, the authors determine the radial magnetic fluctuation level at the plasma edge in the L-mode to be (B r /B 0 ) ~ 2 X 10~4. Scalings of W and B r /B 0 are deduced from parameter scans. From a comparison of these results with the predictions of various theoretical models, it is concluded that skin depth turbulence, electromagnetic drift wave turbulence, rippling modes, and microtearing modes are inferior candidates and that resistive ballooning modes offer the best possibility for a consistent interpretation of the data.

New mechanism of runaway electron diffusion due to microturbulence in tokamaks

“…The reduced radial transport near the low-order rational drift surfaces is related to the gaps in their density there. As shown in Appendix, the gap width Dw is inversely proportional to the maximal toroidal mode number n max $ n c for which the corresponding terms R n in the function (15) are not negligible. One would expect that with increasing the energy of the electrons the transport barriers become wider and deeper since the number of the toroidal modes n > n c contributing to the transport effectively decays.…”

“…This fact has been used to study the magnetic turbulence in tokamaks by the measurement of runaway electron diffusion. 15,16 Theoretical studies of the runaway electron transport due to magnetic turbulence have been based on the assumption that it is mainly caused by the stochastic diffusion in a braided magnetic field, i.e., due to a magnetic diffusion. 17 Thereby, the diffusion coefficients of runaway electrons, D r , can be expressed via the magnetic field line diffusion coefficient, D M (Refs.…”

Diagnostic techniques for measuring suprathermal electron dynamics in plasmas (invited)

“…41 The transport properties of runaway electrons have also been investigated in a number of studies with the aid of HXR emission. 30,42 On the technological side, the development of solid-state detectors paved the way to more compact HXR setups. Initially, the primary detector materials were Ge and Si, which owing to a modest bandgap, require liquid-nitrogen cooling in most cases of practical interest in the laboratory.…”

Pitch angle scattering and synchrotron radiation of relativistic runaway electrons in tokamak stochastic magnetic fields

“…1 Due to the decrease of the Coulomb collision frequency with the electron energy, runaway electrons are essentially collisionless and, hence, strongly sensitive to the magnitude and nature of the fluctuations. Transport in stochastic magnetic fields, associated with the free streaming of the electrons along the field lines, 2 including corrections due to the drift of the runaway orbit when its energy increases, has been claimed to explain the measured radial runaway losses in many devices, [3][4][5][6] and anomalous runaway losses associated with spontaneous and/or externally induced magnetic perturbations have been demonstrated to be effective in realizing runaway avoidance during disruptive events. 7,8 Stochastic magnetic fields do not only cause anomalous electron losses but, if the level of fluctuations is intense enough, the electrons may escape before entering the runaway regime altering the runaway generation process itself.…”