Runaway Electron Diffusion Research Articles

Retraction Note to: Controlling the Diffusion of Runaway Electrons by Safety Factor Changes in IR-T1 Tokamak

Retraction Note to: Controlling the Diffusion of Runaway Electrons by Safety Factor Changes in IR-T1 Tokamak

Self-Consistent Simulation of the Excitation of Compressional Alfvén Eigenmodes and Runaway Electron Diffusion in Tokamak Disruptions.

Alfvénic modes in the current quench (CQ) stage of the tokamak disruption have been observed in experiments. In DIII-D the excitation of these modes is associated with the presence of high-energy runaway electrons (REs), and a strong mode excitation is often associated with the failure of RE plateau formation. In this work we present results of self-consistent kinetic-MHD simulations of RE-driven compressional Alfvén eigenmodes (CAEs) in DIII-D disruption scenarios, providing an explanation of the CQ modes. Simulation results reveal that high energy trapped REs can have resonance with the Alfvén mode through their toroidal precession motion, and the resonance frequency is proportional to the energy of REs. The mode frequencies and their relationship with the RE energy are consistent with experimental observations. The perturbed magnetic fields from the modes can lead to spatial diffusion of REs including the nonresonant passing ones, thus providing the theoretical basis for a potential approach for RE mitigation.

Effect of tearing modes on the confinement of runaway electrons in Experimental Advanced Superconducting Tokamak

The effect of tearing modes on the confinement of runaway electrons is studied in Experimental Advanced Superconducting Tokamak (EAST). The general tendency of the radial diffusion coefficient of runaway electrons (REs) D r is derived based on the time response relation between the tearing modes and runaway electrons. The results indicate that, the magnetic fluctuations of tearing modes will enhance the radial diffusion of runaway electrons when the magnetic island is small. Following the increasing of the magnetic fluctuations of the tearing modes, the formed large magnetic island may weaken the radial diffusion of runaway electrons. The results can be important to understand the confinement of runaway electrons when large magnetic islands exist in the plasma.

RETRACTED ARTICLE: Controlling the Diffusion of Runaway Electrons by Safety Factor Changes in IR-T1 Tokamak

The high-energy current of runaway electrons during a major disruption in tokamak reactors can cause serious damage to the first wall of the reactor and reduce its lifetime. Therefore, it is important to find methods for decreasing the generation of runaway electrons and their energy. The safety factor plays an important role in determining the stability criteria for a wide range of MHD modes. Since runaway electrons suffer only rarely from collisions and are hardly sensitive to electrostatic turbulence, their transport is governed by the magnetic lines structure. On the other hand, since the safety factor is related to the magnetic lines structure, changes in safety factor may have important effects on the diffusion of runaway electrons. In this paper, the generation of runaway electrons and their transport is investigated theoretically. Moreover, by changing the discharge voltage of ohmic and toroidal capacitors, different values of the edge safety factor is generated. In fact, in this experiment, the researchers try to increase the diffusion of runaway electrons by using safety factor changes in the IR-T1 tokamak.

Simulation of runaway electron generation and diffusion during major disruptions in the HL-2A tokamak

The generation and diffusion of runaway electrons (REs) during major disruptions in the HL-2A tokamak has been studied numerically. The diffusion caused by the magnetic perturbation is especially addressed. The simulation results show that the strong magnetic perturbation (δB/B∼1.0×10−3) can cause a significant loss of REs due to the radial diffusion and restrain the RE avalanche effectively. The results also indicate that the REs are generated initially in the plasma core during disruptions, and that the toroidal electric field does not exhibit a centrally hollow phenomenon. In addition, it is found that the toroidal effects have little impact on the generation of RE and the evolution of toroidal electric field.

New mechanism of runaway electron diffusion due to microturbulence in tokamaks

Chaotic transport of runaway electrons in a toroidal system in the presence of a weak small-scale magnetic turbulent field with a wide mode spectrum is studied. Using a fast running mapping, the radial profiles of turbulent diffusion coefficients are calculated. It is found that at large Kubo numbers the chaotic transport of the electrons is described by a fractal-like radial dependence of the diffusion coefficients with reduced or zero values near low-order rational drift surfaces which form transport barriers. The latter can be one of the main reasons of the improved confinement of runaway electrons in tokamaks. One can expect that this effect may lead to the formation of the nested beams of runaway electrons.

Comparative study of runaway electron diffusion in the rise phase of low q a and normal q a discharges in the SINP tokamak

The behaviour of runaway electrons in the SINP tokamak, which can be operated in a normal edge safety factor (qa) (NQ) discharge configuration as well as in a low qa (LQ) configuration, was experimentally investigated, during the initial plasma generation phase. An energy analysis of the runaway electron dynamics in the rise phase of the SINP tokamak discharges was also made. A comparison of the runaway electron diffusion coefficients in NQ and LQ is carried out in this paper.

Runaway electron transport via tokamak microturbulence

The mechanisms found for the magnetic transport of fast ions in the work of Hauff et al. [Phys. Rev. Lett. 102, 075004 (2009)] are extended to the diffusion of runaway electrons. Due to their smaller mass and larger energy, they behave strongly relativistically, for which reason the scaling laws defined previously have to be modified. It is found that due to these changes, the regime of constant magnetic transport does not exist anymore, but diffusivity scales with E−1 for magnetic transport, or even with E−2 in the case that finite gyroradius effects become important. It is shown that the modified analytical approaches are able to explain the surprisingly small values found in experiments, although it cannot be excluded that possibly other reduction mechanisms are present at the same time.

Runaway electrons as a diagnostic of plasma internal magnetic fluctuations

The transport of runaway electrons in a high-temperature plasma is relatively easy to measure in a steady state experiment and a perturbation experiment, which provides runaway electron diffusion coefficient Dr. This diffusion coefficient is determined by internal magnetic fluctuations, so it can be interpreted in terms of a magnetic fluctuation level. The internal magnetic fluctuation level (r/BT) is estimated to be about (2–4)×−4 in the HL-1M plasma. The results presented here demonstrate the effectiveness of using runaway electron transport techniques to determine internal magnetic fluctuations. A profile of magnetic fluctuation level in the HL-1M plasma can be estimated from Dr.

Plasma transport in mixed magnetic topologies

A simple model is introduced to illustrate some features concerning anomalous transport associated with magnetic turbulence. For magnetic topologies that are described as bands of stochasticity separated by regions with good flux surfaces, the transport coefficients deviate significantly from those describing completely stochastic magnetic fields. It is possible to have the electron heat diffusivity exceed a runaway electron diffusion coefficient, despite the existence of widespread magnetic stochasticity. Comparing the ratios of transport coefficients is not a sufficient method to dismiss the possibility that electromagnetic fluctuations play a prominent role in anomalous plasma transport.

Runaway electrons as a diagnostic of magnetic fluctuations (invited)

The transport of runaway electrons in a hot plasma can be comparatively easily measured by perturbation experiments which provide a runaway electron diffusivity D. The diffusion can be interpreted in terms of a magnetic fluctuation level, from which an electron thermal diffusivity can be deduced. We find the runaway electron diffusion coefficient is determined by intrinsic magnetic fluctuations rather than electrostatic fluctuations because of the high energies involved. The results presented here demonstrate the efficacy of using runaway transport techniques for determining intrinsic magnetic fluctuations.

Runaway electrons as a diagnostic of magnetic fluctuations (abstract)

The transport of runaway electrons in a hot plasma can be comparatively easily measured by perturbation experiments. The runaway electron diffusion coefficient is determined by intrinsic magnetic fluctuations rather than electrostatic fluctuations because of the high energies involved. The results presented here demonstrate the efficacy of using runaway transport techniques for determining intrinsic magnetic fluctuations. This work was supported in part by U. S. Department of Energy under grant DE-FG05-88ER-53267 and the Texas Advanced Research Program.

Effect of drifts on the diffusion of runaway electrons in tokamak stochastic magnetic fields

The quasilinear diffusion of runaway electrons in tokamak stochastic magnetic fields is examined. Previous models are generalized with respect to the spatial location and coherency of the perturbing magnetic fields, treatment of the radial as well as poloidal drift motion of the electrons, and the role of sidebands that arise from the beating of the electron drift motion with the applied perturbing fields. It is found that drift effects act to reduce the level of quasilinear diffusion by an amount that depends on the poloidal distribution of the magnetic turbulence. The results are employed to estimate the internal magnetic fluctuation levels at the edge during recent experiments on the TEXT tokamak [Phys. Fluids B 3, 2038 (1991)], where the drift modification effects are shown to be small. It is inferred that intrinsic magnetic turbulence controls runaway diffusion, but not the thermal diffusivity of the background electrons.

Diffusion of runaway electrons in a tokamak

It is shown that the presence of a sufficient number of accelerated electrons in a plasma leads to anomalous diffusion of the electrons across the magnetic field. The diffusion of the particles is associated with the presence of waves driven unstable by the runaway electrons via anomalous and Doppler effect. Expressions are obtained for the diffusion coefficient, the oscillation spectrum and the distribution function of the accelerated electron beam.

Quasi-linear relaxation of runaway electrons in a HF heated tokamak plasma

The theory of quasi-linear diffusion of runaway electrons in the field of electromagnetic waves (HF field) excited by external sources is given. The frequency of the waves is somewhat less than the electron cyclotron frequency. The time-averaged distribution function of resonant electrons is derived. The presence of the toroidal electric field changes the process of quasi-linear relaxation considerably. It also leads to a continuous absorption of energy and momentum from the HF field. On the other hand, the HF field increases the number of high-energy electrons. As a consequence, beam instabilities can arise. A comparison with experimental results is given.