Runaway Electrons In Tokamaks Research Articles

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.

Effect of limiter biasing on runaway electrons in tokamaks

Tokamak limiter biasing is one of the methods for controlling the radial electric field and can induce a transition to an improved confinement state. Moreover, the mean energy of runaway electrons and therefore emitted hard x-rays depends on fluctuations of plasma current and loop voltage, which can be controlled using limiter biasing. In this paper, we studied the effects of a biased limiter on loop voltage and hard x-rays. Hard x-ray emissions are produced by the collision of runaway electrons with limiters. Experiments were done by discharges in the range of a positive 180–380 V biased limiter.

Parametric upconversion of lower hybrid wave by runaway electrons in tokamak

A kinetic formalism of parametric decay of a large amplitude lower hybrid pump wave into runaway electron mode and an upper sideband mode is investigated. The pump and the sideband exert a ponderomotive force on runaway electrons, driving the runaway mode. The density perturbation associated with the latter beats with the oscillatory velocity due to the pump to produce the sideband. The finite parallel velocity spread of the runaway electrons turns the parametric instability into a stimulated Compton scattering process where growth rate scales as the square of the pump amplitude. The large phase velocity waves thus generated can potentially generate relativistic electrons.

Momentum-space study of the effect of bremsstrahlung radiation on the energy of runaway electrons in tokamaks

The dynamics of runaway electrons in tokamak plasmas is studied in momentum space. In this analysis the electrons are accelerated by an electric field while they are decelerated by collisional drag, synchrotron radiation, and bremsstrahlung radiation. It is shown that the inclusion of bremsstrahlung losses into the calculations is important for the interpretation of disruption-mitigation experiments where large amounts of high Z are injected into the plasma. The inclusion of bremsstrahlung reduces the maximum energy that the runaway electrons can gain from the electric field.

Publisher’s Note: Role of Bremsstrahlung Radiation in Limiting the Energy of Runaway Electrons in Tokamaks [Phys. Rev. Lett.94, 215003 (2005)

Publisher’s Note: Role of Bremsstrahlung Radiation in Limiting the Energy of Runaway Electrons in Tokamaks [Phys. Rev. Lett.94, 215003 (2005)

Role of Bremsstrahlung Radiation in Limiting the Energy of Runaway Electrons in Tokamaks

Bremsstrahlung radiation of runaway electrons is found to be an energy limit for runaway electrons in tokamaks. The minimum and maximum energy of runaway electron beams is shown to be limited by collisions and bremsstrahlung radiation, respectively. It is also found that a massive injection of a high-Z gas such as xenon can terminate a disruption-generated runaway current before the runaway electrons hit the walls.

Runaway electrons in a tokamak: A free-electron maser

In ohmic divertor plasma discharges of the ASDEX upgrade tokamak containing a small population of runaway electrons, fluctuating emission in the microwave region with a very narrow bandwidth is observed. The radiation can be explained by relativistic runaway electrons, which are captured in a ripple resonance of the tokamak and are thus made monoenergetic enough that they can undergo the collective instability of a free-electron maser. From the frequency of the maser, the energy of the runaway electrons, and from the linewidth and energy per radiation pulse, the particle density of the runaway electrons is determined locally. Observing this maser radiation is thus a different diagnostic for runaway electrons in tokamaks.

Effect of ripple on runaway electrons in tokamaks

A model of runaway electron generation and orbit predictions is described and applied to the problem of determining the influence of magnetic field ripple on the radiation loss and energy limits of runaway electrons. In particular the prediction of orbits and energy limits for a proposed ITER design are discussed. It is found that a resonance between the electron gyrofrequency and the fundamental ripple frequency can lead to large synchrotron radiation losses and create an upper bound on runaway energy which is strongly dependent on the pitch angle. Interactions with the second harmonic of the ripple field are very sensitive to the ripple amplitude and may lead to a further reduction in runaway energy. In ITER, this effect can limit the runaway energy to values of 270 MeV.

Electric field effects on relativistic charged particle motion in Tokamaks

The authors consider the relativistic guiding center motion of charged particles (charge e, rest mass m0) in magnetic fields consistent with toroidal Tokamak equilibrium. The particle acceleration due to the toroidal electric field-essential to Tokamak operation-is taken into account. For the case of weak acceleration (eU/m0C2<<1, where U is the loop voltage), the poloidal locus of charged particle motion is found to be an envelope of closed drift orbits which, in the case of large aspect-ratio, are characterized by adiabatic invariance of their cross-sectional area. This allows, in the absence of collisions, a complete description of the motion and confinement of runaway electrons in Tokamaks. For peaked current distributions and sufficiently high energies drift separatices exist. They are calculated and their relevance to charge particle confinement and injection is considered.

Nonlinear Evolution of Runaway-Electron Distribution and Time-Dependent Synchrotron Emission from Tokamaks

Stability of the runaway electrons in tokamaks is analyzed. The distinction between high-density and low-density operation of tokamak discharges is interpreted in terms of the stability condition obtained. In the unstable case, the temporal evolution of the distribution function of the runaway electrons is obtained by solving the quasilinear equation. Time-dependent synchrotron emission from the runaway electrons is calculated. (AIP)