On the dynamics of runaway electrons

Two topics in the kinetic theory of runaway electrons are considered here. The first one is the effect of radial diffusion, caused by small-scale magnetic fluctuations, on runaway avalanches produced by dose collisions between fast and thermal electrons. It is found that diffusion reduces the growth rate of such avalanches, and they are predicted to be prevented altogether by magnetic fluctuations exceeding about deltaB/B similar to 10(-3) in large tokamaks. The second topic concerns the damping of runaway land other strongly relativistic) electron beams by Coulomb collisions and emission of synchrotron radiation. Pitch-angle scattering of beam electrons initially moving parallel to a magnetic field causes them to acquire gyro-motion and thus to emit synchrotron radiation. The reaction force damps the beam at a rate which is calculated by including the Abraham-Lorentz force in the kinetic equation, and broadly agrees with the observed decay of post-disruption runaway currents in JET.