Pitch angle scattering of runaway electrons in tokamaks due to cyclotron resonance in the ripple

The aim of this paper is to propose a mechanism which can drastically enhance the perpendicular velocity of electrons in such a way that R<<Ro (R is the instantaneous radius of curvature of the electron trajectory, Ro is the tokamak major radius): the cyclotron interaction of runaway electrons with the field modulation due discrete number of toroidal coils (ripple). The effect of this interaction can be conveniently evaluated using a Lorentz transformation: in its guiding center frame the electron experiences a dramatic cyclotron heating (close to what is obtained with the undulator of some free electron lasers). Going back to the laboratory frame it appears that this heating becomes an exchange between perpendicular and parallel momentum. The associated angular diffusion coefficient is then evaluated to access the efficiency of the proposed mechanism