Modelling and comparison with experiment of radial profiles in a Tokamak with magnetic field stochastization

A deliberate stochastization of the magnetic field in tokamaks with Ergodic Divertors (ED) modi.es essentially the transport properties at the plasma edge by permitting a radial transfer of particles and heat along stochastic field lines and affecting anomalous transport perpendicular to them. A model for the effective transport coefficients in a stochastic magnetic field, taking into account both parallel flows and perpendicular transfer of anomalous nature, has been incorporated into the 1.5D transport code RITM for self-consistent calculations of the radial profiles of plasma parameters. Computations have been performed for the tokamak TEXTOR to simulate phases in an ohmic shot without and with the Dynamic Ergodic Divertor (DED). Results show that the increase of the outward convective transport in the stochastic boundary layer leads to a drop of the line average density. The experimentally observed density variation is reproduced by adjusting the amplitude of the stochastization characteristics with their radial dependence predicted by the field line mapping. The decrease of the diamagnetic energy after DED activation is in qualitative agreement with the experimental data from the diamagnetic loop measurements. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)