Effect of suprathermal electrons flux on the ambipolar electric field in stellarators

Abstract

Low collisionality plasmas in non-axisymmetric toroidal magnetic devices have significant radial particle fluxes due to trapped particles in the magnetic ripples. The associated ambipolar radial electric field cannot be quantitatively reproduced by standard neoclassical models since the measured values are systematically larger. Here the contribution of suprathermal electrons is included in the radial electric field. The radial flux of suprathermal particles is calculated following a kinetic analysis of the particle transport in which a non-Maxwellian velocity distribution is assumed for the high velocity electrons in the low collisionality regime. The electric field modification produced by the suprathermal electrons is then obtained and an analytical expression for the radial electric field is found which is compared with the one obtained from the thermal particles. An estimate of the needed contribution of the suprathermal electrons is given. Experimental data of the TJ-II Heliac-stellarator are taken for the computations since good Heavy Ion Beam Probe measurements are available.