Numerical determination of the ambipolar electric field in a stellarator-reactor plasma

A numerical parametric study of the radial ambipolar electric field in a stellarator reactor has been undertaken. With the numerical neoclassical code FLOCS (Flow Code for Stellarators), which is capable of handling both ions and electrons of all relevant kinetic energies, the radial ambipolar field (E_r)_AMB is determined from the algebraic condition that ion and electron fluxes are equal. As expected, the potential is of the same order of magnitude as the temperature. Somewhat surprisingly at first sight, however, the potential does not change much with the temperature (in the parameter range under consideration), being somewhat insensitive to moderate variations of T. An explanation for this behaviour is presented. Finally, the radial particle fluxes, consistent with the obtained (E_r)_AMB, and the particle confinement time are computed.