Cross‐Field Diffusion by Charge Changing Processes

Abstract

AbstractThe particle transport induced by ionization, recombination and charge exchange processes in magnetically confined plasmas is analyzed. For ions with charge numbers Z ≧ 2 a random walk is caused by the fluttering of the gyro‐radius as a result of successive recombination and ionization events. The corresponding diffusion coefficient, however, is very small and may be neglected even in the case of high‐energy alpha particles being produced in fusion plasmas. Single and neutral particles are subjected to joint transport across the magnetic field due to their unimpeded motion during the short atomic phases. It is found that this mechanism is more important than classical transport over a wide temperature and density range. The corresponding diffusion coefficient D1 = f(Te)Ti/mine is independent of the magnetic field strength and shows an electron temperature dependence which can be expressed in terms of the rate coefficients for ionization, recombination and charge exchange. The latter effect leads to a strong reduction of the diffusion process in hydrogen plasmas. Diffusion coefficients comparable with anomalous values of about 1m2/s are therefore reached only for cool (divertor) plasmas with T < 5eV.