Ambipolar electric field and the ambipolar diffusion in a stochastic magnetic field

The transport of plasmas in a stochastic magnetic field must give rise to an ambipolar electric field. In this paper, the transition probability of particles in the stochastic magnetic field is derived when an electric field is also present. The transport fluxes and the ambipolar electric field can then be calculated. The result shows that the ambipolar electric field is insensitive to collisions (Vertical Bar eVertical Bar E/T/sub e/ = -A(n/sub e//dx)/n/sub e/, A is related to collisions, but Aapprox. =1). It is pointed out that the ambipolar electric field can be used as a necessary (but not sufficient) condition to ascertain the importance of the stochastic magnetic field. The ambipolar transport in the stochastic magnetic field results in a larger flow velocity of impurity ions than that of the hydrogen ions [v/sub I//v/sub i/approx. =(m/sub i//m/sub I/)/sup 1/2/Z/sub I/>1] It can explain the impurity outward transport during the internal disruption in the tokamak experiments. The effect of collisions and the conditions for the validity of the treatment are discussed. The higher collisionality gives a smaller diffusion flux. The corresponding dynamic equation in the stochastic magnetic field is derived and it is somewhat different from that obtained by Molvig et al. in Ref. 3