Neoclassical current in a toroidally-confined multispecies plasma

The parallel currents in a toroidally-confined plasma is shown to consist of the current in response to the parallel electric field, the Pfirsch–Schlüter current which is required by charge neutrality, and the bootstrap current which flows to maintain a balance between the viscous and friction forces along the magnetic field. The steady state momentum equation is used to obtain an Ohm’s law relating the neoclassical current to the cross-field fluxes of particles and heat in a multiple ion species plasma. It is found that the presence of impurities does not alter the magnitude of the bootstrap current. Thus, the neoclassical beta limit established by the bootstrap current is insensitive to the impurity content of the plasma. The magnitudes of the bootstrap current and conjugate Ware fluxes are calculated in the transition from the long mean-free-path to collisional regime.