The charged-particle distribution function in tokamaks modified by a magnetic field gradient

Abstract

With the help of the standard neoclassical formalism, the particle guiding-centre distribution function in tokamaks is derived, taking into account the effect of magnetic field gradients. It is shown that the radial gradient of the parallel velocity, which depends on the magnetic field strength gradient, produces a parallel current additive to the conventional neoclassical bootstrap current. The existence of the parallel current on the magnetic axis may reduce or eliminate the need for a seed current in a tokamak reactor. The radial gradient of magnetic field also produces thermal and particle pinches. The thermal pinch significantly decreases the net heat flux in the vicinity of the magnetic axis, and this result is found to be in agreement with experimental trends. The particle pinch can play an important role in impurity transport.