Test Particle Diffusion and the Failure of Integration along Unperturbed Orbits

This paper discusses a new effect in the kinetic theory of single species plasmas in the guiding-center regime rcλD. If two guiding centers collide once, their velocities parallel to the magnetic field are eventually reversed through interactions with surrounding particles, and the same two guiding centers then collide several times. These multiple collisions are ignored in the method of integration along unperturbed orbits (IUO) that is usually applied in plasma kinetic theory problems. This effect leads to a factor of 3 enhancement in the predicted rate of collisional test particle diffusion as compared to theory based on IUO. The new prediction is in agreement with recent experiments.