Parallel velocity diffusion and slowing-down rate from long-range collisions in a magnetized plasma

This paper derives an expression for the rate of collisional slowing of charges in a magnetized plasma for which rc < λD, where rc is the mean thermal cyclotron radius and λD is the Debye length. The rate depends on a new fundamental length scale d that separates collisions into two impact parameter ranges that yield different slowing rates: a Boltzmann rate due to isolated binary collisions for impact parameters ρ < d and a Fokker-Planck rate due to multiple small scatterings for ρ > d. Slowing due to Boltzmann collisions is also shown to depend on the sign of the Coulomb interaction: for repulsive interactions, the slowing is enhanced by “collisional caging,” while for attractive interactions the Boltzmann slowing rate is zero.