The classical diffusion of plasma confined by the magnetic field refers to the collision dominated transport that follows the 1/B2 scaling, where B is the magnetic field strength.
Diffusion is a random walk process that can be quantified by the two key parameters: Δx, the step size, and Δt, the time interval when the walker takes a different step. Thus, the diffusion coefficient is defined as D≡(Δx)2/(Δt).
In a uniform magnetic field, a particle undergoes random walk across the field lines by the step size of gyroradius ρ≡vth/Ω, where vth denotes the thermal velocity, and Ω≡qB/mc, the gyrofrequency. The steps are randomized by the collisions to lose the coherence. Thus, the time step, or the decoherence time, is inverse of the collisional frequency νc. The rate of diffusion is given by νcρ2, with the rather favorable B−2 scaling law.