Dependence of radial thermal diffusivity on parameters of toroidal plasma affected by resonant magnetic perturbations.

We investigate how the neoclassical thermal diffusivity of an axisymmetric toroidal plasma is modified by the effect of resonant magnetic perturbations (RMPs), using a drift-kinetic simulation code for calculating the radial thermal diffusivity of ion in the perturbed region under an assumption of zero electric field. Here, the perturbed region is assumed to be generated on and near the resonance surfaces, and is wedged in between the regular closed magnetic surfaces. We find that the dependence of the radial thermal diffusivity on parameters of the toroidal plasma is represented as Χ_r = Χ_r⁽⁰⁾ {1 + c₀ (ω_b/ν_eff∆_b²) <∥δB_r∥²>/|B_t0|²}, where Χ_r⁽⁰⁾ is the neoclassical thermal diffusivity and c₀ is a positive coefficient. Here, ω_b is the bounce frequency, ν_eff is the effective collision frequency, ∆_b is the banana width, <∥δB_r∥²^1/2 is the strength of the RMPs in the radial directions, and |B_t0| is the strength of the magnetic field on the magnetic axis. The value of c₀ := c₀/(qR_ax/ √∈_t)² is significantly reduced to c₀ ~ 10^-4 in the simulations because of the drift motion affected by the Coulomb collision, as contrasted with c₀ = π predicted by the so-called field-line diffusion theory, where q is the safety factor, R_ax is the major radius of the magnetic axis, and ∈_t is the inverse aspect ratio. [ABSTRACT FROM AUTHOR]