Spectral scheme for an energetic Fokker-Planck equation with $κ$-distribution steady states

The concern of the present paper is the design of efficient numerical schemes for a specific Fokker-Planck equation describing the dynamics of energetic particles occurring in thermonuclear fusion plasmas (runaway electrons for example). In the long-time limit, the velocity distribution function of these particles tends towards a thermal non-equilibrium $\kappa$-distribution function which is a steady-state of the considered Fokker-Planck equation. These $\kappa$-distribution functions have the particularity of being only algebraically decaying for large velocities, thus describing very well suprathermal particle populations. Our aim is to present two efficient spectral methods for the simulation of such energetic particle dynamics. The first method will be based on rational Chebyshev basis functions, rather than on Hermite basis sets, which are the basis of choice for Maxwellian steady states. The second method is based on a different polynomial basis set, constructed via the Gram-Schmidt orthogonalisation process. These two new spectral schemes, specifically adapted to the here considered physical context, shall permit to cope with the long-time asymptotics without significant numerical costs.