The inverse problem for collisionless plasma equilibria

Abstract

Vlasov-Maxwell equilibria are described by the self-consistent solutions of the time-independent Maxwell equations for the real-space dynamics of electromagnetic fields, and the Vlasov equation for the phase-space dynamics of particle distributions in a collisionless plasma. These two systems (macroscopic and microscopic) are coupled via the source terms in Maxwell's equations, which are sums of velocity-space 'moment' integrals of the particle distribution function. This paper considers 'the inverse problem for collisionless equilibria' (IPCE), viz. "given information regarding the real-space/macroscopic configuration of a specific collisionless plasma equilibrium, what self-consistent equilibrium distributions exist?" We develop the constants of motion approach to IPCE using the assumptions of a 'modified Maxwellian' distribution function, and a strictly neutral and spatially one-dimensional plasma. In such circumstances, IPCE formally reduces to the inversion of Weierstrass transformations (Bilodeau, 1962), such as those transformations that feature in the initial value problem for the heat/diffusion equation. We discuss the various mathematical conditions that a candidate solution of IPCE must satisfy. One method that can be used to invert the Weierstrass transform is expansions in Hermite polynomials. Building on the results of Allanson et al. (2016), we establish under what circumstances a solution obtained by these means converges, and allows velocity moments of all orders. Ever since the seminal work by Bernstein et al. (1957), on 'stationary' electrostatic plasma waves, the necessary quality of non-negativity has been noted as a feature that any candidate solution of IPCE will not $\textit{a priori}$ satisfy. We also discuss this problem in the context of for our formalism, for magnetised plasmas.