Theory Seminar: The failure of the Chapman-Enskog expansion in a collisional plasma

Archie Bott

Princeton University

Thursday, March 5, 2020



PSFC Seminars

In this talk I will discuss the kinetic stability of classical, collisional plasmas. Fluid equations are typically used to describe such plasmas, since their distribution functions are close to being Maxwellian. The small deviations from the Maxwellian distribution are calculated via the Chapman-Enskog (CE) expansion, and determine macroscopic momentum and heat fluxes in the plasma. Such a calculation is only valid if the underlying CE distribution function is stable at collisionless scales. I will demonstrate that at sufficiently high plasma beta, the CE distribution function can be subject to numerous microinstabilities across a wide range of scales, the most significant of which shall be characterized. Of specific note is the discovery of the previously uncharacterized `whisper-wave instability', whose growth rate in certain parameter regimes is large compared to other instabilities. Our approach enables us to construct a kinetic stability map of classical, two-species collisional plasma in terms of the mean free path, the electron skin depth and the plasma beta. Our work highlights that collisional plasmas can be kinetically unstable; in this situation, the determination of transport coefficients with the standard CE expansion is not necessarily valid. Evidence for discrepancies from standard models of collisional plasma transport arising in recent laser-plasma experiments on the National Ignition Facility will also be discussed.