A Review of Recent Progress on Energy Conversion in Plasmas Beyond Fluid Models

The mechanisms for energy conversion in plasmas, especially irreversible energy conversion into internal energy, are well understood for plasmas describable as a simple fluid. However, many plasmas are not describable as a fluid with conventional closures because collisions are slower than dynamical time scales. Examples abound in naturally occurring and laboratory plasmas - the solar corona, interplanetary space, planetary magnetospheres, interstellar space, compact astrophysical objects, magnetically confined fusion, high energy density plasmas, and low temperature plasmas. Such plasmas are regularly not in local thermodynamic equilibrium (LTE) and require a kinetic theory description. How energy is converted in plasmas out of LTE is a forefront research area across the plasma sciences. Since many plasmas are far from LTE, non-perturbative approaches are desired. In this review, we discuss three recently developed non-perturbative first-principles approaches. (1) The field-particle correlation (Klein and Howes, ApJL, 826, L30, 2016) describing energy conversion between electromagnetic fields and charged particles, (2) The pressure-strain interaction (e.g., Yang et al., Phys. Plasmas, 24, 072306, 2017) describing conversion between bulk flow and internal energy, and (3) Energy conversion associated with non-LTE moments (Cassak et al., PRL, 130, 085201, 2023; Barbhuiya et al., submitted). For the latter, it was recently emphasized that the fundamental variable to describe the thermodynamics of non-LTE systems is the phase space density rather than its moments, and an entropic approach was developed to quantify it; this effectively extends the first law of thermodynamics to kinetic theory. For each approach, we describe kinetic simulations and satellite and/or laboratory measurements in the context of Landau damping, magnetic reconnection, plasma turbulence, and/or collisionless shocks. We conclude with a discussion of open questions.