A kinetic–moment framework for electron energy dynamics in capacitively coupled plasmas: absorption, conversion, transport, and dissipation

Understanding electron energy dynamics in low-temperature plasmas such as capacitively coupled plasmas (CCPs), including energy absorption, conversion, transport, and dissipation, is essential for interpreting discharge physics and process applications. We propose a kinetic-moment framework based on particle-in-cell/Monte Carlo collision (PIC/MCC) simulations. The framework reconstructs the first three velocity moments of the Boltzmann equation directly from PIC/MCC data and enables a quantitative, self-consistent description of electron energy dynamics in low-pressure CCPs. Importantly, to clarify energy conversion among electromagnetic energy, electron fluid kinetic (mechanical) energy, and electron thermal (internal) energy, we further separate the total energy transport equation into kinetic- and thermal-energy equations. This decomposition identifies, in addition to direct collisional processes, an important kinetic-to-thermal conversion channel through pressure–strain interaction. We find that, under the present low pressure condition, electrons gain directed kinetic energy in the sheath and convert it locally into thermal energy through pressure–strain interaction and collisions, after which the thermal energy is transported into the bulk and dissipated mainly through inelastic electron–neutral collisions. The pressure–strain interaction can be further decomposed into isotropic and anisotropic contributions, where the former corresponds to reversible energy conversion associated with volumetric compression and expansion, whereas the latter, associated with incompressible deformation, dominates the net dissipation of kinetic energy into thermal energy. More broadly, the results show coexistence of localized energy conversion near the sheath and nonlocal energy transport from the sheath to the bulk dominated by microscopic heat flux. The heat flux deviates strongly from Fourier-type closure based solely on local temperature gradients. Overall, this framework combines kinetic fidelity with fluid-level interpretability and extends power-absorption analysis to a comprehensive description of energy evolution in nonequilibrium plasmas.