Electron kinetics in low temperature plasmas *

Low-temperature plasma (LTP) can be broadly defined as a quasi-neutral mixture of charged and neutral species with electron mean energy about the ionization potential of atoms and molecules. Its highly non-equilibrium nature is the source of numerous instabilities and collective effects observed in nature and the foundation for the widespread use of LTP in modern technologies. Collective behavior of magnetized space plasmas (associated with wave-particle interactions, turbulence, shock waves) and collisionless effects in plasma reactors (associated with stochastic electron heating and anomalous skin effect) call for a unified picture. This paper uncovers common features of electron kinetics at vastly different time- and space scales, from solar wind plasma to semiconductor manufacturing and nano-technologies. Nonlocal electron kinetics in low-pressure radio frequency (RF) discharges resemble collisionless effects in space science, fusion, terahertz technology, and plasmonics. Understanding scaling laws and closure rules for selecting kinetic and fluid models is critical for efficient plasma modeling and full utilization of the beneficial LTP properties. We describe common effects associated with the formation of electron groups in LTPs due to electron trapping by ambipolar electric fields, velocity filtration via magnetic focusing, and the nonlocal electro-dynamic related to violation of Ohm’s law. Using kinetic models of the solar wind and gas discharges as examples, we illustrate the formation of double layers and the associated appearance of different electron groups in glow discharges and in expanding plasmas of solar wind and magnetic nozzles. We also briefly discuss the runaway electrons responsible for a wide range of physical phenomena from nano- and picoscale breakdown of dielectrics to lightning initiation. Understanding the electron kinetics of LTPs could promote scientific advances in several topics of plasma physics and accelerate modern plasma technologies.