A model of a double sheath between two plasmas is presented in which reflected particles and the initial velocities of accelerated particles are considered. The model shows that, in addition to the voltage drop across the double sheath, the ions from the plasma sac acquire kinetic energy (comparable with the electron thermal energy) in a presheath region. The ratio of the ion and electron current densities can depart significantly from Langmuir's (M/M)k.

Theory of a double sheath between two plasmas

The 2022 Roadmap is the next update in the series of Plasma Roadmaps published by Journal of Physics D with the intent to identify important outstanding challenges in the field of low-temperature plasma (LTP) physics and technology. The format of the Roadmap is the same as the previous Roadmaps representing the visions of 41 leading experts representing 21 countries and five continents in the various sub-fields of LTP science and technology. In recognition of the evolution in the field, several new topics have been introduced or given more prominence. These new topics and emphasis highlight increased interests in plasma-enabled additive manufacturing, soft materials, electrification of chemical conversions, plasma propulsion, extreme plasma regimes, plasmas in hypersonics, data-driven plasma science and technology and the contribution of LTP to combat COVID-19. In the last few decades, LTP science and technology has made a tremendously positive impact on our society. It is our hope that this roadmap will help continue this excellent track record over the next 5–10 years.

The 2022 Plasma Roadmap: low temperature plasma science and technology

Sheaths in low temperature collisionless and weakly collisional plasmas are often viewed as simple examples of nonlinear physics. How well do we understand them? Closer examination indicates that they are far from simple. Moreover, many predicted sheath properties have not been experimentally verified and even the appropriate “Bohm velocity” for often encountered two-ion species plasma is unknown. In addition, a variety of sheathlike structures, e.g., double layers, can exist, and many two- and three-dimensional sheath effects have not been considered. Experimental studies of sheaths and presheaths in weakly collisional plasmas are described. A key diagnostic is emissive probes operated in the “limit of zero emission.” Emissive probes provide a sensitive diagnostic of plasma potential with a resolution approaching 0.1V and a spatial resolution of 0.1cm. Combined with planar Langmuir probes and laser-induced fluorescence, they have been used to investigate a wide variety of sheath, presheath, and sheathlike structures. Our experiments have provided some answers but have also raised more questions.

Sheaths: More complicated than you think

Plasma jets are sources of repetitive and stable ionization waves, meant for applications where they interact with surfaces of different characteristics. As such, plasma jets provide an ideal testbed for the study of transient reproducible streamer discharge dynamics, particularly in inhomogeneous gaseous mixtures, and of plasma–surface interactions. This topical review addresses the physics of plasma jets and their interactions with surfaces through a pedagogical approach. The state-of-the-art of numerical models and diagnostic techniques to describe helium jets is presented, along with the benchmarking of different experimental measurements in literature and recent efforts for direct comparisons between simulations and measurements. This exposure is focussed on the most fundamental physical quantities determining discharge dynamics, such as the electric field, the mean electron energy and the electron number density, as well as the charging of targets. The physics of plasma jets is described for jet systems of increasing complexity, showing the effect of the different components (tube, electrodes, gas mixing in the plume, target) of the jet system on discharge dynamics. Focussing on coaxial helium kHz plasma jets powered by rectangular pulses of applied voltage, physical phenomena imposed by different targets on the discharge, such as discharge acceleration, surface spreading, the return stroke and the charge relaxation event, are explained and reviewed. Finally, open questions and perspectives for the physics of plasma jets and interactions with surfaces are outlined.

https://iopscience.iop.org/article/10.1088/1361-6595/ac61a9/pdf

Physics of plasma jets and interaction with surfaces: review on modelling and experiments

Numerous applications such as micro- and nanoelectromechanical systems, microplasmas, and directed energy increasingly drive device miniaturization to nanoscale and from vacuum to atmospheric pressure. This wide range of operating conditions and relevant mechanisms complicates the derivation of a single scaling law for electron emission and gas breakdown; therefore, theoretical studies often unify two or three mechanisms piecemeal. This study defines a common set of scaling parameters across the range of dominant mechanisms to derive a theory that links electron emission and breakdown mechanism theories from quantum scales to Paschen's law and yields asymptotic solutions for quantum space-charge limited emission (QSCL), classical space-charge limited emission (CSCL), space-charge limited emission with collisions (MG), Fowler–Nordheim field emission (FN), field emission driven gas breakdown, and classical gas breakdown defined by Paschen's law (PL). These non-dimensionalized equations are universal (true for any gas) across all regimes except for PL, which contains a single, material-dependent parameter. This approach reproduces various nexuses corresponding to the transitions across multiple mechanisms, such as QSCL to CSCL, CSCL to FN, CSCL to MG to FN, and field emission-driven breakdown as described by FN to PL, using a single non-dimensionalization scheme to facilitate experimental designs concerned with crossing these regimes. Furthermore, we demonstrate the conditions for more complicated nexuses, such as matching QSCL, CSCL, MG, and FN. This provides valuable information to experimentalists concerning regimes where slight perturbations in conditions may alter the electron emission mechanism and to theorists concerning the applicability of the asymptotic solutions or reduced nexus theories.

Linkage of electron emission and breakdown mechanism theories from quantum scales to Paschen's law

The most fundamental response of a solid to a plasma and vice versa is electric. An electric double layer forms with a solid-bound electron-rich region—the wall charge—and a plasma-bound electron-depleted region—the plasma sheath. However, it is only the plasma sheath that has been studied extensively ever since the beginning of plasma physics. The wall charge received much less attention. Particularly, little is known about the operando electronic structure of plasma-facing solids and how it affects the spatiotemporal scales of the wall charge. The purpose of this Perspective is to encourage investigations of this terra incognita by techniques of modern surface physics. Using our own theoretical explorations of the electron microphysics at plasma–solid interfaces and a proposal for measuring the wall charge by infrared reflectivity to couch the discussion, we hope to put together enough convincing reasons for getting such efforts started. They would open up—at the intersection of plasma and surface physics—a new arena for applied as well as fundamental research.

/pdf/electron-microphysics-at-plasma-solid-interfaces-58l5jo6hve.pdf

Electron microphysics at plasma–solid interfaces

For a collisionless plasma in contact with a dielectric surface, where with unit probability electrons and ions are, respectively, absorbed and neutralized, thereby injecting electrons and holes into the conduction and valence bands, we study the kinetics of plasma loss by nonradiative electron-hole recombination inside the dielectric. We obtain a self-consistently embedded electric double layer, merging with the quasineutral, field-free regions inside the plasma and the solid. After a description of the numerical scheme for solving the two sets of Boltzmann equations, one for the electrons and ions of the plasma and one for the electrons and holes of the solid, to which this transport problem gives rise to, we present numerical results for a $p$-doped dielectric. Besides potential, density, and flux profiles, plasma-induced changes in the electron and hole distribution functions are discussed, from which a microscopic view on plasma loss inside the dielectric emerges.

Kinetic modeling of the electric double layer at a dielectric plasma-solid interface.

We show that the charge accumulated by a dielectric plasma-facing solid can be measured by infrared spectroscopy. The approach utilizes a stack of materials supporting a surface plasmon resonance in the infrared. For frequencies near the Berreman resonance of the layer facing the plasma the reflectivity dip--measured from the back of the stack, not in contact with the plasma--depends strongly on the angle of incidence making it an ideal sensor for the changes of the layer's dielectric function due to the polarizability of the trapped surplus charges. The charge-induced shifts of the dip, both as a function of the angle and the frequency of the incident infrared light, are large enough to be measurable by attenuated total reflection setups.

http://iopscience.iop.org/article/10.1209/0295-5075/124/25001/pdf

Measuring the plasma-wall charge by infrared spectroscopy

The most fundamental response of a solid to a plasma and vice versa is electric. An electric double layer forms with a solid-bound electron-rich region-the wall charge-and a plasma-bound electron-depleted region-the plasma sheath. But it is only the plasma sheath which has been studied extensively ever since the beginning of plasma physics. The wall charge received much less attention. Especially little is known about the in-operando electronic structure of plasma-facing solids and how it affects the spatio-temporal scales of the wall charge. The purpose of this perspective is to encourage investigations of this terra incognito by techniques of modern surface physics. Using our own theoretical explorations of the electron microphysics at plasma-solid interfaces and a proposal for measuring the wall charge by infrared reflectivity to couch the discussion, we hope to put together enough convincing reasons for getting such efforts started. They would open up-at the intersection of plasma and surface physics-a new arena for applied as well as fundamental research.

Electron microphysics at plasma-solid interfaces

We propose to measure the surface charge accumulating at the interface between a plasma and a dielectric by infrared spectroscopy using the dielectric as a multi-internal reflection element. The surplus charge leads to an attenuation of the transmitted signal from which the magnitude of the charge can be inferred. Calculating the optical response perturbatively in first order from the Boltzmann equation for the electron-hole plasma inside the solid, we can show that in the parameter range of interest a classical Drude term results. Only the integrated surface charge enters, opening up thereby a very efficient analysis of measured data.

K. Rasek

Papers

Electron microphysics at plasma–solid interfaces

Kinetic modeling of the electric double layer at a dielectric plasma-solid interface.

Measuring the plasma-wall charge by infrared spectroscopy

Electron microphysics at plasma-solid interfaces

Infrared spectroscopy of surface charges in plasma-facing dielectrics.