Electrons In Low-temperature Plasma Research Articles

Electron kinetics in low-temperature plasmas

This article presents an overview of recent advances in the field of electron kinetics in low-temperature plasmas (LTPs). It also provides author's views on where the field is headed and suggests promising strategies for further development. The authors have selected several problems to illustrate multidisciplinary nature of the subject (space and laboratory plasma, collisionless and collisional plasmas, and low-pressure and high-pressure discharges) and to illustrate how cross-disciplinary research efforts could enable further progress. Nonlocal electron kinetics and nonlocal electrodynamics in low-pressure rf plasmas resemble collisionless effects in space plasma and hot plasma effects in fusion science, terahertz technology, and plasmonics. The formation of electron groups in dc and rf discharges has much in common with three groups of electrons (core, strahl, and halo) in solar wind. Runaway electrons in LTPs are responsible for a wide range of physical phenomena from nano- and picoscale breakdown of dielectrics to lightning initiation. Understanding electron kinetics of LTPs could promote scientific advances in a number of topics in plasma physics and accelerate modern plasma technologies.

Distribution of the potential and concentration of electrons in low-temperature plasma with hollow microparticles

Using approximation of a uniform background (the jellium model) for a condensed dispersed phase, the analytical expressions describing a spatial distribution of the potential of the electric field and electron concentration in the low-temperature plasma at equilibrium which contains hollow spherical microparticles are obtained. The influence of heating temperature of plasma on the above distributions is studied, and the dependencies of the charge on microparticle radius, the size of the microparticle cavity and the absolute temperature of plasma are calculated. It is shown that electrons can be emitted not only into the surrounding plasma but also into the cavity of the particles.

Experimental investigation of electron transport across a magnetic field barrier in electropositive and electronegative plasmas

In this paper we experimentally investigate the drift of electrons in low temperature plasmas containing a magnetic field barrier; a plasma configuration commonly used in gridded negative ion sources. A planar Langmuir probe array is developed to quantify the drift of electrons over the cross-section of the ion-extraction region of an ion–ion plasma source. The drift is studied as a function of pressure using both electropositive plasmas (Ar), as well electronegative plasmas (Ar and SF6 mixtures), and is demonstrated to result from an interaction of the applied magnetic field and the electric fields in the sheath and pre-sheath near the transverse boundaries. The drift enhances electron transport across the magnetic field by more than two orders of magnitude compared with simple collisional transport, and is found to be strongly dependant on pressure. The lowest pressure resulted in the highest influence of the drift across the extraction area and is found to be 30%.

Binary and ternary recombination of and ions with electrons in low temperature plasma

Measurements of recombination rate coefficients of binary and ternary recombination of and ions with electrons in a low temperature plasma are described. The experiments were carried out in the afterglow plasma in helium with a small admixture of Ar and parent gas (H2 or D2). For both ions a linear increase of measured apparent binary recombination rate coefficients (αeff) with increasing helium density was observed: αeff = αBIN + K He[He]. From the measured dependencies, we have obtained for both ions the binary (αBIN) and the ternary (K He) rate coefficients and their temperature dependence. For the description of observed ternary recombination a mechanism with two subsequent rate determining steps is proposed. In the first step, in + e− (or + e−) collision, a rotationally excited long-lived Rydberg molecule (or ) is formed. In the following step (or ) collides with a He atom of the buffer gas and this collision prevents autoionization of (or ). Lifetimes of the formed (or ) and corresponding ternary recombination rate coefficients have been calculated. The theoretical and measured binary and ternary recombination rate coefficients obtained for and ions are in good agreement.

Application of NIR – CRDS for state selective study of recombination of para and ortho H3+ ions with electrons in low temperature plasma

We present a study of H3+ recombination performed at 77 K on the two lowest rotational levels of this ion, which belong to its two different nuclear spin states of the studied ion. A near infrared cavity ring-down spectrometer (~1381 nm, CRDS arrangement) has been used to obtain the time evolution of concentration of both states. From the overall ion density decay during the afterglow we obtained the binary recombination rate coefficient αbin (77 K) = 1.2×10−7 cm3s−1. We have also observed ternary helium assisted recombination of both para and ortho H3+. The process is very slow (at 77 K) and the obtained ternary recombination rate coefficient is in contradiction with the theoretical prediction. It is the first time that the binary and ternary H3+ recombination rate coefficient was measured at a known population of para and ortho H3+ ions in decaying plasma.

Recombination of KrH+ and XeH+ ions with electrons in low temperature plasma

Reported is Flowing Afterglow study of recombination of KrH+ and XeH+ ions with electrons at low temperature. In the experiment the plasma was created in a microwave discharge in the upstream part of the flow tube in a fast flow of helium carrier gas at a pressure ∼ 1 600 Pa. Further downstream He-Kr-H2 and He-Ar-Xe-H2 were added to form KrH+ and XeH+ ions, respectively. The electron number density along the flow tube was monitored by an axially movable Langmuir probe. The corresponding recombination rate coefficients were obtained from the rate of plasma decay during the KrH+ (or XeH+) dominated afterglow. The kinetic model was used to optimize dominance of KrH+ and XeH+ ions and to minimize the formation of H+3 ions and fast recombining cluster ions. Since the proton affinity of krypton is only 0.02 eV greater than that of H2, the partial pressures of these reactants have to be adjusted very carefully to obtain plasma with a maximal population of KrH+. Obtained rate coefficients for recombination of KrH+ and XeH+ ions with electrons at 250 K are 2 × 10−8 cm3s−1 and 8 × 10−8 cm3s−1, respectively.

Temporal and spatial relaxation of electrons in low temperature plasmas

A brief introduction into the large importance of relaxation processes in the electron gas for the understanding of various transient processes in low temperature plasmas is given. Based on comprehensive, strictly non-hydrodynamic studies on the electron relaxation by using kinetic approaches the large variety of the temporal and spatial electron relaxation in atomic and molecular plasmas is elaborated and demonstrated under different plasma conditions. The analysis has been performed particularly with respect to the relaxation behavior of the electron energy distribution function and the associated energy and momentum dissipation in electron collisions as well as the resultant characteristic overall relaxation times and relaxation lengths. Especially the temporal and spatial relaxation, respectively, is considered under field-free conditions in decaying plasmas, under the action of constant electric fields leading to the formation of steady states and under the action of temporally or spatially periodic fields. Moreover, the complex spatiotemporal transient behavior of the electron gas into a spatially structured, time-independent state is considered. To simplify the representation, all results on the temporal and spatial relaxation have been deduced using corresponding two-term approximations, but could be performed as well with available multi-term approaches without effecting significant changes in the deduced basic relaxation characteristics.

Theory of negative differential conductivity of electrons caused by electron–electron scattering in low-temperature plasma

A simplified quantitative analysis of the mechanism of negative differential conductivity (NDC) of electrons in low-temperature plasma due to electron–electron scattering (EES) is presented for the first time. On the basis of an analytical treatment of the Boltzmann equation, it is shown that with the constraint νε≪νee≪νm the necessary condition for displaying of the N- and S-type NDC induced by EES can be written as ν̂mU=δ̂U>0.5 and <−0.5, respectively, where νε=νε(U), νee=νee(U), and νm=νm(U) are the total electron energy exchange, effective electron–electron, and momentum transfer collision frequencies, respectively [U is the electron mean energy, δ=(E/N)/W, where E/N is the reduced electric field (E=E(U) is the intensity of the electric field, N is the gas number density), W=W(U) is the drift velocity of electrons, and ŷx=d ln y/d ln x]. Simple analytical criteria for prediction of the EES induced NDC are obtained. It is shown that the EES induced NDC may be responsible for triggering of a broad category of spatial and temporal instabilities taking place in various gases and gas mixtures in glow-discharge plasma. The validity of the proposed theory is confirmed by comparison with numerous experimental and numerical works of other authors.

The effect of chemical reactions on the velocity distribution of electrons in low-temperature plasma

The effect of chemical reactions on the velocity distribution of electrons in low-temperature plasma