Microwave and RF plasmas are finding increasing use in materials processing, plasma chemistry, chemical analysis, and other fields. This is stimulating the search for suitable plasma sources. In the 1970s, electromagnetic surface waves were put to use to sustain plasmas and an efficient microwave device, called a surfatron, was developed for this purpose. Recent work has shown that such discharges can also operate at radio frequencies. A large number of on surface-wave plasmas experimental data have been accumulated-their modelling is well advanced and they have found applications in various fields of research and technology. This paper reviews the physical principles of operation and the design of surface-wave plasma sources. Since the wave launcher is the central component of the source, this review presents a unified description of several compact, efficient, and easy to operate launchers specifically intended for plasma generation that have been developed over the past fifteen years. It is now possible to sustain such plasmas at frequencies ranging from 1 MHz to 10 GHz, in a pressure domain extending from 10-5 Torr up to few times atmospheric pressure, and in a rich variety of plasma vessels and reaction chambers.

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Plasma sources based on the propagation of electromagnetic surface waves

A self-consistent kinetic model is developed to study dc flowing glow discharges in N/sub 2//O/sub 2/ mixtures. This model includes the calculation of electron energy distribution functions and electron rate coefficients coupled with detailed vibrational kinetics of N/sub 2/ molecules, chemical kinetics taking into account a large set of neutral, excited and charged species, interaction of N and O atoms at the discharge tube wall, and the thermal balance of the discharge. The results of this model agree reasonably well with the measurements of the electronic density, the gas temperature, the reduced electric field, the vibrational temperature of N/sub 2/ and the concentration of O, N atoms, NO molecules, N/sub 2/(C), N/sub 2//sup +/(B), and NO(/spl gamma/) excited states. The comparison was performed in a N/sub 2/-O/sub 2/ discharge at pressure p=2 Torr, for discharge currents I=15, 30, and 80 mA, a flow rate Q=100 sccm, and O/sub 2/ percentages ranging from 0 up to 100%,. >

Kinetic model of a low-pressure N/sub 2/-O/sub 2/ flowing glow discharge

A one‐dimensional fluid simulation of a 13.56 MHz argon glow discharge including metastable species was performed as an example of a coupled glow‐discharge/neutral‐transport‐reaction system. Due to the slow response time of metastables (∼10 ms) direct time integration of the coupled system requires ∼105 rf cycles to converge. This translates to prohibitively long computation time. An ‘‘acceleration’’ scheme was employed using the Newton–Raphson method to speed up convergence, thereby reducing the computation time by orders of magnitude. For a pressure of 1 Torr, metastables were found to play a major role in the discharge despite the fact that their mole fraction was less than 10−5. In particular, metastable (two‐step) ionization was the main mechanism for electron production to sustain the discharge. Bulk electric field and electron energy were lower, and a smaller fraction of power was dissipated in the bulk plasma when compared to the case without metastables. These results suggest that neutral transport and reaction must be considered in a self‐consistent manner in glow discharge simulations, even in noble gas discharges.

Fluid simulations of glow discharges: Effect of metastable atoms in argon

This paper presents a critical review of the basic data concerning the physics and chemistry of low pressure SiH 4 glow discharges used to deposit hydrogenated amorphous silicon films (a-Si:H). Starting with an updated table of thermochemical data, we analyze the gas-phase elementary processes consisting of i) electron-molecule collisions, ii) ion-molecule collisions, iii) neutral-neutral collisions, iv) other electron and ion collisions involving electron-ion and ion-ion recombination, electron attachment on radicals and detachment of anions, and v) cluster growth kinetics in dusty plasmas. Experimental data or theoretical estimates are given and discussed in terms of cross-sections. collision and reaction rate constants, and transport coefficients. We also analyze the surface processes and reaction probabilities of ions, radicals and molecules.

Cross-Sections, Rate Constants and Transport Coefficients in Silane Plasma Chemistry

A review is presented of the studies in the former Soviet Union and in the USA of the mutual interactions of plasmas and high speed flows and shocks. There are reports from as early as the 1980s of large changes in the standoff distance ahead of a blunt body in ballistic tunnels, significantly reduced drag and modifications of travelling shocks in bounded weakly ionized gases. Energy addition to the flow results in an increase in the local sound speed that leads to expected modifications of the flow and changes to the pressure distribution around a vehicle due to the decrease in local Mach number. The critical question was, did a plasma provide a significant energy multiplier for the system? There have been a large number of experimental studies on the influence of a weakly ionized plasma on relatively low Mach number shocks and inherently also on the influence of the shock on the plasma. This literature is reviewed and illustrated with representative examples. The convergence through more controlled experiments and improved modelling to a physics understanding of the effects being essentially due to heating is outlined. It is demonstrated that the heating in many cases is global; however, tailored experiments with positive columns, dielectric barrier discharges and focused microwave plasmas can produce very localized heating. The latter appears more attractive for energy efficiency in flow control. Tailored localized ionization and thermal effects are also of interest for high speed inlet shock control and for producing reliable ignition for short residence time combustors, and work in these areas is also reviewed.

https://iopscience.iop.org/article/10.1088/0022-3727/38/4/R01/pdf

Plasmas in high speed aerodynamics

The contribution of the ionization from the two metastable and the two resonance levels of argon to the total ionization rate in a low‐pressure argon positive column is investigated theoretically and experimentally. A simple yet self‐consistent discharge model is developed by coupling the balance between the electron total production and loss rates to the steady‐state rate balance equations for the excited states. The predictions of this model, i.e., the maintenance field for the positive column and the populations in the individual excited states as a function of the gas pressure and the discharge current are compared with the experiment (0.05<p<1 Torr; 0.2<I<20 mA; φ=2.6‐cm‐i.d. discharge tube). It is shown that stepwise ionization becomes predominant for pressures larger than a few tenths of a Torr even for the lowest currents of this experiment. The predicted values of the maintenance field are considerably lower than those that would be required to sustain the discharge by ground‐state ionization and compare favorably with the measurements. The calculated populations in the metastable states also compare reasonably well with the experiment. The discrepancies are, however, quite large in the case of the resonance levels owing, presumably, to the lack of accuracy of the theoretical escape factors for the resonance radiation.

Populations in the metastable and the resonance levels of argon and stepwise ionization effects in a low‐pressure argon positive column

The electron energy distribution function in N2 and the vibrational distribution function of N2 molecules in the electronic ground state were calculated self-consistently for steady state conditions by solving the Boltzmann equation coupled to a system of rate balance equations for the fibration levels including e-V, V-V, and V-T energy exchange processes. This formulation provides a relationship between the degree of vibrational excitation, as measured by a characteristic vibrational temperature theta , the ratio of the electric field to the gas density E/N, and the degree of ionisation ne/N. The values of theta have only a small dependence on E/N in the range 3*10-16-3*10-15 V cm2 and increase with ne/N from about 2*103K to 6*103K in the range 10-11<or=ne/N<or=10-5. The inclusion of superelastic e-V collisions in the Boltzmann equation strongly affects the electron energy distribution function and results in a significant increase in the electron rate coefficients for excitation especially at the lower E/N values. The authors present calculations of electron transport parameters and excitation rates, of the power transferred by the electrons through the various collisional mechanisms, of the rate of dissociation, and of the net power transferred to the gas translational mode by the V-T and the non-resonance V-V energy exchanges, and by electron collisions.

https://iopscience.iop.org/article/10.1088/0022-3727/19/1/007/pdf

Coupled electron energy and vibrational distribution functions in stationary N2 discharges

A theoretical analysis of the characteristics of high-frequency and direct-current argon discharges at low pressures is carried out and a comparison of the basic properties of these discharges is presented. This analysis uses recent calculations of electron energy distributions and collision rate coefficients in argon under the action of uniform AC and DC electric fields together with a steady-state discharge model expressing the balance between collisional ionisation of the gas and the loss of electrons to the wall. The electric field strength, E, required for the steady-state operation of planar and cylindrical discharges is calculated as a function of the gas density N, the angular frequency omega , and the diffusion length for the discharge tube Lambda , assuming predominant direct ionisation.

https://iopscience.iop.org/article/10.1088/0022-3727/17/6/014/pdf

Characteristics of high-frequency and direct-current argon discharges at low pressures: a comparative analysis

The electron energy distribution function in H2 and the vibrational distribution function of H2(X, v) molecules have been self-consistently calculated for the typical operating conditions of a low-pressure, moderate current, hydrogen positive column by solving the Boltzmann equation together with a system of rate balance equations for the vibrational levels. This system takes into account e-V, V-V and V-T processes, the latter including both intermolecular and molecule-atom processes. The formulation provides a relationship between the characteristic vibrational temperature, Tv, the degree of ionisation, delta e, the fractional concentration of dissociated atoms, delta a, and the ratio of the electric field to the gas density, E/N. At low E/N the superelastic e-V collisions strongly enhance the high-energy tail of the electron energy distribution which results in a significant increase in the electron excitation rates, especially in the rate of dissociation by electron impact. The authors present calculations of electron transport parameters and excitation rates, of the power transfer by the electrons via the various collisional mechanisms, of the total power transferred to the translational and rotational modes by electron collisions and V-T and non-resonant V-V processes, and of the total dissociation rate.

https://iopscience.iop.org/article/10.1088/0022-3727/22/11/017/pdf

Electron and vibrational kinetics in the hydrogen positive column

The populations of the N2(A 3 Sigma u+, nu =0-5) states in a pure nitrogen glow discharge have been determined from measurements of the absorption of the first positive system bands for gas pressures in the range 0.1-2 Torr and discharge current densities up to about 30 mA cm-2. The observed vibrational distributions are highly non-Boltzmann and an inversion of population between the levels nu =2 and nu =3 was found. The total population in the nu =0-5 levels saturates with increasing current, the saturated population densities ranging from about 3 to 6*1012 cm-3, depending on pressure. An interpretation of the experimental data is given using a kinetic model which includes excitation of individual N2(A, nu ) vibrational levels by electron impact with ground-state molecules and by cascading from the B 3 Pi g to the A state, diffusion to the wall, quenching by nitrogen atoms, metastable-metastable energy pooling, vibration-vibration energy transfer between N2(A) and N2(X) molecules, and collisionally induced transitions between the B 3 Pi g and A 3 Sigma u+ states. The predictions are in good agreement with the measurements. However the observed inversion of population between the nu =2 and nu =3 levels cannot be explained by the above mechanisms. It is suggested that is due to vibrationally dependent quenching.

https://iopscience.iop.org/article/10.1088/0022-3700/17/21/030/pdf

C M Ferreira

Papers

Populations in the metastable and the resonance levels of argon and stepwise ionization effects in a low‐pressure argon positive column

Coupled electron energy and vibrational distribution functions in stationary N2 discharges

Characteristics of high-frequency and direct-current argon discharges at low pressures: a comparative analysis

Electron and vibrational kinetics in the hydrogen positive column

Vibrational populations of N2(A 3Σu+) in a pure nitrogen glow discharge