Plasma columns sustained by electromagnetic (EM) surface waves (SWs) not only yield stable and unambiguously reproducible gaseous discharges, but can also provide them over an unrivaled wide range of operating conditions. It means that the operating conditions of most existing radio-frequency (RF) and microwave discharges can be matched over their specific range by those of surface-wave discharges (SWDs). Such a unique attribute of SWDs allows parametric studies, i.e. the possibility of maintaining all but one of the operating conditions constant and scrutinizing the variation of that sole condition over a large enough range, such as to determine the actual influence of any given operating condition. The concept of power absorbed/lost per electron , which was initially introduced based on SWD properties, is herein developed to reveal new insightful aspects of the power balance, which can then be extended to all kinds of discharges. This approach emerged from the way that the EM SW expends its power flow P ( z ) as it propagates: it gets absorbed as d P ( z )/d z in the gas (plasma) slab delineated between the differential position z and z + d z , inducing, in that same axial interval (and not outside it), a corresponding electron density in the range of n , n + d n . In other words, the wave power is spent locally within the differential axial segments in which it is absorbed; because the plasma radius is most generally small with respect to the column length, the various phenomena occurring along the radius of the plasma column can be considered non-local and analyzed through radial averages. This distinct behavior of SWDs suggested setting θ _A as the wave power absorbed per electron on average in a radial cross-section between z and z + d z from the EM E -field sustaining plasma, and θ _L , the power lost in the discharge, expressed on a per electron basis, within the same corresponding axial segment. The power per electron quantities provide insight into the physical properties of DC, RF, and microwave discharges previously unknown, ignored, or misunderstood. Some corresponding examples are presented herein: (i) consider the case of the power absorbed from the EM E -field, θ A ( E ) = e 2 E 2 ¯ ν / [ m e ( ν 2 + ω 2 ) ] , where ν is the electron collision frequency for the momentum transfer, ω , the wave angular frequency, e / m _e , the electron charge to mass ratio, and E 2 ¯ , the mean squared value of the EM E - field. The θ _A value is shown to adjust so as to compensate exactly for θ _L , which is thus the dominant power parameter; as a result, the intensity of the maintenance E - field sustaining the discharge comes out as an internal parameter, i.e. it is operator independent, in contrast to what is generally believed whatever the kind of E -field sustained discharges; (ii) the smaller the volume in which power is absorbed with respect to the volume in which it is spent, the higher the intensity of the maintenance E - field: this leads to higher atomic (molecular) excitation rates inside than outside the absorption region (such is the case in microdischarges); (iii) the value of θ _L varies significantly with the SW field frequency, decreasing with increasing frequency from the RF domain to that of microwaves; (iv) the power lost θ _L , which is the power cost to sustain an electron–ion pair in an SWD, can serve to evaluate the efficiency of the various discharge regimes (diffusion and volume recombination with either single or step-wise ionisation); (v) the EM E - field intensity under the electron cyclotron resonance (ECR) condition passes through a minimum, not a maximum, contrary to what is generally claimed; (vi) similarity laws, originally derived with DC discharges, are generalized to include RF and microwave discharges. For example , θ _A / p as a function of pR ( p is the gas pressure and R the discharge tube inner radius) replaces advantageously the widely used E / p versus pR similarity law since θ _A is more easily measured than E ² and further it prevents considering the latter as an external parameter, etc. The important EM power loss due to space-wave radiation in the immediate vicinity of the SW launching interstice has escaped the attention of those working with SWDs. This effect is documented experimentally, showing in addition that it can be significantly reduced by using a cylindrical conducting enclosure, coaxial with the discharge tube, having a small enough radius such that it acts as a circular waveguide at cut-off. A survey limited to underlining specific properties and innovative features of SWDs other than plasma columns is conducted, which include planar SWDs and microwave discharges of the torch type sustained at atmospheric pressure, for instance, with the TIA and TIAGO E- field applicators: it is shown that their flame actually results from an SWD.

Results of chemical kinetics modeling in methane subjected to the microwave plasma at atmospheric pressure are presented in this paper. The reaction mechanism is based on the methane oxidation model without reactions involving nitrogen and oxygen. For the numerical calculations 0D and 1D models were created. 0D model uses Calorimetric Bomb Reactor whereas 1D model is constructed either as Plug Flow Reactor or as a chain of Plug Flow Reactor and Calorimetric Bomb Reactor. Both models explain experimental results and show the most important reactions responsible for the methane conversion and production of H₂, C₂H₂, C₂H₄ and C₂H₆ detected in the experiment. Main conclusion is that the chemical reactions in our experiment proceed by a thermal process and the products can be defined by considering thermodynamic equilibrium. Temperature characterizing the methane pyrolysis is 1,500–2,000 K, but plasma temperature is in the range of 4,000–5,700 K, which means that methane pyrolysis process is occurring outside the plasma region in the swirl gas flowing around the plasma.

The propagation characteristics of microwave frequency surface waves along atmospheric pressure plasma columns sustained in capillary tubes by these same waves are studied analytically and numerically. In contrast to what has been done to date in such a pressure range, our analysis includes the modelling of the discharge processes in order to account for their interdependence with the wave's propagation. To describe these processes and to determine the plasma parameters, we turn to a two-temperature model for non-equilibrium plasmas. The analysis of the wave dispersion is performed using a cold plasma, linear dielectric description; it assumes the local uniformity approximation, which is scrutinized. Also, we assume that the phase and attenuation coefficients of the wave, under given discharge conditions, depend only on the amount of power dissipated in the plasma per unit length. The obtained results are compared with known experimental data.

The characteristics of electromagnetic waves propagating along dense plasma filaments, as encountered in atmospheric pressure discharges, are examined in the microwave frequency range; they turn out to be surface waves. Results of numerical calculations of the dependence of the phase and attenuation coefficients on the plasma parameters are presented. In the limit of large electron densities, this guided wave is akin to a Sommerfeld wave and the propagation can be described in an analytical form.

Three-dimensional electric field distributions in a waveguide-based microwave plasma source (MPS) have been determined numerically. Tuning characteristics of the MPS have been calculated using a direct and newly proposed two-port network method. A method for easy assessment of the quality of a set of the tuning characteristics is presented. Optimization of the plasma source has been performed using these characteristics to ensure good power coupling and stability of the plasma source operation. The calculated tuning characteristics have been compared with experimental ones. Good agreement has been found.