Microwave discharges: generation and diagnostics

Abstract

Microwave discharges are widely used for generation of quasi-equilibrium and nonequilibrium plasma for different applications. Microwave plasma can be generated at pressures from 10-5 Torr up to atmospheric pressure in the pulse and continuum wave regimes at incident powers ranged between several Watts and hundreds of kW. The plasma absorbed power can be high enough and runs up to 90% of the incident power. Plasma density usually exceeds the critical n ec (n ec [cm-3 ]≈1,24•10 10 f 2 [GHz]). The used wavelengths of microwaves are in the range from millimeters up to several tens of centimeters and should correspond to permitted microwave frequencies for industrial, medical and scientific applications. The frequency 2.45 GHz is the most commonly used. Main advantages of microwave plasma are summarized. Methods of microwave plasma generation are briefly reviewed (microwave waveguide plasma generators, microwave cavity plasma generators, plasma generators with plasma waves, slow wave plasma generators, plasma generators with distributed energy input, wave beam produced plasmas, electrode microwave plasma generators, initiated microwave plasmas, combined microwave and other frequencies sources plasma generators, microwave plasmas with magnetic fields). Non-uniformity is the inherent feature of microwave plasma and this fact should be taken into account both in plasma diagnostics and in design of microwave-to-plasma applicators for specific targets. Influences of external discharge parameters on plasma uniformity are illustrated. The plasma non-uniformity defines the theoretical approaches used for processing of diagnostic data to obtain the plasma parameters. Examples of selforganization of microwave unbounded plasma is also presented. Selected aspects of microwave plasma diagnostics by means of the probe technique, emission optical spectroscopy and high speed cameras are discussed. Peculiarities of probe measurements in microwave plasma are analyzed. Examples of results of microwave plasma diagnostics are presented together with results of modeling. Comprehensive information on microwave discharges is contained in the Proceedings of the periodical International Workshop on Microwave Discharges: Fundamentals and Applications which is held since 1992.

Measurement of atomic oxygen in the middle atmosphere using solid electrolyte sensors and catalytic probes

“…The PHLUX sensors were calibrated for the incident heat flux density using the NISI (non-integer system identification) method (Löhle et al, 2013(Löhle et al, , 2007. A microwave generator (2.45 GHz, Sairem, France) is used to produce a low-pressure oxygen plasma as a source of atomic oxygen, a technique widely employed in various technical and scientific applications and well studied in the literature (Lebedev, 2010). The pure-oxygen plasma is sustained within a cylindrical quartz tube at an oxygen pressure of 1.0 mbar and an incident microwave power of up to 300 W. The tube has a length of 30 cm with an outer diameter of 50 mm and a wall thickness of 2.5 mm and is mounted onto a vacuum chamber.…”

Understanding Microwave Surface‐Wave Sustained Plasmas at Intermediate Pressure by 2D Modeling and Experiments

“…Microwave (MW) induced discharges are well known for their high degree of non‐equilibrium ( T e >> T gas ), the high level of applied energy absorption by plasma electrons, and relatively easy operation . Based on the configuration reactors and the way of coupling the MW energy into the plasma, MW discharges can be divided into two general types: localized, that is, plasma created in resonant cavity reactors and plasma torches; and traveling‐wave discharges, that is, surface‐wave (SW) plasma columns .…”

“…The first application of EM surface waves in the MW frequency range for sustaining plasma was developed by Moisan et al in 1974 . Since then SW sustained discharges attracted significant attention due to their flexibility in terms of continuous or pulsed operation regimes, in a wide range of gas pressure (10 −2 –10 5 Pa), applied frequency (300 MHz–10 GHz), and geometry size and shape . A considerable amount of research has been done in a number of groups leading to a vast number of publications (cf.…”

“…Examples of SW plasma generators are the surfatron, and the surfaguide launchers . We have applied a surfaguide sustained discharge at intermediate pressure for plasma‐catalytic greenhouse gas conversion into valuable chemicals .…”

“…[1][2][3][4] Based on the configuration reactors and the way of coupling the MW energy into the plasma, MW discharges can be divided into two general types: localized, that is, plasma created in resonant cavity reactors and plasma torches; and traveling-wave discharges, that is, surface-wave (SW) plasma columns. [1][2][3][4] They find application in a wide range of technology fields such as surface treatment, [5,6] material processing and (nanoparticle) synthesis, [7][8][9] plasma diagnostics, [10] plasma medicine (sterilization, skin treatment), [11][12][13][14][15][16] in atomic and molecular spectrometry, [17,18] and in dissociation of greenhouse gases. [19][20][21] A SW sustained plasma, which is the topic of the present research, is created by electromagnetic (EM) waves propagating along the boundary between the plasma and a dielectric (usually a quartz tube).…”