Surface microwave discharges in air
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EMPERATURE PLASMA
Surface Microwave Discharges in Air V. M. Shibkov, S. A. Dvinin, A. P. Ershov, R. S. Konstantinovskiœ, O. S. Surkont, V. A. Chernikov, and L. V. Shibkova Moscow State University, Vorob’evy gory, Moscow, 119992 Russia Received December 5, 2005; in final form, April 3, 2006
Abstract—A microwave discharge excited on the outer surface of a dielectric antenna has been investigated. The transverse and longitudinal dimensions and propagation velocities of the discharge have been measured as functions of the air pressure and the power and duration of the exciting microwave pulse. The spatial distributions and time evolution of the gas temperature, electron density, and radiation intensity of the discharge have been determined. It is shown that the degree of ionization of the discharge plasma can exceed 10%. The spatial distribution of the electron density is found to depend strongly on the air pressure. PACS numbers: 52.80.-s, 52.40.Fd, 52.50.Dg, 51.50.+v DOI: 10.1134/S1063780X07010096
1. INTRODUCTION It was shown in [1–6] that, when a microwave discharge is excited inside a gas-filled dielectric tube, the electromagnetic energy input into the system transforms into a surface wave. The wave propagates while its energy is sufficient to produce a plasma with the electron density of no lower than the critical density nec = m(ω2 + ν2)/(4πe2), where e and m are the charge and mass of an electron, ω is the microwave circular frequency, and ν is the collision frequency of electrons with neutral gas molecules. In the region where the electron density is lower than nec, the wave does not propagate and the surface discharge is not excited. This method, which has been thoroughly analyzed, underlies the operation of devices called surfatrons and has found wide application, e.g., in plasma chemistry. In [7–9], a new type of microwave discharge excited by a wave propagating along a dielectric surface was considered. This is a self-consistent system in the form of a plasma waveguide produced by a surface wave propagating along this waveguide; i.e., the plasma so produced is required for the very existence of the surface wave. In this way, a fairly long plasma region of various shapes can be produced in a wide range of the microwave frequency and power. A specific feature of this type of discharge is that it is capable of producing a large-size plasma with a fairly high density because the discharge can exist only at electron densities ne higher than the critical density nec. It was suggested that this type of discharge be used in supersonic plasma aerodynamics at air pressures as high as p = 50–300 Torr [10–16]. In this case, the pulsed microwave power needed for producing a surface discharge does not exceed 102 kW and the discharge plasma occupies a thin (h ≈ 1 mm) surface layer that entirely or partially covers the antenna surface,
depending on the gas pressure and the duration and power of the microwave pulse. The propagation velocity of the discharge can reach 100 km/s, which is several orders of magnitude higher
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