Parameters of microwave discharge plasmas in powder mixtures
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MPERATURE PLASMAS
Parameters of Microwave Discharge Plasmas in Powder Mixtures G. M. Batanov, N. K. Berezhetskaya, V. A. Kop’ev, I. A. Kossyi, and A. N. Magunov Prokhorov Institute of General Physics, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia Received July 3, 2007
Abstract—A study is made of the optical emission spectra of a plasma produced under the action of a microwave beam with a field amplitude of about 3 kV/cm on a metal–dielectric mixture at atmospheric pressure. In the initial stage of the discharge, the plasma is localized in microscale sites at the target surface, the characteristic size of the glowing regions in the target plane being less than 1 mm. In this stage, the target material is evaporated and atomic spectral lines with excitation energies of up to 6–8 eV are emitted. The population temperature of the excited levels, which was determined from the intensity ratios of atomic spectral lines, can be interpreted as the temperature of the atmospheric-pressure plasma. The temperatures determined from different pairs of lines belonging to the same or different elements (Cu, Mo, Li, Fe) fall within the range 7000–9000 K. In the later stage of the discharge, when the characteristic plasma size reaches a few centimeters, no atomic lines are present in the spectrum, which corresponds to a Planckian continuum with a temperature of 2400–3200 K. PACS numbers: 52.50.Sw, 52.70.Kz, 78.70.Gg DOI: 10.1134/S1063780X08040090
1. INTRODUCTION The experiments on the irradiation of powder mixtures with short (microsecond) microwave pulses in atmospheric-pressure air have shown that the development of a low-threshold microwave breakdown in heterogeneous metal–dielectric composites begins with the generation of optical emission, whose spectrum consists of metal atomic lines [1]. Sources of optical emission from mixtures irradiated with short microwave pulses are microscale sites (sparks) randomly distributed over the target surface. The emission spectra and parameters of surface sparks arising under the action of microwave field have not yet been studied quantitatively. The low threshold of a microwave breakdown (the threshold intensity is about 10 kW/cm2) at a metal– dielectric contact has been used to initiate discharges since the 1970s; however, the mechanism for the local energy release, which is responsible for the appearance of free metal atoms in a gas and for the population of their excited levels, is still unclear. For a metal to be rapidly evaporated, it is necessary that its temperature reach the boiling point (above 4000 K for refractory metals, such as Mo and W), and even a higher temperature is required to populate levels with excitation energies ε ≈ 6–8 eV. Powder mixtures irradiated with 3- to 5-ms microwave pulses emit intense optical radiation with a continuous Planckian spectrum corresponding to a temperature of 2000–3000 K [2]. The radiation source is the plasma filling the space between powder grains. The
size of the glowing region in the target plane is comparable with the
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