An investigation of electromagnetic wave propagation in plasma by shock tube

Abstract

This paper presents the electromagnetic wave propagation characteristics in plasma and the attenuation coefficients of the microwave in terms of the parametersn _e ,v, ω,L, ω _b . The ϕ800 mm high temperature shock tube has been used to produce a uniform plasma. In order to get the attenuation of the electromagnetic wave through the plasma behind a shock wave, the microwave transmission has been used to measure the relative change of the wave power. The working frequency isf=(2∼35) GHz (ω=2πf, wave length λ=15 cm ∼ 8 mm). The electron density in the plasma isn _e =(3×10¹⁰∼1×10¹⁴) cm⁻³. The collision frequencyv=(1×10⁸∼6×10¹⁰) Hz. The thickness of the plasma layerL=(2∼80) cm. The electron circular frequency ω _b =eB ₀/m _e, magnetic flux densityB ₀=(0∼0.84) T. The experimental results show that when the plasma layer is thick (such asL/λ ≥10), the correlation between the attenuation coefficients of the electromagnetic waves and the parametersn _e ,v, ω,L determined from the measurements are in good agreement with the theoretical predictions of electromagnetic wave propagations in the uniform infinite plasma. When the plasma layer is thin (such as when bothL and λ are of the same order), the theoretical results are only in a qualitative agreement with the experimental observations in the present parameter range, but the formula of the electromagnetic wave propagation theory in an uniform infinite plasma can not be used for quantitative computations of the correlation between the attenuation coefficients and the parametersn _e ,v, ω,L. In fact, if ω < ω _p ,v ² ≪ ω², the power attenuationsK of the electromagnetic waves obtained from the measurements in the thin-layer plasma are much smaller than those of the theoretical predictions. On the other hand, if ω > ω_p,v ² ≪ ω² (justv≈f), the measurements are much larger than the theoretical results. Also, we have measured the electromagnetic wave power attenuation value under the magnetic field and without a magnetic field. The result indicates that the value measured under the magnetic field shows a distinct improvement.