In this experimental research, the effect of electrical waveforms including direct current (DC), sinusoidal, sawtooth, triangular, modified sinusoidal and square wave on the plasma electron density and collision frequency of the positive column of low-pressure gas discharge (PAr = 10 Torr, PHg = 6 × 10−3 Torr, R = 18 mm, Irms = 400 mA) was examined. Time-resolved plasma electron density and collision frequency were measured by fast time-resolved microwave interferometry with simultaneous current and voltage measurements. In the case of direct current excitation, the striation with 800 Hz fluctuation was observed. In a sine wave, the striation was observed at 50 Hz while it was disappeared at 5 kHz. In the square waveform, the collision frequency that keeps the plasma density constant could be controlled by tuning the excitation frequency. It was also found that a rapid change in the electrical current signal, like the sawtooth waveform, causes a total reduction in the electron density and collision frequency. Studying the rectangular pulse periods, showed that the maximum density and collision frequency, as well as their evolutions, could be controlled. Regarding the obtained results, it was concluded that one could effectively control (increase or reduce) the electron density and collision frequency with respect to the critical time intervals for practical applications such as plasma antennas and plasma RCS reduction.
In this experimental research, the effect of tunable low-pressure gas discharge plasma was examined on radar cross -section (RCS) reduction. A quintet array of plasma tubes (P Ar = 10 torr, P Hg ≈ 2-6 × 10 −3 torr, R in = 18 mm) next to each other was used to reduce the RCS of a planar metallic target. Plasma columns were optimized to have minimum axial and temporal inhomogeneity. The plasma electron density, plasma frequency, and collision frequency of the plasma tubes were measured by fast time-resolved microwave interferometry with simultaneous measurement of current and voltage. The influence of the discharge current (I rms = 50-1500 mA) as a regulator of the plasma parameters was investigated. RCS reduction changes were observed by tuning plasma parameters at a frequency range of 2-18 GHz for the normal and oblique incidence of TE and TM polarizations. The assembled plasma tubes depicted broadband RCS reduction (<−10 dB) in TM polarization for a 300 mA discharge current (42.5% total bandwidth) and in TE polarization for a 1400 mA discharge current (34% total bandwidth) for normal incidence. It was shown that the measurement is in good agreement with the simulation. It was concluded that by performing bistatic measurements, the plasma cylinders behave as a wave absorber and scattering object in the TE and TM polarizations, respectively.
On the basis of kinetic theory along with self-consistent field equations, the expressions for dielectric tensor of radially inhomogeneous magnetized plasma columns are obtained. The study of dielectric tensor characteristics allows the accurate analysis of the inhomogeneous properties, beyond limitations that exist in the conventional method. Through the Bessel-Fourier transformation, the localized form of material equations in a radially inhomogeneous medium are obtained. In order to verify the integrity of the model and reveal the effect of inhomogeneity, a special case of a cylindrical plasma waveguide completely filled with inhomogeneous magnetized cold plasma was considered. The dispersion relation curves for four families of electromagnetic (EH and HE) and electrostatic (SC and C) modes are obtained and compared with the findings of the conventional model. The numerical analysis indicates that the inhomogeneity effect leads to coupling of electromagnetic and electrostatic modes each having different radial eigen numbers. The study also reveals that the electrostatic modes are more sensitive to inhomogeneous effects than the electromagnetic modes.
The damping decrement of Landau damping and the effect of thermal velocity on the frequency spectrum of a propagating wave in a bounded plasma column are investigated. The magnetized plasma column partially filling a cylindrical metallic tube is considered to be collisionless and non-degenerate. The Landau damping is due to the thermal motion of charge carriers and appears whenever the phase velocity of the plasma waves exceeds the thermal velocity of carriers. The analysis is based on a self-consistent kinetic theory and the solutions of the wave equation in a cylindrical plasma waveguide are presented using Vlasov and Maxwell equations. The hybrid mode dispersion equation for the cylindrical plasma waveguide is obtained through the application of appropriate boundary conditions to the plasma-vacuum interface. The dependence of Landau damping on plasma parameters and the effects of the metallic tube boundary on the dispersion characteristics of plasma and waveguide modes are investigated in detail through numerical calculations.
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