Research on the method of dual-frequency microwave diagnosis of plasma for solving phase integer ambiguity

In this work,microwaves and terahertz waves have performed a dual-frequency combineddiagnosis in high-temperature,large-scale plasma.According to the attenuation and phase shift of electromagnetic waves in the plasma,the electron density and collision frequency of theplasma can be inversely calculated.However,when the plasma size is large and the electron density is high,the phase shift of the electromagnetic wave is large (multiple times 2π period).Due to the limitations of the test equipment,the true phase shift is difficult to test accurately or to recover reality.That is,there is a problem of phase integer ambiguity.In order to obtain a phase shift of less than 180°,a higher electromagnetic wave frequency (terahertz wave with 890 GHz)is used for diagnosis.However,the attenuation of the terahertz wave diagnosis is too small (less than 0.1 d B),only the electron density can be obtained,and the collision frequency cannot be accurately obtained.Therefore,a combined diagnosis was carried out by combining twofrequencies (microwave with 36 GHz,terahertz wave with 890 GHz) to obtain electron density and collision frequency.The diagnosis result shows that the electron density is in the range of(0.65–1.5)×10~(19)m~(-3),the collision frequency is in the range of 0.65–2 GHz,and the diagnostic accuracy is about 60%.     

Microwave method based on curve fitting method for high-precision collision frequency diagnosis

In this work, the results of plasma microwave transmission diagnosis were analyzed. According to the attenuation and phase shift of the electromagnetic wave propagating in the plasma, the electron density and collision frequency of the plasma can be diagnosed. Since part of the electromagnetic wave is reflected or diffracted when propagating in the plasma, and is not absorbed by the plasma, and this part of the attenuation is still included in the measured attenuation, the attenuation is distorted. Therefore, a curve fitting method is proposed to remove the attenuation caused by the plasma reflection, thereby improving the accuracy of the diagnosis of the collision frequency. The calibration effect of this method on plasmas with different electron densities and collision frequencies is analyzed, and a diagnostic frequency band with good calibration results is given. The curve fitting method is verified by experiment and simulation. After adopting the newly proposed method, the diagnosis accuracy of collision frequency can be increased by 30%. This method can be widely used in various types of plasma diagnosis and provides a new idea for plasma diagnosis.     

Experimental Studies of Microwave Reflection and Attenuation by Plasmas Produced by Burning Chemicals in Atmosphere

A series of chemicals are designed and prepared. With the method of thermodynamics, the average electron densities of the plasmas generated by burning chemicals are calculated. The reflection and attenuation of the microwaves, in a frequency band of 2 GHz to 15 GHz, by the plasma are measured. The results of measurements indicate that the plasma can absorb the energies of the microwaves in a broad band and reflect them faintly. Moreover, theoretical discussion reveals that the electron-neutral collision is the major factor that results in the absorption in the wide band. By using Appleton equations, average collision frequencies and electron densities are calculated from the attenuations of microwaves.     

Wave Propagation in a Coaxial Glow Discharge

The propagation of electromagnetic radiation from 500 to 4200 Mc in a coaxial cage transmission line passing through a cylindrical discharge chamber is investigated experimentally. Measurements were conducted in argon, helium, and nitrogen gas at pressures between 0.09 mm Hg and 2 mm Hg for a discharge-electromagnetic radiation interaction length of 30.48 cm. The results are compared with theory using a Lorentzian model to describe the electron motion in the presence of the RF field. A graphical method for determining the plasma frequency and electron momentum transfer collision frequency from measurements of the power transmission coefficient at two different radio frequencies is discussed. A comparison is made between the plasma frequency determined by this method and that obtained by the use of a cylindrical Langmuir probe. The electron momentum transfer collision probability is calculated from the RF and probe data.     

Experimental Investigation on Electromagnetic Attenuation by Low Pressure Radio-Frequency Plasma for Cavity Structure

This paper reports on an experiment designed to test electromagnetic(EM)attenuation by radio-frequency(RF)plasma for cavity structures.A plasma reactor,in the shape of a hollow cylinder,filled with argon gas at low pressure,driven by a RF power source,was produced by wave-transmitting material.The detailed attenuations of EM waves were investigated under different conditions:the incident frequency is 1-4 GHz,the RF power supply is 13.56 MHz and1.6~(-3) k W,and the argon pressure is 75-200 Pa.The experimental results indicate that 5-15 d B return loss can be obtained.From a first estimation,the electron density in the experiment is approximately(1.5-2.2)×1016m~(-3)and the collision frequency is about 11~(-3)0 GHz.The return loss of EM waves was calculated using a finite-difference time-domain(FDTD)method and it was found that it has a similar development with measurement.It can be confirmed that RF plasma is useful in the stealth of cavity structures such as jet-engine inlet.     

Impact of the Collisional Plasma on the Propagation of Millimeter Waves

The plasma generated in the low-altitude atmosphere is of high collision frequencies. In this paper, the transmission coefficients of millimeter(MM) waves normally incident upon the plasma with high collision frequencies are calculated and analyzed. The experimental results of reflection and attenuation are presented for the eight-millimeter waves propagating through the plasma. Both the calculated experimental results indicate that the MM-waves concerned are attenuated significantly and reflected weakly, when propagating through the plasma of high collision frequencies.     

Simulation of propagation of the HPM in the low-pressure argon plasma

The propagation of the high-power microwave (HPM) with a frequency of 6 GHz in the low-pressure argon plasma was studied by the method of fluid approximation.The two-dimensional transmission model was built based on the wave equation,the electron drift-diffusion equations and the heavy species transport equations,which were solved by means of COMSOL Multiphysics software.The simulation results showed that the propagation characteristic of the HPM was closely related to the average electron density of the plasma.The attenuation of the transmitted wave increased nonlinearly with the electron density.Specifically,the growth of the attenuation slowed down as the electron density increased uniformly.In addition,the concrete transmission process of the HPM wave in the low-pressure argon plasma was given.     

Propagation Properties of Electromagnetic Wave in a Plasma Slab

In this paper, the calculated results about the propagation properties of electromag-netic wave in a plasma slab are described. The relationship of the propagation properties with frequencies of electromagnetic wave, and parameters of plasma (electron temperature, electron density, dimensionless collision frequency and the size of the plasma slab) is analyzed.     

Microwave Measurements of High Electron Densities Using the Complex Reflection Coefficient

The determination of electron densities in the high density range has become increasingly important in recent years and the use of the complex reflection coefficient at microwave frequencies as a plasma diagnostic tool has been reported by several authors. However, the sensitivity of the reflected phase to the electron density profile composing the boundary of the plasma reduces the effectiveness of the sharply bounded uniform slab as a plasma model. Since this model finds frequent use, the resultant errors should be considered. Indeed, even when the width of the electron-gradient boundary zone is very small, serious error in the determination of the plasma number density can occur when the reflection coefficient is used. This paper suggests a method by which electron-gradient-induced errors in number density calculations can be overcome. Briefly, it is shown that if measurements of the complex reflection coefficient can be made at several frequencies, the data can be extrapolated to find the electron density in a uniform plasma region regardless of the width of the boundary zone of the plasma or the associated plasma density profile. The region where ?P>? is emphasized.     

Modeling Errors in Microwave Plasma Diagnostics Employing the Reflection Coefficient

The use of the complex reflection coefficient, R, at microwave frequencies as a plasma diagnostic tool in the high electron density range (1012 electrons/cm3) has been reported by several authors. A common assumption is that at microwave frequencies the boundary between free space and a stationary plasma is best described as a step function in electron density. Actually, many practical boundaries consist of a finite-width region wherein the electron density has a finite gradient. The complex reflection coefficient is derived for the finite width boundary and comparisons are made with an assumed sharp boundary. It is shown that improper modeling of the plasma boundary can result in significant error in electron density measurements when R is used as a diagnostic tool.     

Microwaves Scattering by Underdense Inhomogeneous Plasma Column

The scattering characteristics of microwaves(MWs) by an underdense inhomogeneous plasma column have been investigated.The plasma column is generated by hollow cathode discharge(HCD) in a glass tube filled with low pressure argon.The plasma density in the column can be varied by adjusting the discharge current.The scattering power of X-band MWs by the column is measured at different discharge currents and receiving angles.The results show that the column can affect the properties of scattering wave significantly regardless of its plasma frequency much lower than the incident wave frequency.The power peak of the scattering wave shifts away from 0°to about ±15odirection.The finite-different time-domain(FDTD) method is employed to analyze the wave scattering by plasma column with different electron density distributions.The reflected MW power from a metal plate located behind the column is also measured to investigate the scattering effect on reducing MW reflectivity of a metal target.This study is expected to deepen the understanding of plasma-electromagnetic wave interaction and expand the applications concerning plasma antenna and plasma stealth.     

Numerical and Experimental Investigation on Electromagnetic Attenuation by Semi-Ellipsoidal Shaped Plasma

Some reports presented that the radar cross section(RCS) from the radar antenna of military airplanes can be reduced by using a low-temperature plasma screen. This paper gives a numerical and experimental analysis of this RCS-reduction method. The shape of the plasma screen was designed as a semi-ellipsoid in order to make full use of the space in the radar dome.In simulations, we discussed the scattering of the electromagnetic(EM) wave by a perfect electric conductor(PEC) covered with this plasma screen using the finite-difference-time-domain(FDTD)method. The variations of their return loss as a function of wave frequency, plasma density profile, and collision frequency were presented. In the experiments, a semi-ellipsoidal shaped plasma screen was produced. Electromagnetic attenuation of 1.5 GHz EM wave was measured for a radio frequency(RF) power of 5 k W at an argon pressure of 200-1150 Pa. A good agreement is found between simulated and experimental results. It can be confirmed that the plasma screen is useful in applications for stealth of radar antenna.     

Research on phase shift characteristics of electromagnetic wave in plasma

The phase shift characteristics reflect the state change of electromagnetic wave in plasma sheath and can be used to reveal deeply the action mechanism between electromagnetic wave and plasma sheath. In this paper, the phase shift characteristics of electromagnetic wave propagation in plasma were investigated. Firstly, the impact factors of phase shift including electron density,collision frequency and incident frequency were discussed. Then, the plasma with different electron density distribution profiles were employed to investigate the influence on the phase shift characteristics. In a real case, the plasma sheath around the hypersonic vehicle will affect and even break down the communication. Based on the hypersonic vehicle model, we studied the electromagnetic wave phase shift under different flight altitude, speed, and attack angle. The results indicate that the phase shift is inversely proportional to the flight altitude and positively proportional to the flight speed and attack angle. Our work provides a theoretical guidance for the further research of phase shift characteristics and parameters inversion in plasma.     

Numerical study on the modulation of THz wave propagation by collisional microplasma photonic crystal

In order to demonstrate the modulation of terahertz wave propagation in atmospheric pressure microplasmas, in this work, the band structure and the transmission characteristics of a onedimensional collisional microplasma photonic crystal are investigated, using the transfer matrix method. For a lattice constant of 150 μm and a plasma width of 100 μm, three stopbands of microplasma photonic crystal are observed, in a frequency range of 0.1–5 THz. Firstly, an increase in gas pressure leads to a decrease in the central frequency of the stopband. When the gas pressure increases from 50.5 kPa to 202 kPa, the transmission coefficient of the THz wave first increases and then decreases at high frequency, where the wave frequency is much greater than both the plasma frequency and the collision frequency. Secondly, it is interesting to find that the central frequency and the bandwidth of the first THz stopband remain almost unchanged for electron densities of less than 10¹⁵ cm^–3, increasing significantly when the electron density increases up to 10¹⁶ cm^–3. A central frequency shift of 110 GHz, and a bandgap broadening of 200 GHz in the first stopband are observed. In addition, an atmospheric pressure microplasma with the electron density of 1 × 10¹⁵–6 × 10¹⁵ cm^–3 is recommended for the modulation of THz wave propagation by plasma photonic crystals.     

FDTD Analysis of Reflection of Electromagnetic Wave From a Conductive Plane Covered with Inhomogeneous Time-Varying Plasma

A finite-difference time-domain (FDTD) algorithm is applied to study the electromagnetic reflection of conduction plane covered with inhomogeneous time-varying plasma, homogeneous plasma and inhomogeneous plasma. The collision frequency of plasma is a function of electron density and plasma temperature. The number density profile follows a parabolic function. A discussion on the effect of various plasma parameters on the reflection coefficient is presented. Under the one-dimensional case, transient electromagnetic propagation through various plasmas has been obtained, and the reflection coefficients of EM wave through various plasmas are calculated under different conditions. The results illustrate that a plasma cloaking system can successfully absorb the incident EM wave.     

Feasibility of applying the lower cut-off frequency for the density radial coverage extension in EAST reflectometry measurement

The extraordinary mode (X-mode) lower cut-off frequency is proposed for use in the reflectometry diagnostic on ITER for the electron density profile measurement, which is a trade-off between extreme plasma parameters and the accessible probing frequency. In contemporary experiments, the lower cut-off frequency can be identified at the probing frequency below the electron cyclotron frequency (fce) under certain plasma conditions. We provide here, for the first time, the experimental validation of the use of the lower cut-off frequency for the density profiles via the reflectometry measurement on EAST. The corresponding group delay of the lower cut-off frequency evolves continuously with the upper one, revealing a reasonable radial coverage extension of reflectometry measurement toward the plasma core. It is concluded that the lower cut-off frequency can be used as a supplement to the upper one in the density profile inversion process, which is of particular interest in the high magnetic field and/or density discharge to extend the radial coverage of reflectometry measurement.     

The Propagation of an Electromagnetic Wave through a Diffusing Plasma

The propagation of an electromagnetic wave through a fully ionized diffusing plasma slab is investigated. The slab is considered to be diffusing in an ambipolar fashion, with the electron-ion collision frequency proportional to the spatial and time varying ion density. The propagation characteristics of the wave are shown to be dependent on the relationship between np and nc, the densities where the signal frequency is equal to the plasma frequency and collision frequency respectively. This relationship is temperature dependent. The reflection and transmission coefficients of the plasma are calculated as a function of signal frequency, and their behavior as the slab diffuses is investigated.     

Plasma Diagnostics in Supersonic Flow Containing Cesium and Potassium

Electron density, collision frequency and electron extinction rates have been computed from microwave absorption measurements on the exhaust plumes of solid propellant rocket motors burning CsNO3:Al and KNO3:Al The plasma was generated by burning the fuel at a chamber pressure above 100 psi, and expanding the gases through an orifice to an ambient pressure of 1 Torr. The plume consists of both gases and solid particles which modify the aerodynamic flow. High speed movies together with grid wires in the flow field were used to define the plume. Microwaves operating at frequencies up to 70 gc were used for measuring the plasma properties.     

Theoretical demonstration of surface plasmon polaritons in plasma/vacuum interface in GHz frequency

Negative permittivity of the material may lead to the enhanced radiation of an antenna embedded in a finite plasma,which suggests a potential way to solve blackout problem in space technology.However,the enhanced radiation phenomenon is still lack of strict theoretical investigations of surface plasmon polaritons(SPPs) in plasma in GHz frequency.In this paper,we demonstrate the SPPs excited at a plasma/vacuum interface in GHz frequency by the consistency of the simulated and theoretical results.With SPPs,plasma layer thicker than skin depth can be penetrated with w w,pwhich is a complement of wave propagation theory in plasma.We also discuss the influences of thickness d,collision frequency Γ,and different plasma frequencies on SPPs.For plasma frequencies with large difference,common numerical methods have difficulties in result comparison under the same mesh size because of the computer capacity and memory.The analytical multilayer method used in the paper does not need to generate mesh,so the studies of plasma frequencies with large difference can be carried out.The simulation shows that the SPPs can be excited for an arbitrary plasma frequency.We believe the study will be beneficial for the problem of wave propagation in plasma science and technology.     

Monopole Antenna Probe for Density Measurements in Cold Plasmas

A simple diagnostic tool for density measurements in plasma with a certain spatial resolution is proposed in the this paper. It uses the emission characteristics of monopole antenna to determine the dielectic property of plasmaε=1-ƒ_p²/ƒ², withƒ_pthe electron plasma frequency related to plasma density. We immersed a monopole antenna probe into plasma and introduced a microwave signal via a network analyzer. When the emitted power is maximized, the reflected power is minimized and there occurs a resonance. Sinceε can be derived from the resonant frequency, this is actually a method to measure the absolute electron density. Validated by a comparison with the amended Langmuir double-probe method, the monopole antenna probe is valuable. In addition, it is free from the difficulties, such as fluctuation in plasma potential.