Time-varying Plasma Research Articles

Experimental study on correlation between amplitude and phase of electromagnetic wave affected by time-varying plasma by amplitude-modulated radio frequency plasma generator

The time-varying plasma channel can increase the bit error rate of the phase modulation signal. The essential reason is that there is a correlation between the amplitude variation and phase variation caused by time-varying plasma. It can cause constellation rotation, and traditional communication methods cannot deal with this special interference. In this paper, the relationship between the amplitude and phase of electromagnetic wave affected by time-varying plasma is investigated. The correlation coefficient is derived. The result shows that the close correlation exists and the signs of correlation coefficients are different under different parameters. A verification experiment is also designed, and the time-varying transmission coefficients, constellations of QPSK signals, and correlation coefficients are measured under different conditions. The experimental results and the theoretical results are in good agreement. The conclusions can provide prior knowledge for communication methods and contribute to improving communication efficiency.

A Deep-Learning-Based Method for Diagnosing Time-Varying Plasma Adopting Microwaves

Performing as an important technology, microwave interferometry helps understand and analyze plasma characteristics. However, the relatively appropriate diagnosing methods being discovered for investigating fast time-varying plasma have met with limited success so far. The crucial problem of this task lies in eliminating the interference of additive noise while compensating for phase ambiguity of the initial point and of the phase jump points. In particular, randomly varying electron density and noise make it impossible to directly detect these points. Theoretical analysis has found that the time-varying electron density of plasma causes the received signal to rotate on the complex plane. Inspired by this fact, a deep learning (DL) method is proposed to track the time-varying plasma and solve the above two problems, during which the deep neural network (DNN) is utilized to extract the curve from the data, thereby eliminating the noise and detecting the initial point and the phase jump points according to the projection of the data on the curve. Experimental results suggest that the learned curve is in good consistency with the ground-truth curve. Meanwhile, the time-varying electron density and the collision frequency can be accurately diagnosed using the learned curve.

Microwave Reflectometry to Characterize the Time-Varying Plasma Generated in the Shock Tube

To investigate the behavior of plasma generated in the shock tube, microwave reflectometry is proposed to extract the permittivity $\epsilon _{r}$ of plasma. To remove the influence of parasitic reflections caused by the surroundings, a calibration process is introduced and the unknown calibration coefficients are determined by utilizing microwave interferometry as the reference technique. The shock tube is modeled as a three-layered medium to calculate the reflection coefficient. A time-dependent reconstruction algorithm is applied and theoretically validated to eliminate the multiple solutions in the inverse problem. By comparing the permittivities extracted with microwave reflectometry and interferometry, the effects of plasma diffusion are demonstrated with a modified analytical model in the beginning time region of experiments. In addition, the nonuniform flow in the generated plasma located near the end time region is also observed. The determination of the effective time region for electron density $N_{e}$ and collision frequency $v_{e}$ extraction is discussed as well. Finally, the differences between microwave reflectometry and interferometry in terms of averaged $N_{e}$ $1{\times }10^{17} m^{-3}$ and averaged $v_{e}$ $1.5{\times }10^{9} s^{-1}$ are investigated in the effective time region

Short-Frame Fountain Code for Plasma Sheath With “Communication Windows”

The surface of a hypersonic vehicle traveling at a speed exceeding Mach 10 is covered by a plasma sheath. The plasma sheath is a complex dynamic random medium that seriously attenuates electromagnetic (EM) waves and generates the so-called blackout phenomenon. However, owing to changes in vehicle flight parameters, such as the angle of attack, speed, and altitude, or the effects of plasma weakening control measures, the attenuation effect of the plasma sheath on EM waves weakens for a short time. At times, the amplitude of the EM waves is higher than the communication threshold, and the EM waves penetrate the plasma sheath in a so-called “communication window”. However, the random temporal variance of the plasma sheath parameters causes the required parameters of the “communication windows” to appear randomly, resulting in the traditional communication methods that needs to keep the communication link open for a long time being no longer applicable. To maximize the use of these “communication windows”, this paper proposes a short-frame fountain code (SFFC), constructs a time-varying plasma sheath channel model, and studies the relationship between the SFFC parameters and the transmission performance of this particular channel. Simulation and experimental results show that the use of SFFC improves the reliability of communication through the plasma sheath. The uninterrupted and correct transmission of information can be achieved even when the plasma equivalent frequency is close to the carrier frequency.

Cold-hot coupled waves in a flowing magnetized plasma

Nonlinear coupling of cold and hot waves in a flowing magnetized plasma is analyzed with the Vlasov equation. An analytical solution is obtained for cold waves of a small amplitude (weak flow) and a long wavelength. The distribution function is obtained by integrating the kinetic equation along a perturbed phase-space trajectory for a time-varying plasma flow. The kinetic description presents a generalized dispersion relation that involves resonances depending on cold and hot wave dispersions. Coherent fluid motion leads to radiation peaks in addition to the cyclotron harmonics, where the wavenumber of the cold wave determines the peak frequencies. The peaks appear narrow when the wave propagates perpendicular to the time-averaged flow while they become broad due to the Doppler effect when the wave propagates parallel to the flow. Fully kinetic particle-in-cell simulations corroborate the theoretical predictions. The dispersion relation and resulting wave spectra provide information about plasma parameters and flow properties.

A Novel Demodulation Method Based on Spectral Clustering for Phase-Modulated Signals Interrupted by the Plasma Sheath Channel

The phenomenon of radio blackout due to a plasma sheath during reentry has attracted much attention over the last several decades. However, radio blackout has long puzzled aerospace researchers and has not yet been completely resolved. Owing to the effects of the time-varying plasma sheath channel environment, the constellation of received signals exhibits multiple manifolds, making the traditional algorithms based on Euclidean distance unable to perform demodulation. In the current study, we proposed a novel demodulation method based on spectral clustering for phase-modulated signals interrupted by the plasma sheath channel. Experimental results revealed that as long as the receiving radio waves were above the floor of receiver’s dynamic range, the proposed algorithm effectively classified received signals after time-varying plasma. The parasitic modulation interference caused by time-varying plasma was restrained effectively through the novel algorithm. In contrast to the traditional communication method, the proposed method can be applied to telemetry, track and command, as well as communication between reentry vehicles and near-space hypersonic vehicles in future.

Plasma magnetic time-varying nonlinear robust control system for the Globus-M/M2 tokamak

This paper is devoted to the development of the magnetic plasma control system for the Globus-M/M2 spherical tokamak (ST) when minor disruptions occur. The novelty of the research is the design of robust cascade multivariable control systems for time-varying plasma models in the presence of nonlinear thyristor current inverters as actuators for plasma position control. A new architecture of coordinated control of plasma position and shape is proposed. Quantitative Feedback Theory is used for the synthesis of robust Single-Input Single-Output (SISO) controllers for plasma position and PF-currents. Multiple-Input Multiple-Output (MIMO) H∞ control is applied to control the plasma shape. The simulation results of the gaps-based MIMO plasma shape control system with the plasma equilibrium reconstruction code in the feedback are shown. The robust stability of the control system is estimated.

Numerical Simulation of Melt-Pool Hydrodynamics in μ-EDM Process

The machining efficiency and productivity of the micro-electrical discharge machining (μ-EDM) process can be enhanced by the fundamental insights on the plasma-material interaction, which can be appraised via experimental and numerical analysis of discharge crater formation. The high-power density with low pulse-duration results in the transformation of the solid workpiece into liquid and even vaporisation phase to generate a crater. The crater dimension largely depends on the extent of vaporization induced recoil pressure as well as the pressure exerted by the expanding plasma channel. Efforts have been made by the researchers to analyse the plasma characteristics viz. its temperature, pressure and the role of plasma pressure on the material removal. However, there is a scarcity of the realistic data pertaining to precise plasma pressure which consequences in less accurate estimation of the numerically simulated crater. Moreover, the extent of evaporation is delayed due to activation of plasma pressure. Therefore, it is imperative to conduct a sensitivity analysis of variation of plasma pressure to improve the predictive nature of three-phase multi-physics numerical modelling of the μ-EDM.In the present paper, an endeavour has been made to consider the multi-phase transformation corresponding to the high-power density applied to the workpiece. Further, the effect of parameters such as Marangoni convection, Young Laplace surface tension pressure, vapour induced recoil pressure and a time-varying plasma pressure on the melt-pool hydrodynamics are considered. A 2D phase-field numerical model for the machining of Ti-6Al-4V has been formulated considering the aforementioned parameters. The numerical model is implemented in the commercial finite element method (FEM) software COMSOL Multiphysics. The simulation results reveal that the material removal in the initial phase of pulse-duration is dominated by vaporisation along with the melt expulsion due to recoil pressure which governs the crater size. The vaporisation of the Ti-6Al-4V is delayed to 4250 K due to active plasma pressure in the melt-pool. In the remaining pulse period, the surface tension induced backflow of molten metal along with Marangoni convection result in non-uniform perturbation at the bottom of the crater. Finally, the simulated crater is validated with the experimentally determined one-off crater using a dedicated single spark EDM circuit.

A Joint Demodulation and Estimation Algorithm for Plasma Sheath Channel: Extract Principal Curves With Deep Learning

For reentry communication, owing to the influence of the time-varying plasma sheath, the received IQ baseband signals are severely rotated on constellation. Researches have shown that the frequency of electron density varies from 20kHz to 100 kHz which is on the same order as the symbol rate of most TT&C communication systems and traditional estimation algorithms cannot track the variation of channel. In this letter, motivated by principal curve analysis, we propose a deep learning (DL) algorithm which is called symmetric manifold network (SMN) to extract the curves on the constellation and classify the signals based on the curves. The key advantage is that SMN can achieve joint optimization of demodulation and estimation. From our simulation results, the new algorithm significantly reduces the symbol error rate (SER) compared to existing algorithms and enables accurate estimation of fading with extremely high bandwidth utilization rate.

Researches on the scattering characteristics in THz band of conductor cylinder coated with parabolic distribution and time-varying plasma media

It is widely acknowledged that inhomogeneous and time-varying plasma can effectively reduce the back scattering radar cross section (RCS) and THz wave has the potential to penetrate the plasma and against the plasma stealth. Therefore, the backscattering and bistatic radar cross section (RCS) of conductor cylinder coated with parabolic distribution and time-varying plasma media in THz wave band are studied by the Runge-Kutta Exponential Time Differencing-Finite Difference Time Domain (RKETD-FDTD) method in this paper. The influences of plasma parameters are analyzed. From both the backscattering and bistatic RCS simulations, it is obviously that inhomogeneous and time-varying plasma cloaking system makes a significant difference on reducing the RCS and improve the plasma stealth while electromagnetic wave in THz region can successfully weaken its influence and contribute to the anti-stealth.

Microwave frequency downshift in the time-varying collision plasma

Microwave frequency downshift in the time-varying collision plasma has been demonstrated by particle-in-cell simulations. The simulation results are consistent with the theoretical analysis, and the preconditions for microwave frequency downshift are that the collision frequency needs to be greater than the incident wave frequency, and the plasma frequency is two times greater than the incident wave frequency. Finally, the simulation results are compared with the reported experimental results indicating good agreement.

Bit Error Rate and Channel Capacity Performance of Telemetry Modulation Methods Under Typical Reentry Plasma Sheath Channel

Due to the severe impact of plasma turbulence on electromagnetic waves, the dynamic plasma sheath can degrade communication quality, even resulting in communication blackouts. This article studies the effect of different frequency bands and two typical telemetry modulation methods on communication signals under a time-varying plasma sheath channel. First, considering plasma dynamics on different scales, a typical reentry plasma sheath channel response is simulated and analyzed. Then, the average bit error rate (BER) and channel capacity of $M$ -ary phase-shift keying (MPSK) and $M$ -ary frequency-shift keying (MFSK) evaluating the telemetry communication performance are derived theoretically and simulated numerically based on the typical reentry plasma sheath channel response. The simulation results show that BPSK and noncoherent-MFSK (NC-MFSK) in the Ka -band and above bands have a preferable communication performance; when bit-to-noise ratio > 10 dB, BER $ and channel capacity loss $ for both BPSK and NC-MFSK ( $M\ge 8$ ) modulations in the especial reentry plasma condition (ERPC). Comparing BPSK and NC-MFSK in the Ka -band and above bands, the performances of BER and channel capacity follow the order: NC-MFSK ( $M \ge 8$ ) > BPSK > NC-4FSK > NC-2FSK. In particular, results indicate that under the dynamic plasma channel, terahertz communication can effectively improve communication performance and alleviate communication blackout.

Effects of Plasma Sheath on Parameter Estimations of Linear Frequency Modulation Pulse Signal

A space vehicle traveling in the atmosphere at hypersonic speed is enveloped by a time-varying plasma sheath, and the performance of onboard radar reconnaissance system will be significantly interrupted. Plasma sheath would generate parasitic modulations of the amplitude and phase of radar signals. Therefore, based on the transmission coefficient of radar wave in time-varying plasma sheath, the Wigner–Hough transform and autocorrelation time-domain signal detection method are used to estimate the frequency- and time-domain parameters of the linear frequency modulation (LFM) pulse signal statistically. A variation rule of the estimation errors of signal parameters is established under various plasma characteristics parameter and frequency and bandwidth of LFM waves. The critical signal-to-noise ratio that is the threshold value for failure of signal parameter estimation has been accurately obtained for different electron densities and radar frequencies. The calculation results could provide useful information for onboard electronic reconnaissance system in further applications.

Research on the Propagation Characteristics of THz Waves in Spatial Inhomogeneous and Time-Varying and Weakly Ionized Dusty Plasma

Dusty plasma differs from ordinary plasma, and its unique properties have always been an active research branch of plasma science. In this article, a finite-difference time-domain (FDTD) method is used to study the propagation characteristics of terahertz (THz) waves in spatially inhomogeneous and time-varying weakly ionized and weakly colliding dusty plasma. The reflection and transmission coefficients are analyzed for the concentration and radius of dust particles, the time-varying rate of electron density, and the effective collision frequency. Results show that increasing the concentration and radius of dust particles will enhance the adsorption ability of the dust particles toward electrons, thereby weakening the electron density in dusty plasma, which will increase the penetration ability of THz waves into the dusty plasma layer. The increase in the effective collision frequency in the dusty plasma will enhance the collisional absorption of dusty plasma and restrain the ability of electromagnetic (EM) waves to penetrate the plasma layer. However, the ability of EM waves to penetrate the dusty plasma layer can be effectively improved by increasing incident wave frequency. Therefore, using THz technology to penetrate spatially inhomogeneous and time-varying dusty plasma is feasible.

Investigation on reason of range profile distortion of plasma sheath target

The range profile of the plasma sheath target is always found to be distorted when using the linear frequency modulation signal as the radar detection signal. As a result, the detection and tracking of the target became hard, or even failure. Here, three possible reasons of the range profile distortion is investigated in detail: (i) The broadening of the particles in the radial direction of the radar detection when the plasma sheath is produced; (ii) The scatterers in one range gate have different radial velocities that the range profile is dispersion due to the range-Doppler coupling effect; (iii) The amplitude and frequency of the echo signal are modulated by the time-varying plasma sheath. Here, the influence of the above three mechanisms on the range profile is simulated. In addition, for the accurate detection and tracking of these targets, the reason of the distortion should be distinguished. Therefore, several methods are proposed to distinguish the different mechanisms for the range profile distortion.

Analysis of microwave propagation in a time-varying plasma slab with particle-in-cell simulations

Continuous microwave propagation through a time-varying plasma and frequency up-conversion has been demonstrated by particle-in-cell (PIC) simulation. In principle, it is possible to transform a 2.45 GHz source radiation to an arbitrary larger frequency radiation. The energy conversion is also obtained by the theoretical analysis and has been testified by PIC simulation. The source wave was propagating in a parallel plate waveguide locally filled with the ionized gas. In this paper we would discuss the effects of the rise time, the plasma length, the switching time and the collision frequency on the energy conversion, and the methods to improve the upshift wave energy are proposed. We also put forward the new concept of the critical values of the rise time and the source wave amplitude to provide a theoretical basis for the selection of parameters in the experiments.

Intra-pulse modulation of the linear frequency modulated pulse caused by time-varying plasma

The effects of time-varying plasma on the propagation of a linear frequency modulated (LFM) pulse are investigated. An experiment on the propagation of LFM pulse in continuous time-varying plasma is described and a spectrum broadening phenomenon is observed. The phenomenon is due to intra-pulse amplitude and phase modulation of the LFM pulse by time-varying plasma; this explanation is verified by a comparison of the results of the experiment and theoretical analysis. The theoretical result is obtained by measuring the time-varying electron density that the intra-pulse modulation depends on and applying the measured time-varying electron density into calculation method presented in this paper. The effects of the bandwidth of the LFM pulse and oscillation frequency of electron density on the intra-pulse modulation phenomenon are then explored by another two experiments. Time-frequency analysis employing a smooth pseudo Wigner–Ville distribution (SPWD) is adopted to further analyze the phenomenon and results of time-frequency analysis confirm the earlier theoretical analysis and conclusions. The effect of intra-pulse modulation on the LFM pulse due to time-varying plasma should be emphasized in the application of the radar and communication systems on a vehicle under a plasma sheath.

Spatiotemporal data fusion and manifold reconstruction in Hall thrusters**Distribution Statement A: Approved for Public Release. Distribution is Unlimited. PA# 18635.

A critical breakthrough in the understanding of Hall effect thrusters (HETs) has been the emergence of mapping techniques to fuse multiple time-varying plasma measurements from spatially distributed probes into a unified reconstruction describing the full spatiotemporal dynamics of the HET plasma. The most basic requirement for generating such a spatiotemporal reconstruction is a robust and accurate method to build a map between a single input and a single output signal. This work describes a new nonlinear phase space based mapping technique, the shadow manifold interpolation (SMI) technique, and assesses its performance versus traditional linear FFT-based mapping. Results demonstrate the superiority of the SMI reconstruction for short, noisy and aperiodic signals. Analysis in conjunction with the convergent cross mapping methodology (Sugihara et al 2012 Science 338 496–500) provides theoretical justification for the physical relevance of SMI application to a given signal reconstruction.

Influence of plasma channel impedance model on electrohydraulic shockwave simulation

The simulation of electrohydraulic shockwaves is the key technology for their application and device design. The analytical approach of underwater pulsed discharge is described using the dimensionless analysis method, which combines energy based hydrodynamic equations and the electrical differential equation describing the discharge characteristics of the circuit. A constant plasma channel impedance model based on the experimental results and an improved time-varying plasma channel impedance model based on the Braginskii equation are proposed and presented. The plasma channel impedance has a great influence on the deposited energy into the plasma channel and then affects the calculation and analysis of the shockwave propagation. The electrical characteristics and fluid physical properties are taken into consideration, and the results are compared. The application range of different impedance models is summarized, and the time-varying impedance model can provide a more accurate result in the simulation of shockwave propagation. The dimensionless analysis method, together with a time-varying impedance model, could be a useful tool for evaluation of the electrohydraulic shockwave in practical applications and applied physics research of underwater spark discharge.

Passive Radar Jamming: A Novel Method Using Time-Varying Plasma

Based on the resonance absorption effect of plasma, a novel time-varying plasma passive jamming method against one dimensional range profile of radar detection is proposed in this paper. The jamming mechanism of time-varying plasma structure is presented and verified. Based on the transmission line analogy, an instantaneous method is developed to calculate the radar time domain echo and the pulse compression of a target coated by the time-varying plasma. Then, the distortions of radar waveform and spectrum are numerically calculated. Finally, the jamming effect of the time-varying plasma on the one-dimensional range profile of a target is studied systematically for different regulating amplitude and frequency of electron density, radar bandwidth and pulse width parameters. The range profile result indicates that single radar target coating by time-varying plasma structure will generate cloaking effects and produce two or more false radar targets, and the distance between real and false target as well as reduction of echo intensity are related to the radar and regulating plasma parameters. The results show that this jamming method could have a great impact on the one-dimensional range profile, and the results could provide many useful reference to radar countermeasure.