Effects Of Plasma Sheath Research Articles

Instantaneous polarization statistic property of EM waves incident on time-varying reentry plasma

An analytical method is proposed in this paper to study the effect of time-varying reentry plasma sheath on the instantaneous polarization statistic property of electromagnetic (EM) waves. Based on the disturbance property of the hypersonic fluid, the spatial-temporal model of the time-varying reentry plasma sheath is established. An analytical technique referred to as transmission line analogy is developed to calculate the instantaneous transmission coefficient of EM wave propagation in time-varying plasma. Then, the instantaneous polarization statistic theory of EM wave propagation in the time-varying plasma sheath is developed. Taking the S-band telemetry right hand circularly polarized wave as an example, effects of incident angle and plasma parameters, including the electron density and the collision frequency on the EM wave's polarization statistic property are studied systematically. Statistical results indicate that the lower the collision frequency and the larger the electron density and incident angle is, the worse the deterioration of the polarization property is. Meanwhile, in conditions of critical parameters of certain electron density, collision frequency, and incident angle, the transmitted waves have both the right and left hand polarization mode, and the polarization mode will reverse. The calculation results could provide useful information for adaptive polarization receiving of the spacecraft's reentry communication.

A layered fluctuation model of electron density in plasma sheath and instability effect on electromagnetic wave at Ka band

The effects of plasma sheath on communication signals are not only power attenuation but also multiplicative noise addition because of the sheath's instability. The fluctuation properties of time-varying electron density are crucial to comprehensively understand the blackout problem. In this paper, the fluctuation laws of time-varying electron density are first comprehensively revealed in space–time–frequency domain. A layered fluctuation model of time-varying electron density is then proposed based on these fluctuation laws. The effects of dynamic plasma sheath on electromagnetic (EM) waves at Ka band are calculated by Monte Carlo quasistatic EM numerical method. Results show that the amplitude and phase of time-varying transmission coefficient both follow Gaussian distribution, whereas the spectrum curves follow a bi-Gauss function because of the effect of second mode instability in the hypersonic boundary layer. Furthermore, the means of amplitudes increase with the increasing of the incident EM wave frequency and the decreasing of the peak electron density, while the standard deviations decrease with the increasing of incident EM wave frequency, the decreasing of the peak electron density and the decreasing of electron density standard deviation.

Transmission Channel Characteristics of Relay Dual-Polarization MIMO System for Hypersonic Vehicles Under Plasma Sheath

The blackout problem caused by plasma sheath is a serious obstacle to the communication system of hypersonic vehicles. The uplink relay transmission method is an effective means of mitigating serious attenuation caused by plasma sheath. This paper proposes a Ka-band $2\times2$ dual-polarization multiple-input multiple-output (MIMO) scheme by adopting a left-/right-hand circular polarized antenna to increase channel capacity and signal quality. The relay dual-polarized MIMO channel model between the hypersonic vehicle and satellite is established by emphasizing the plasma sheath effect, especially the cross-polar coupling and polar correlation under plasma sheath. Useful numerical results on the relay dual-polarized MIMO ergodic channel capacity are analyzed. Thus, significant capacity gains with respect to the conventional single-input single-output case are illustrated based on this analysis. Simulation results show that the relay dual-polarized MIMO method can increase the channel ergodic capacity by 1.9–2 times under most circumstances, and the angle between the selected relay satellite and the vehicle normal direction should be minimal. This statistical channel model can be applied to the wireless communication system design and evaluation for hypersonic vehicles.

Effects of a reentry plasma sheath on the beam pointing properties of an array antenna

The reduction in the gain of an on-board antenna caused by a reentry plasma sheath is an important effect that contributes to the reentry “blackout” problem. Using phased array antenna and beamforming technology could provide higher gain and an increase in the communication signal intensity. The attenuation and phase delay of the electromagnetic (EM) waves transmitting through the plasma sheath are direction-dependent, and the radiation pattern of the phased array antenna is affected, leading to a deviation in the beam pointing. In this paper, the far-field pattern of a planar array antenna covered by a plasma sheath is deduced analytically by considering both refraction and mutual coupling effects. A comparison between the analytic results and the results from an electromagnetic simulation is carried out. The effect of the plasma sheath on the radiation pattern and the beam pointing errors of the phased array antenna is studied systematically, and the derived results could provide useful information for the correction of pointing errors.

A Multiscale Model of Reentry Plasma Sheath and Its Nonstationary Effects on Electromagnetic Wave Propagation

During reentry, plasma sheath is affected by many flight factors, such as trajectory, velocity, angle of attack, and fluid turbulence. As a result, the plasma parameters are time-varied and nonstationary. The nonstationary effects of reentry plasma sheath on electromagnetic (EM) wave propagation are investigated in this paper. First, the factors that affect the plasma sheath are analyzed, and the plasma sheath variation is divided into three scales. A multiscale reentry plasma sheath electron density model is established. A compatible modeling method is proposed, and this method is proven effective. Finally, we analyze the effects of plasma sheath on EM wave at 1.57542, 2.2, and 5 GHz, which are generally used in GPS and telemetry, tracking, and command systems. Results show that the statistical parameters of complex transmission coefficients are time-varied and nonstationary.

Influence of Time-Varying Plasma Sheath on the Lock Condition of Phase-Locked Loop for TT&C Carrier Tracking System

Time-varying plasma sheath likely causes a parasitic phase modulation effect on communication signals. It is essential to understand the effect of plasma sheath on the tracking telemetry and command (TT&C) systems for blackout mitigation. This paper comprehensively investigates the impact of parasitic phase modulation effect on the TT&C tracking system of Ka band signals, especially single-frequency and phase-modulated signals. A fluctuant electron density model in the space-time domain is initially established. The phase noise caused by the time-varying plasma sheath is then obtained through wave impedance numerical calculation method. The effect of the time-varying plasma sheath with different fluctuant intensities on the carrier tracking performance is investigated and quantitatively analyzed by imposing phase noise on a phase-locked loop model. Simulation results show that a large fluctuant intensity corresponds to a great impact on the lockout threshold. A critical fluctuant intensity is observed in the three typical TT&C signals. The effect of the time-varying plasma sheath on phase-modulated signals is more severe than that on single-frequency signals. The maximum tolerable fluctuant intensities are 12%, 10%, and 6% for single-frequency signals, binary phase shift keying signals, and quadrature phase shift keying signals, respectively. With the critical fluctuant intensity, their lockout thresholds are increased to 0, 6, and 4 dB, respectively.

The modeling and simulation of plasma sheath effect on GNSS system

Plasma sheath can potentially degrade global navigation satellite system (GNSS) through signal attenuation as well as phase noise when a hypersonic vehicle reenters the Earth’s atmosphere. Modeling and simulation method of GNSS system disturbed by plasma sheath is introduced in this paper by means of electromagnetic wave propagation theory combined with the satellite signal simulation technique. The transmission function of the plasma sheath with stratified model is derived utilizing scattering matrix method. The effects of the plasma sheath on GPS signal reception and positioning performance are examined. Experimental results are presented and discussed, partly supporting the validity of the analytical method proposed.

MITIGATION OF COMMUNICATION BLACKOUT DURING RE-ENTRY USING STATIC MAGNETIC FIELD

During re-entry into earth's atmosphere, a spacecraft suffers from loss of communication with the ground control station, known as communication blackout, due to formation of plasma around the re-entry spacecraft. This paper presents the theory and analysis of the communication blackout and its mitigation using static magnetic field method. The interaction between electromagnetic waves and plasma in presence as well as absence of magnetic field is described to determine the effects of plasma sheath on the spacecraft re-entering into the atmosphere. An analysis is done to determine the effectiveness of this mitigation technique for a typical re-entry spacecraft and the strength of magnetic field required to establish the communication link between the re-entry spacecraft and the ground station is obtained.

Effects of Reentry Plasma Sheath on GPS Patch Antenna Polarization Property

A plasma sheath enveloping a reentry vehicle would affect performances of on-board antenna greatly, especially the navigation antennas. This paper studies the effects of reentry plasma sheath on a GPS right-hand circularly polarized (RHCP) patch antenna polarization property during a typical reentry process. Utilizing the algorithm of finite integration technique, the polarization characteristic of a GPS antenna coated by a plasma sheath is obtained. Results show that the GPS RHCP patch antenna radiation pattern distortions as well as polarization deteriorations exist during the entire reentry process, and the worst polarization mismatch loss between a GPS antenna and RHCP GPS signal is nearly 3 dB. This paper also indicates that measures should be taken to alleviate the plasma sheath for maintaining the GPS communication during the reentry process.

Analysis of the Impact of Plasma Sheath on GPS Antenna

The electromagnetic simulation software CST was used to analyze the effects of reentry plasma sheath on the GPS navigation antenna. The Impedance and radiation characteristics of antenna were studied on condition that the antenna was coated with uniform and nonuniform electron density distribution plasma sheath respectively. The results show that, the antenna coated with the uniform plasma sheath, the plasma electron density increasing, the antenna operating frequency moves to high-frequency and that the directivity decreases as well; when the antenna was coated with nonuniform plasma, with the higher electron peak density of plasma sheath, besides that the operating frequency also moves to high-frequency, the bandwidth stretches wide and the return loss reduces; the antenna radiation pattern distorts seriously at the electron peak density of 1018m-3.

Uniformity enhancement of incident dose on concave surface in plasma immersion ion implantation assisted by beam-line ion source

Plasma immersion ion implantation (PIII) is a promising surface treatment technique for the irregular-shaped components. However, it is difficult to achieve uniform implantation along the surface of a concave sample due to the propagation and overlapping effect of plasma sheath. In this paper, a new ion implantation process is presented for improving the dose uniformity, especially for enhancing the lateral dose of the samples with concavities. In PIII enhanced by beam-line ions process, a beam-line ion source with certain energy is introduced from an external source into the concavity to suppress the sheath propagation and consequently to improve the dose uniformity. The time-dependent evolution of the potential, electrical field and the particle movement surrounding the surface of concave sample is studied by a particle-in-cell/Monte Carlo collision (PIC/MCC) simulation during a single bias high voltage (HV) pulse. The simulation results show that the plasma sheath propagation surrounding the concave sample is suppressed effectively by beam-line ions, and can be quasi-steady state during a single HV pulse. The influence of the energy of induced beam-line ions on the incident ion dose and energy distribution is discussed. Compared with the traditional PIII process, the dose uniformity of the sample surface is improved obviously due to the increase of the ions implanted into the lateral surface.

Heating and Plasma Sheath Effects in Low‐Temperature, Plasma‐Assisted Growth of Carbon Nanofibers

AbstractPlasma sheath, nanostructure growth, and thermal models are used to describe carbon nanofiber (CNF) growth and heating in a low‐temperature plasma. It is found that when the H2 partial pressure is increased, H atom recombination and H ion neutralization are the main mechanisms responsible for energy release on the catalyst surface. Numerical results also show that process parameters such as the substrate potential, electron temperature and number density mainly affect the CNF growth rate and plasma heating at low catalyst temperatures. In contrast, gas pressure, ion temperature, and the C2H2:H2 supply ratio affect the CNF growth at all temperatures. It is shown that plasma‐related processes substantially increase the catalyst particle temperature, in comparison to the substrate and the substrate‐holding platform temperatures. magnified image

Progress Towards an End-to-End Model of an Electrothermal Chemical Gun

Much progress has been made in the last few years toward the development of an end-to-end computer model of an electrothermal chemical gun. Phenomena that must be considered in an electrothermal chemical gun model include the initial capillary plasma properties, the plasma-air interaction, plasma sheath effects, and the plasma-propellant interaction itself. This paper presents an overview of recent progress made modeling these various phenomena. Also discussed are methods under consideration to incorporate surface chemistry effects into the plasma-propellant interaction model.

Plasma sheath structures around a radio frequency antenna

A one‐dimensional particle‐in‐cell (PIC) simulation code is developed to investigate plasma sheath structures around a high‐voltage transmitting antenna in the inner magnetosphere. We consider an electrically short dipole antenna assumed to be bare and perfectly conducting. The oscillation frequency of the antenna current is chosen to be well below the electron plasma frequency but higher than the ion plasma frequency. The magnetic field effects are neglected in the present simulations. Simulations are conducted for the cases without and with ion dynamics. In both cases, there is an initial period, about one‐fourth of an oscillation cycle, of antenna charging because of attraction of electrons to the antenna and the formation of an ion plasma sheath around the antenna. With the ion dynamics neglected, the antenna is charged completely negatively so that no more electrons in the plasma can reach the antenna after the formation of the sheath. When the ion dynamics are included, the electrons impulsively impinge upon the antenna while the ions reach the antenna in a continuous manner. In such a case, the antenna charge density and electric field have a brief excursion of slightly positive values during which there is an electron sheath. The electron and ion currents collected by the antenna are weak and balance each other over each oscillation cycle. The sheath–plasma boundary is a transition layer with fine structures in electron density, charge density, and electric field distributions. The sheath radius oscillates at the antenna current frequency. The calculated antenna reactance is improved from the theoretical value by 10%, demonstrating the advantage of including the plasma sheath effects self‐consistently using the PIC simulations. The sheath tends to shield the electric field from penetrating into the plasma. There is, however, leakage of an electric field component with significant amplitude into the plasma, implying the applicability of the high‐voltage antennas in whistler wave transmission in the inner magnetosphere.

Effects of Plasma Power and Plasma Sheath on Field Emission Properties of Carbon Nanotubes

With a nickel catalyst, carbon nanotubes (CNTs) were prepared by microwave plasma-enhanced chemical vapor deposition (MPCVD). Transmission electron microscopy (TEM) images reveal the center hollowness and multiwall structure of CNTs. The tip-growth mechanism of the CNTs prepared by MPCVD is confirmed by the Ni particles enclosed at the tips of the CNTs. The degree of CNT graphitization increases with plasma power up to about 1000 W and then reaches the upper limit. This is attributed to an insufficient carbon feedstock for CNT growth. The field emission (FE) efficiency of the CNTs increases with plasma power. For plasma powers not greater than 1000 W, the increase in graphitization degree and the decrease in the number of defects in a CNT emitter array enhance FE performance. The defects produced by the large split catalysts remaining in the CNTs at low plasma powers (700 and 800 W) also result in a low FE efficiency. Although the CNTs grown at moderate to high plasma powers (1000 to 1200 W) have similar graphitization degrees, a low plasma power results in split catalysts and hence defects, as evidenced by TEM observation. These defects accumulate electrons, block electronic transport, and hence reduce the FE efficiency of the CNTs. The effect of kinked CNTs produced by the a plasma sheath on FE efficiency is negligible.

Plasma-sheath effects on the Debye screening problem

The classical Debye-Hückel screening effect of the electrostatic field generated by isolated charged particles immersed in a plasma is reviewed. The validity of the underlying mathematical model, and particularly of the weak-field approximation, is analyzed. It is shown that the presence of the plasma sheath around test particles and the resulting effect of charge screening are essential for the description of plasmas that are strongly coupled.

Rocket measurements of ionospheric electron density from Brazil in the last two decades

Several rockets carrying plasma diagnostic experiments were launched from the Brazilian rocket launching stations in Natal (5.9°S, 35.2°W Geog. Lat.) and Alcantara (2.31°S, 44.4°W Geog. Lat.). Langmuir Probes (LP) were used to measure the height profiles of electron density ( n e) and electron temperature ( T e) and high frequency capacitance (HFC) probes were used to measure n e. The main objectives of the studies reported here are: (1) to compare the observed n e profiles with the Model predictions and (2) to look into the possible reasons for the deviations of the observed n e profiles from the model predictions. An attempt is made to obtain information as to what are the physical parameters responsible for the observed deviations in the profiles and thereby to suggest some possible improvements in the methods used for model predictions for low latitudes. The electron density profiles estimated from the Langmuir and the HFC probe measurements, qualitatively, match with the IRI-95 model profiles especially during daytime or during nighttime when the F-region base is rather at a low level. However, quantitatively, the model profiles deviate considerably from the observations. These deviations can be attributed to one or more of the following: (a) inadequacy of the IRI-95 model to represent the equatorial ionosphere over Brazilian region which is not adequately represented in the data base used in the IRI model formulation; (b) the non-inclusion of the effects of electrodynamic forces in the IRI model. These processes are extremely important in the low latitude ionosphere; (c) non-inclusion of the effect of electron temperature variation on the saturation electron current and thereby on the proportionality constant that relates the sensor current collected with the electron number density, that can result in an overestimation of the electron density in regions of lower electron temperature; (d) non-inclusion of the negative plasma sheath effect in the sensor current electron density relationship.

Aligned Carbon Nanotube Formation via Radio-Frequency Magnetron Plasma Chemical Vapor Deposition

The plasma-enhanced chemical vapor deposition method, which has such benefits as low synthesis temperature and vertical alignment control, was successfully applied to the single-walled carbon nanotube (SWNT) formation stage. By means of plasma sheath effects, the individual SWNT with a freestanding form was grown on a silicon-based flat substrate surface. The detailed plasma effects are also discussed in order to achieve more precise control of the growth of the carbon nanotube.

Experimental evidence for sheath effects at the ICRF antenna and ensuing changes in the plasma boundary during ICRF on textor

Measurements of a generated dc current between a Faraday-shielded ICRF antenna and the wall (liner) were performed on the ICRF-heated TEXTOR plasma. It is observed that the dc current, corresponding to electrons drawn by the antenna, increases by a factor of 8, reaching a value of 80 A for 200 kW ICRF power applied to the antenna. It is shown that this increased dc current is mainly due to the plasma sheath effect at the ICRF antenna. The resulting changes in the plasma edge are very similar to those observed during negative biasing of the ALT-I [1] limiter on TEXTOR. The most important consequences of the sheath effect are the occurrence of a density bump in the scrape-off layer (SOL) due to particle transport changes, and the generation of waves at harmonics of the RF generator frequency.

An experimental study of plasma sheath effects on antennas

Abstract : A plasma simulation technique was developed which can be used to study the effects of homogeneous or inhomogeneous plasma sheath on the radiation pattern deterioration and input impedance of microwave antennas. A tank was designed and constructed for use in the simulation technique which can reproduce by means of real dielectric materials the dielectric constant encountered in plasma covered antenna research. The radiation patterns and the input impedances of an annular slot and a thin and long rectangular slot have been successfully measured in the presence of a simulated loss-less, homogeneous and isotropic plasma layer of varied thickness. Comparison with the available theoretical data indicates a generally good agreement, although some differences exist. In the case of the radiation patterns these differences are attributed to the finite distance between the radiator and the receiving antenna on one hand and the inherent inaccuracy of the saddle point method of integration in certain regions on the other hand.