Variation Of Plasma Parameters Research Articles

Reflection of Fast Magnetosonic Waves near a Magnetic Reconnection Region.

Magnetic reconnection in the solar corona is thought to be unstable with the formation of multiple interacting plasmoids, and previous studies have shown that plasmoid dynamics can trigger MHD waves of different modes propagating outward from the reconnection site. However, variations in plasma parameters and magnetic field strength in the vicinity of a coronal reconnection site may lead to wave reflection and mode conversion. In this paper we investigate the reflection and refraction of fast magnetoacoustic waves near a reconnection site. Under a justified assumption of an analytically specified Alfvén speed profile, we derive and solve analytically the full wave equation governing the propagation of fast-mode waves in a non-uniform background plasma without recourse to the small wavelength approximation. We show that the waves undergo reflection near the reconnection current sheet due to the Alfvén speed gradient and that the reflection efficiency depends on the plasma-β parameter, as well as on the wave frequency. In particular, we find that waves are reflected more efficiently near reconnection sites in a low-β plasma, which is typical under solar coronal conditions. Also, the reflection is larger for lower-frequency waves while high-frequency waves propagate outward from the reconnection region almost without the reflection. We discuss the implications of efficient wave reflection near magnetic reconnection sites in strongly magnetized coronal plasma for particle acceleration, and also the effect this might have on first ionization potential (FIP) fractionation by the ponderomotive force of these waves in the chromosphere.

On limitations of laser-induced fluorescence diagnostics for xenon ion velocity distribution function measurements in Hall thrusters

Hall thruster operation is characterized by strong breathing oscillations of the discharge current, the plasma density, the temperature, and the electric field. Probe- and laser-induced fluorescence (LIF) diagnostics were used to measure temporal variations of plasma parameters and the xenon ion velocity distribution function (IVDF) in the near-field plasma plume in regimes with moderate (<18%) external modulations of applied DC discharge voltage at the frequency of the breathing mode. It was shown that the LIF signal collapses while the ion density at the same location is finite. The proposed explanation for this surprising result is based on a strong dependence of the excitation cross-section of metastables on the electron temperature. For large amplitudes of oscillations, the electron temperature at the minimum enters the region of very low cross-section (for the excitation of the xenon ions); thus, significantly reducing the production of metastable ions. Because the residence time of ions in the channel is generally shorter than the time scale of breathing oscillations, the density of the excited ions outside the thruster is low and they cannot be detected. In the range of temperature of oscillations, the ionization cross-section of xenon atoms remains sufficiently large to sustain the discharge. This finding suggests that the commonly used LIF diagnostic of xenon IVDF can be subject to large uncertainties in the regimes with significant oscillations of the electron temperature, or other plasma parameters.

The temporal behaviour of MHD waves in a partially ionized prominence-like plasma: Effect of heating and cooling

Context.During heating or cooling processes in prominences, the plasma microscopic parameters are modified due to the change of temperature and ionization degree. Furthermore, if waves are excited on this non-stationary plasma, the changing physical conditions of the plasma also affect wave dynamics.Aims.Our aim is to study how temporal variation of temperature and microscopic plasma parameters modify the behaviour of magnetohydrodynamic (MHD) waves excited in a prominence-like hydrogen plasma.Methods.Assuming optically thin radiation, a constant external heating, the full expression of specific internal energy, and a suitable energy equation, we have derived the profiles for the temporal variation of the background temperature. We have computed the variation of the ionization degree using a Saha equation, and have linearized the single-fluid MHD equations to study the temporal behaviour of MHD waves.Results.For all the MHD waves considered, the period and damping time become time dependent. In the case of Alfvén waves, the cut-off wavenumbers also become time dependent and the attenuation rate is completely different in a cooling or heating process. In the case of slow waves, while it is difficult to distinguish the slow wave properties in a cooling partially ionized plasma from those in an almost fully ionized plasma, the period and damping time of these waves in both plasmas are completely different when the plasma is heated. The temporal behaviour of the Alfvén and fast wave is very similar in the cooling case, but in the heating case, an important difference appears that is related with the time damping.Conclusions.Our results point out important differences in the behaviour of MHD waves when the plasma is heated or cooled, and show that a correct interpretation of the observed prominence oscillations is very important in order to put accurate constraints on the physical situation of the prominence plasma under study, that is, to perform prominence seismology.

Comparison of runaway electron generation parameters in small, medium-sized and large tokamaks—A survey of experiments in COMPASS, TCV, ASDEX-Upgrade and JET

This paper presents a survey of the experiments on runaway electrons (RE) carried out recently in frames of EUROFusion Consortium in different tokamaks: COMPASS, ASDEX-Upgrade, TCV and JET. Massive gas injection (MGI) has been used in different scenarios for RE generation in small and medium-sized tokamaks to elaborate the most efficient and reliable ones for future RE experiments. New data on RE generated at disruptions in COMPASS and ASDEX-Upgrade was collected and added to the JET database. Different accessible parameters of disruptions, such as current quench rate, conversion rate of plasma current into runaways, etc have been analysed for each tokamak and compared to JET data. It was shown, that tokamaks with larger geometrical sizes provide the wider limits for spatial and temporal variation of plasma parameters during disruptions, thus extending the parameter space for RE generation. The second part of experiments was dedicated to study of RE generation in stationary discharges in COMPASS, TCV and JET. Injection of Ne/Ar have been used to mock-up the JET MGI runaway suppression experiments. Secondary RE avalanching was identified and quantified for the first time in the TCV tokamak in RE generating discharges after massive Ne injection. Simulations of the primary RE generation and secondary avalanching dynamics in stationary discharges has demonstrated that RE current fraction created via avalanching could achieve up to 70–75% of the total plasma current in TCV. Relaxations which are reminiscent the phenomena associated to the kinetic instability driven by RE have been detected in RE discharges in TCV. Macroscopic parameters of RE dominating discharges in TCV before and after onset of the instability fit well to the empirical instability criterion, which was established in the early tokamaks and examined by results of recent numerical simulations.

Highly tunable electronic properties in plasma-synthesized B-doped microcrystalline-to-amorphous silicon nanostructure for solar cell applications

Highly controllable electronic properties (carrier mobility and conductivity) were obtained in the sophisticatedly devised, structure-controlled, boron-doped microcrystalline silicon structure. Variation of plasma parameters enabled fabrication of films with the structure ranging from a highly crystalline (89.8%) to semi-amorphous (45.4%) phase. Application of the innovative process based on custom-designed, optimized, remote inductively coupled plasma implied all advantages of the plasma-driven technique and simultaneously avoided plasma-intrinsic disadvantages associated with ion bombardment and overheating. The high degree of SiH4, H2 and B2H6 precursor dissociation ensured very high boron incorporation into the structure, thus causing intense carrier scattering. Moreover, the microcrystalline-to-amorphous phase transition triggered by the heavy incorporation of the boron dopant with increasing B2H6 flow was revealed, thus demonstrating a very high level of the structural control intrinsic to the process. Control over the electronic properties through variation of impurity incorporation enabled tailoring the carrier concentrations over two orders of magnitude (1018–1020 cm−3). These results could contribute to boosting the properties of solar cells by paving the way to a cheap and efficient industry-oriented technique, guaranteeing a new application niche for this new generation of nanomaterials.

Electromagnetic dip and hump solitary structures in oxygen-hydrogen dissipative plasmas

The excitation of low frequency magnetosonic waves in O+−H+−e− and O+−H−−e− collisional plasmas is studied. The light ions (hydrogen) may be positive as well as negative and are warm, and the heavy ions (oxygen) are considered as the cold species. The inertia of isothermal electrons is also considered. The collisions of ions and electrons with neutrals are taken into account. The hydrodynamic equations represent the dynamics of positive ions, negative ions, and isothermal electrons along with Maxwell's equations. The damped Korteweg de Vries equation is derived by employing the reductive perturbation technique and its time dependent solution is presented. Dip magnetosonic solitary structures are observed when both ions are positive and hump structures are seen in the presence of negative ions. The effects of variations of different plasma parameters on magnetosonic solitary structures in the presence of collisions are discussed.

Ion-acoustic shocks in magnetized quantum plasmas with relative density effects of spin-up and spin-down degenerate electrons

Ion-acoustic shock wave propagation in dense magnetized plasmas with relative density effects of spin-up and spin-down degenerate electrons is studied. The ions are classical, and their dissipative effects on plasma dynamics are included via kinematic viscosity. The electrons with spin-up and spin-down states are taken as separate species. The quantum tunneling effects of electrons are also considered in equations of motions of electrons. The Korteweg de Vries Burgers (KdVB) equation is derived, which admits the shock solution. The KdVB equation is solved numerically to study the transition from shock with oscillatory trails at its wave fronts to the monotonic shock structure with respect to variations in different plasma parameters. The parametric role of the spin density polarization ratio in the propagation characteristics of the shock wave structure is discussed.

Enhanced understanding of non-axisymmetric intrinsic and controlled field impacts in tokamaks

An extensive study of intrinsic and controlled non-axisymmetric field (δB) impacts in KSTAR has enhanced the understanding about non-axisymmetric field physics and its implications, in particular, on resonant magnetic perturbation (RMP) physics and power threshold (Pth) for L–H transition. The n = 1 intrinsic non-axisymmetric field in KSTAR was measured to remain as low as δB/B0 ~ 4 × 10−5 even at high-beta plasmas (βN ~ 2), which corresponds to approximately 20% below the targeted ITER tolerance level. As for the RMP edge-localized-modes (ELM) control, robust n = 1 RMP ELM-crash-suppression has been not only sustained for more than ~90 τE, but also confirmed to be compatible with rotating RMP. An optimal window of radial position of lower X-point (i.e. Rx = m) proved to be quite critical to reach full n = 1 RMP-driven ELM-crash-suppression, while a constraint of the safety factor could be relaxed (q95 = 5 0.25). A more encouraging finding was that even when Rx cannot be positioned in the optimal window, another systematic scan in the vicinity of the previously optimal Rx allows for a new optimal window with relatively small variations of plasma parameters. Also, we have addressed the importance of optimal phasing (i.e. toroidal phase difference between adjacent rows) for n = 1 RMP-driven ELM control, consistent with an ideal plasma response modeling which could predict phasing-dependent ELM suppression windows. In support of ITER RMP study, intentionally misaligned RMPs have been found to be quite effective during ELM-mitigation stage in lowering the peaks of divertor heat flux, as well as in broadening the ‘wet’ areas. Besides, a systematic survey of Pth dependence on non-axisymmetric field has revealed the potential limit of the merit of low intrinsic non-axisymmetry. Considering that the ITER RMP coils are composed of 3-rows, just like in KSTAR, further 3D physics study in KSTAR is expected to help us minimize the uncertainties of the ITER RMP coils, as well as establish an optimal 3D configuration for ITER and future reactors.

Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES)

The floating potential variability of the Electric Field Detector (EFD) probes, on board the Chinese Seismo-Electromagnetic Satellite (CSES), has been modeled, and the effects of several structural and environmental elements have been determined. The expected floating potentials of the probes are computed considering the ambient ionospheric plasma parameter variations. In addition, the ion collection variability, due to the different probe attitudes along the orbit, and its effect on each floating potential, are considered. Particular attention is given to the analysis of the shadow produced by the stubs, in order to determine the artificial electric field introduced by instrumental effects which has to be subtracted from the real measurements. The modulation of the altered electric field, due to the effect on shadowing of the ion drift, as measured by the ESA satellite Swarm A in a similar orbit, is also modeled. Such simulations are made in preparation of real EFD data analysis performed during the upcoming flight of CSES.

Three-dimensional Features of the Outer Heliosphere Due to Coupling between the Interstellar and Heliospheric Magnetic Field. V. The Bow Wave, Heliospheric Boundary Layer, Instabilities, and Magnetic Reconnection

Abstract The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW–LISM interaction. By performing three-dimensional, MHD plasma/kinetic neutral atom simulations, we determine the width of the outer heliosheath—the LISM plasma region affected by the presence of the heliosphere—and analyze quantitatively the distributions in front of the heliopause. It is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. We also demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by Interstellar Boundary Explorer.

Damped electrostatic structures in quantum plasmas

In this work, we study the damped nonlinear solitary wave structures in electron ion dense collisional plasmas in the presence of exchange correlation potential. Due to high density and low temperature, these plasmas are considered as quantum plasmas. The quantum mechanical effects due to quantum statistical pressure, quantum tunnelling, and exchange correlation due to 1/2 spin of Fermions are included in a quantum hydrodynamic model. The collisions of plasmas particles with neutrals are taken into account to derive the Damped Korteweg-de Vries equation. A reductive perturbation technique is performed to study nonlinearities and dispersive effects in the plasma system. The comparative importance of the potential due to the degenerate pressure, exchange correlation potential, and the Bohm potential in the linear and nonlinear dispersion is presented. The effects of variations of different plasma parameters on propagation characteristics of damped oscillations in the context of astrophysical objects like neutron stars/pulsar are discussed.

Hydrogen Plasma Characterization at Low Pressure in 2.45 GHz Electron Cyclotron Resonance Proton Ion Source

The plasma parameters in the 2.45 GHz electron cyclotron resonance (ECR) proton ion source have been measured and optimized at low pressures. These parameters are important as they are related to the beam parameters of the ion source. A detailed plasma characterization study has been conducted using hydrogen as a working gas in the operating pressure and power range $10^{-4}$ – $10^{-3}$ mbar, 400–1000 W of the ion source, respectively. The plasma parameters have been measured in the ECR zone of the plasma chamber. An automatic Langmuir probe and data acquisition system has been used to measure these parameters. In the above-mentioned power and pressure range, the plasma density, plasma electron temperature, and plasma potential measured are in the range $4 \times 10^{11}$ – $6.5 \times 10^{11}$ cm $^{-3}$ , 4–11 eV, and 30–40 V, respectively. The variation of plasma parameters with neutral gas pressure is studied for different microwave powers. A 0-D (global) model has been developed to compare the measured plasma parameter data with the calculated ones. The ion species content in the plasma was measured after extraction using a magnetic mass analyzer and a Faraday cup. A plasma-based model has been developed to compare the measured ion species fraction data with the calculated ones by taking the measured ECR plasma parameters as input.

Magnetosonic wave in pair-ion electron collisional plasmas

Low frequency magnetosonic waves in positive and negative ions of equal mass and opposite charges in the presence of electrons in collisional plasmas are studied. The collisions of ions and electrons with neutrals are taken into account. The nonlinearities in the plasma system arise due to ion and electrons flux, Lorentz forces, and plasma current densities. The reductive perturbation method is applied to derive the Damped Korteweg de Vries (DKdV) equation. The time dependent solution of DKdV is presented. The effects of variations of different plasma parameters on propagation characteristics of magnetosonic waves in pair-ion electron plasma in the context of laboratory plasmas are discussed.

A statistical study on hot flow anomaly current sheets

AbstractHot flow anomalies (HFAs) are phenomena frequently observed near Earth's bow shock and form when the interplanetary discontinuities interact with Earth's bow shock. We perform a statistical study to determine what kind of discontinuities are more efficient to generate HFAs. We use strict criteria to identify classic HFAs, excluding similar foreshock phenomena such as spontaneous hot flow anomalies (SHFAs) and foreshock bubbles. Our results show that magnetic field on at least one side of the interplanetary discontinuities has to be connected to the bow shock in order to form HFAs. Discontinuities with large shear angles are more efficient to form HFAs. The thickness of current sheets and the thickness of HFAs are strongly correlated and current sheets with thickness from 1000 km to 3162 km are more efficient to form HFAs. Of the HFAs, 74% have the electric field pointing toward the current sheet on the leading side and 72% have the electric field pointing toward the current sheet on the trailing side. In addition, the variations of plasma parameters and the magnetic field of HFAs with E inward on both sides are more dramatic than those with E inward on only one side. An HFA is more likely to form when the reflected flow from the bow shock is along the discontinuity.

Obliquely propagating dust ion-acoustic solitary waves and double layers in multicomponent plasmas

This research work deals with the propagation characteristics of dust ion-acoustic solitary waves and double layers in a strongly magnetized and rotating plasma comprising of fluid ions, charged dust, superthermal electrons, and positrons. In small amplitude approximation, reductive perturbation technique is employed to derive the Korteweg-de Vries (KdV) equation, and its analytical solution is presented. The combined effects of variation of different plasma parameters like superthermality, dust concentration, magnetic field strength, and rotation of a plasma on the amplitude and width of dust ion-acoustic solitons are analyzed. Both positive and negative potential solitary waves occur in this dusty plasma system. The critical values of plasma parameters for which KdV model is not valid, are examined, and the modified KdV (mKdV) equation is derived. The existence regimes of mKdV solitons and double layers have also been investigated. Positive and negative potential double layers occur in the present plasma system.

Variations in plasma parameters and magnetic field upon magnetopause crossing at the main phase maximum of the magnetic storm of November 14, 2012

Measurements of the plasma parameters and magnetic field upon magnetopause crossings by the THEMIS-А satellite during the large magnetic storm of November 14, 2012, are analyzed. The main specific feature of this event is the magnetopause crossing at the time of the magnetic-storm maximum. An imbalance of total pressure on the magnetopause reaching up to ~40% has been observed. An abrupt turn of the magnetic field immediately on the magnetopause is recorded. Inside the magnetosphere, plasma motions have been observed, both along the magnetopause and inward, at velocities of ~100–300 km/s. Variations in geomagnetic parameters are analyzed before and after the crossing. It is shown that specific features of the observed crossing may be associated with a sharp change in the magnetospheric current systems during the magnetospheric substorm.

Improved ERO modelling for spectroscopy of physically and chemically assisted eroded beryllium from the JET-ILW

Physical and chemical assisted physical sputtering were characterised by the BeI and BeII line and BeD band emission in the observation chord measuring the sightline integrated emission in front of the inner beryllium limiter at the torus midplane. The 3D local transport and plasma-surface interaction Monte-Carlo modelling (ERO code [18]) is a key for the interpretation of the observations in the vicinity of the shaped solid Be limiter. The plasma parameter variation (density scan) in limiter regime has provided a useful material for the simulation benchmark. The improved background plasma parameters input, the new analytical expression for particle tracking in the sheath region and implementation of the BeD release into ERO has helped to clarify some deviations between modelling and experiments encountered in the previous studies [4,5]. Reproducing the observations provides additional confidence in our ‘ERO-min’ fit for the physical sputtering yields for the plasma-wetted areas based on simulated data.

Reflection and backscattering of microwaves under doubling of the plasma density and displacement of the gyroresonance region during electron cyclotron resonance heating of plasma in the l-2M stellarator

Reflection and backscattering of high-power (400 kW) gyrotron radiation creating and heating plasma at the second harmonic of the electronic cyclotron frequency in the L-2M stellarator have been investigated experimentally. The effect of the displacement of the gyroresonance region from the axis of the plasma column under doubling of the plasma density on the processes of reflection and backscattering of microwave radiation has been examined. A near doubling of short-wavelength (k⊥ ≈ 30 cm–1) turbulent density fluctuations squared is observed. The change in the energy confinement time under variations of plasma parameters and characteristics of short-wavelength turbulence is discussed. A discrepancy between the measured values of the reflection coefficient from the electron cyclotron resonance heating region and predictions of the one-dimensional model is revealed.

Anomalous behavior of ionospheric parameters above the Kamchatka peninsula before and during seismic activity

In the present work, searching for new methods of earthquake prediction, variations of ionospheric plasma parameters and ionospheric turbulence before and during seismic activity are studied applying complex radiophysical methods and a theoretical probabilistic approach. The analysis of radiophysical observations of some seismic events of February–March (28.02.–02.03.) 2013 shows, that K-layer formation (appearance of a corpuscular layer due to precipitation of particles from the radiation belts), Es- and F-spread effects as well as an increase of the critical foF2-frequency occurred about one day before earthquakes with a magnitude M ≥ 6.5. F2-spread was even observed at low geomagnetic activity. One seismic shock was preceded by an abnormal increase in the value of the critical frequency foF2, and about 2 h after the shock, foF2 was again well below the median value although a geomagnetic storm happened. Therefore, it may be concluded that, in future, variations of ionospheric plasma parameters and turbulence may be used as additional tool to determine the magnitude of preparing strong earthquakes. The propabilistic method which is applied in the present work is already described in (Bogdanov et al., J. Volcanology and Seismology, 4(6), 412–422, 2010). It allowed to detect a growth of seismic activity in the Kamchatka region in 2013.

Study on the effect of hydrogen addition on the variation of plasma parameters of argon-oxygen magnetron glow discharge for synthesis of TiO2 films

We report the effect of hydrogen addition on plasma parameters of argon-oxygen magnetron glow discharge plasma in the synthesis of H-doped TiO2 films. The parameters of the hydrogen-added Ar/O2 plasma influence the properties and the structural phases of the deposited TiO2 film. Therefore, the variation of plasma parameters such as electron temperature (Te), electron density (ne), ion density (ni), degree of ionization of Ar and degree of dissociation of H2 as a function of hydrogen content in the discharge is studied. Langmuir probe and Optical emission spectroscopy are used to characterize the plasma. On the basis of the different reactions in the gas phase of the magnetron discharge, the variation of plasma parameters and sputtering rate are explained. It is observed that the electron and heavy ion density decline with gradual addition of hydrogen in the discharge. Hydrogen addition significantly changes the degree of ionization of Ar which influences the structural phases of the TiO2 film.