Variation Of Plasma Parameters Research Articles

Spatial dimensions of the electron diffusion region in anti-parallel magnetic reconnection

Abstract. Spatial dimensions of the detailed structures of the electron diffusion region in anti-parallel magnetic reconnection were analyzed based on two-dimensional fully kinetic particle-in-cell simulations. The electron diffusion region in this study is defined as the region where the positive reconnection electric field is sustained by the electron inertial and non-gyrotropic pressure components. Past kinetic studies demonstrated that the dimensions of the whole electron diffusion region and the inner non-gyrotropic region are scaled by the electron inertial length de and the width of the electron meandering motion, respectively. In this study, we successfully obtained more precise scalings of the dimensions of these two regions than the previous studies by performing simulations with sufficiently small grid spacing (1∕16–1∕8 de) and a sufficient number of particles (800 particles cell−1 on average) under different conditions changing the ion-to-electron mass ratio, the background density and the electron βe (temperature). The obtained scalings are adequately supported by some theories considering spatial variations of field and plasma parameters within the diffusion region. In the reconnection inflow direction, the dimensions of both regions are proportional to de based on the background density. Both dimensions also depend on βe based on the background values, but the dependence in the inner region ( ∼ 0.375th power) is larger than the whole region (0.125th power) reflecting the orbits of meandering and accelerated electrons within the inner region. In the outflow direction, almost only the non-gyrotropic component sustains the positive reconnection electric field. The dimension of this single-scale diffusion region is proportional to the ion-electron hybrid inertial length (dide)1∕2 based on the background density and weakly depends on the background βe with the 0.25th power. These firm scalings allow us to predict observable dimensions in real space which are indeed in reasonable agreement with past in situ spacecraft observations in the Earth's magnetotail and have important implications for future observations with higher resolutions such as the NASA Magnetospheric Multiscale (MMS) mission.

Study of the Cosmic-Ray Modulation During the Passage of ICMEs and CIRs

We compare the cosmic-ray response to interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) during their passage in near-Earth space. We study the relative importance of various structures/features identified during the passage of the ICMEs and CIRs observed during Cycle 23 (1995 – 2009). The identified ICME structures are the shock front, the sheath, and the CME ejecta. We isolate the shock arrival time, the passage of the sheath region, the arrival of ejecta, and the end time of their passage. Similarly, we isolate the CIR arrival, the associated forward shock, the stream interface, and the reverse shock during the passage of a CIR. For the cosmic-ray intensity, we utilize the data from high counting rate neutron monitors. In addition to neutron monitor data, we utilize near-simultaneous and same time-resolution data of interplanetary plasma and field, namely the solar-wind velocity, the interplanetary magnetic field (IMF) vector, and its variance. Further, we also utilize some derived interplanetary parameters. We apply the method of the superposed-epoch analysis. As the plasma and field properties are different during the passage of different structures, both in ICMEs and CIRs, we systematically vary the epoch time in our superposed-epoch analysis one by one. In this way, we study the role and effects of each of the identified individual structures/features during the passage of the ICMEs and CIRs. Relating the properties of various structures and the corresponding variations in plasma and field parameters with changes of the cosmic-ray intensity, we identify the relative importance of the plasma/field parameters in influencing the amplitude and time profiles of the cosmic-ray intensity variations during the passage of the ICMEs and CIRs.

Evidence for breathing modes in direct current, pulsed, and high power impulse magnetron sputtering plasmas

We present evidence for breathing modes in magnetron sputtering plasmas: periodic axial variations of plasma parameters with characteristic frequencies between 10 and 100 kHz. A set of azimuthally distributed probes shows synchronous oscillations of the floating potential. They appear most clearly when considering the intermediate current regime in which the direction of azimuthal spoke motion changes. Breathing oscillations were found to be superimposed on azimuthal spoke motion. Depending on pressure and current, one can also find a regime of chaotic fluctuations and one of stable discharges, the latter at high current. A pressure-current phase diagram for the different situations is proposed.

Peculiarities of magnetic barrier formation for southward and northward directions of the IMF

AbstractThe magnetic barrier is a region which is formed in the course of the solar wind flow around the Earth's magnetosphere and is characterized by depleted plasma and an enhanced magnitude of the magnetic field. There is a general point of view that the magnetic barrier can persist only for the northward interplanetary magnetic field (IMF), whereas it is practically absent if the IMF is southward directed. We studied the presence of the magnetic barrier for low‐shear (predominantly northward IMF) and high‐shear (predominantly southward IMF) magnetopause conditions and analyzed 74 events of low‐latitude dayside magnetopause crossings by the Time History of Events and Macroscale Interactions during Substorms satellites. We used the superposed epoch analysis to study variations of magnetic field and plasma parameters near the magnetopause. Results of Phan et al. (1994) that the magnetic barrier is well pronounced for low‐shear magnetopauses and practically absent for high‐shear events have been confirmed by our analysis. However, 5 from 49 high‐shear events show clear signatures of a magnetic barrier. Hence, we conclude that the magnetic barrier is formed in the same manner for all direction of IMF but for high‐shear events it is destroyed by reconnection much faster than it accumulates. Ratio of the events with and without magnetic barrier signatures (5/44) is approximately equal to the ratio of characteristic times of disruption due to reconnection (1–2 min) and the formation of a magnetic barrier (10 min). The reduction of the magnetic field due to reconnection can be used to estimate reconnection rate which turns out to be rather high of the order of 0.3 in 19 from 31 cases.

Equivalent Circuit Model for Frequency-Selective Surfaces Embedded Within a Thick Plasma Layer

In this paper, an equivalent circuit model for a frequency-selective surface (FSS) embedded in a thick plasma layer is introduced. The plasma layer is a lossy and dispersive medium, and therefore, the equivalent model is not purely imaginary and consists of resistive elements as well as inductance and capacitance elements. First, a square patch FSS is simulated in a wide frequency range from 1 to 25 GHz by a full-wave electromagnetic software, and then all the circuit elements are calculated by genetic algorithm optimization to achieve reflection coefficient similar to the one obtained by simulation. It is shown that this model can mimic the simulated behavior with good approximation. Then values of different elements of the model through practical variations of cold plasma parameters are plotted, and the effects of electron density and collision frequency are investigated on circuit elements.

Nonlinear oscillatory and monotonic shocks in dense plasmas with ultra-relativistic degenerate electrons

In this paper we study the ion acoustic oscillatory and monotonic shocks in dissipative homogeneous magnetized plasmas. The dissipation in the plasma system is considered via kinematic viscosity of ions and quantum effects are included through degeneracy pressure of ultra-relativistic electrons. Korteweg de Vries Burgers (KdVB) equation is derived by using reductive perturbation method. Numerical and analytical solutions of KdVB equation are presented. The transition from oscillatory profile to monotonic shock are studied numerically at different values of kinematic viscosity. We also analyzed the effects of variations of different plasma parameters on the strength of the shock structure in dense plasmas. The relevance of the work to astrophysical plasma conditions such as in compact stars is also pointed out.

Case and statistical studies on the evolution of hot flow anomalies

AbstractHot flow anomalies (HFAs) frequently observed near Earth's bow shock are phenomena resulting from the interaction between interplanetary discontinuities and Earth's bow shock. We identify 199 HFA events using Cluster data from 2001 to 2010 and divide these events into four categories according to the dynamic pressure profile, namely, “−+,” “+−,” “M,” and “W” types, where the symbols describe the profile of the dynamic pressure variations. We present case studies to show the main characteristics of each type of HFAs. Normalized superposed epoch analyses show that variations of the magnetic field magnitude of −+ and +− type HFAs are more dramatic than those of M and W types. The statistical study shows that the occurrence percentage of mature HFAs in W type HFAs is higher than that of −+, +−, and M types. Superposed epoch analysis result shows that variations of plasma parameters and magnetic field of mature HFAs are more dramatic than those of young HFAs, except for temperature. M and W type HFAs may be the later evolution stages of −+ and +− type HFAs; on the other hand, four categories of HFAs may be due to the fact that the spacecraft crossed an HFA structure along different paths.

The solar cycle variations of plasma parameters and their correlations at topside ionosphere from DEMETER during 2005–2010

Based on the IAP (Instrument d’Analyse du Plasma) observing data onboard DEMETER satellite, the solar cycle variations in ion density (Ni), ion composition [O+,H+,He+] and ion temperature (Ti) were analyzed respectively in local daytime 10:30 and nighttime 22:30 during 2005–2010 during the 23rd/24th solar cycles, which would greatly contribute to the research of the topside ionospheric physics during this extremely low solar minimum. Based on analyzing the solar cycle variations of these plasma parameters, it was found that longitude-averaged Ni, [O+] and daytime Ti presented positive correlation with solar flux, while [H+], nighttime Ti and [He+] varied negatively to reach their maxima at the solar minimum. Furthermore, [O+], as the main composition at the altitude of DEMETER of 660km, showed typical seasonal variation and close relationship with sub-solar dynamics. Daytime [He+] was the most complex component, showing negative variation with solar activity at low latitudes over Southern Hemisphere and positive correlation at middle latitudes over Northern Hemisphere. The correlation of Ni and Ti in local daytime was dependent on season over two hemispheres, and the results illustrated negative correlation at low latitudes, and positive correlation at middle latitudes especially over Southern Hemisphere in solstitial seasons. While [H+] presented anti-correlation, especially the peak values with Ti valleys at 0–20°N in each year, which all might be related to the thermal diffusion and relative drift among different ions. Furthermore, the asymmetrical features of Ni over two hemispheres in solar minimum were widely revealed at different longitude sectors, latitudes and local times, which might be explained by the concentration of neutral oxygen, neutral winds, and also the lower atmospheric dynamics.

Flow structure and shear generation in the 3D SOL of a circular tokamak plasma in Aditya

The presence of a poloidal ring limiter in the tokamak Aditya, which has a circular plasma, presents an interesting scrape-off layer (SOL) plasma configuration where small to large scale spatial variations of plasma parameters are expected. Flow shears in the Aditya SOL are routinely observed and their strong correlations with experimentally measured edge fluctuations in the floating potential and SOL currents/drifts have been thoroughly investigated in the recent past. The first implementation of the three-dimensional (3D) SOL plasma transport simulation code EMC3-EIRENE allows the possibility of investigating the SOL's complexity and associated short and long scale spatial patterns in the flow and plasma transport, and their impact on the confinement properties of the plasma in the edge region. A detailed investigation of the first results from the 3D plasma transport simulations shows interesting 3D flow patterns and the existence of resulting parallel and radial flow shears in the device's SOL. In this work the origin of these complex flow patterns is studied and their correlation with the observed behavior of the flows in the device is discussed.

On the pressure effect in energetic deposition of Cu thin films by modulated pulsed power magnetron sputtering: A global plasma model and experiments

The modulated pulsed power magnetron sputtering (MPPMS) discharge processes are numerically modeled and experimentally investigated, in order to explore the effect of the pressure on MPPMS discharges as well as on the microstructure of the deposited thin films. A global plasma model has been developed based on a volume-averaged global description of the ionization region, considering the loss of electrons by cross-B diffusion. The temporal variations of internal plasma parameters at different pressures from 0.1 to 0.7 Pa are obtained by fitting the model to duplicate the experimental discharge data, and Cu thin films are deposited by MPPMS at the corresponding pressures. The surface morphology, grain size and orientation, and microstructure of the deposited thin films are investigated by scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. By increasing the pressure from 0.1 to 0.7 Pa, both the ion bombardment energy and substrate temperature which are estimated by the modeled plasma parameters decrease, corresponding to the observed transition of the deposited thin films from a void free structure with a wide distribution of grain size (zone T) into an underdense structure with a fine fiber texture (zone 1) in the extended structure zone diagram (SZD). The microstructure and texture transition of Cu thin films are well-explained by the extended SZD, suggesting that the primary plasma processes are properly incorporated in the model. The results contribute to the understanding of the characteristics of MPPMS discharges, as well as its correlation with the microstructure and texture of deposited Cu thin films.

Investigation in the synthesis process of plasma polymerisation: new polystyrene

Radiofrequency inductively coupled plasma is used to form new polystyrene. In order to gain insights into the process of radiofrequency styrene plasma polymerisation, the gas phase plasmas were investigated by optical emission spectroscopy. The pulsed plasma-deposited films were characterised by transfer Fourier infrared spectroscopy and X-ray photoelectronic spectroscopy. With the variation of different plasma parameters, characters of plasma polymer film vary and parameters of plasma have a certain tendency. The optical emission spectra acquired in the plasma gas showed the variation of specie intensity with the plasma parameters. In this paper, the correlation between the plasma gas and the character of the deposited films was discussed, and the functional thin film was carried out by the parameters of plasma.

Morphological and spectroscopic characterization of laser-ablated tungsten at various laser irradiances

The variation in surface morphology and plasma parameters of laser irradiated tungsten has been investigated as a function of irradiance. For this purpose, Nd:YAG laser (1064 nm, 10 ns, 10 Hz) is employed. Tungsten targets were exposed to various laser irradiances ranging from 6 to 50 GW/cm2 under ambient environment of argon at a pressure of 20 Torr. Scanning electron microscope analysis has been performed to analyze the surface modification of irradiated tungsten. It revealed the formation of micro- and nanoscale surface structures. In central ablated area, distinct grains and crack formation are observed, whereas peripheral ablated areas are dominated by cones and pinhole formation. It was observed that at irradiances exceeding a value of 13 GW/cm2, the morphological trend of the observed structures has been changed from erosion to melting and re-deposition dominant phase. Ablation efficiency as a function of laser irradiance has also been investigated by measuring the crater depth using surface profilometry analysis. It is found to be maximum at an irradiance of 13 GW/cm2 and decreases at high laser irradiances. In order to correlate the accumulated effects of plasma parameters with the surface modification, laser-induced breakdown spectroscopy analysis has been performed. The electron temperature and number density of tungsten plasma have been evaluated at various laser irradiances. Initially with the increase of the laser irradiance up to 13 GW/cm2, an increasing trend is observed for both plasma parameters due to enhanced energy deposition. Afterward, a decreasing trend is achieved which is attributed to the shielding effect. With further increase in irradiance, a saturation stage comes and insignificant changes are observed in plasma parameters. This saturation is explainable on the basis of the formation of a self-regulating regime near the target surface. Surface modifications of laser irradiated tungsten have been correlated with plasma parameters.

Tungsten dust impact on ITER-like plasma edge

The impact of tungsten dust originating from divertor plates on the performance of edge plasma in ITER-like discharge is evaluated using computer modeling with the coupled dust-plasma transport code DUSTT-UEDGE. Different dust injection parameters, including dust size and mass injection rates, are surveyed. It is found that tungsten dust injection with rates as low as a few mg/s can lead to dangerously high tungsten impurity concentrations in the plasma core. Dust injections with rates of a few tens of mg/s are shown to have a significant effect on edge plasma parameters and dynamics in ITER scale tokamaks. The large impact of certain phenomena, such as dust shielding by an ablation cloud and the thermal force on tungsten ions, on dust/impurity transport in edge plasma and consequently on core tungsten contamination level is demonstrated. It is also found that high-Z impurities provided by dust can induce macroscopic self-sustained plasma oscillations in plasma edge leading to large temporal variations of edge plasma parameters and heat load to divertor target plates.

Magneto-seismological insights into the penumbral chromosphere and evidence for wave damping in spicules

The observation of propagating magneto-hydrodynamic kink waves in magnetic structures and measurement of their properties (amplitude, phase speed) can be used to diagnose the plasma conditions in the neighbourhood of the magnetic structure via magneto-seismology (MS). We aim to reveal properties of the chromosphere/Transition Region above the sunspot penumbra using this technique. Hinode observed a sunspot as it was crossing the limb, providing a unique side on view of the sunspot atmosphere. The presence of large spicule-like jets is evident in \ion{Ca}{II} H images. The jets are found to support transverse wave motions that displace the central axis, which can be interpreted as a kink wave. The properties of a wave event are measured and used to determine the magnetic and density stratification along the structure. We also measure the width of the spicule and the intensity profile along the structure. The measured wave properties reveal an initial rapid increase in amplitude with height above the solar surface, followed by a decrease in amplitude. The MS inversion suggests this initial increase corresponds to large changes in density and magnetic field strength. In addition, we provide the first measurements of spicule width with height, which confirm that the spicule under goes rapid expansion. The measured expansion shows good agreement with the results from the MS. The observed variations in plasma parameters are suggested to be partly due to the presence of a gravitational stratified, ambient atmosphere. Combining width measurements with phase speed measurements implies the observed decrease in wave amplitude at greater heights can be explained by wave damping. Hence, we provide the first direct evidence of wave damping in chromospheric spicules and the quality factor of the damping is found to be significantly smaller than estimated coronal values.

Optical Layout and Alignment Methods for Visible Tomography and Emission Spectroscopy Diagnostics in SPIDER

SPIDER, a prototype negative ion source with low voltage extraction for the ITER neutral beam injector will be equipped with a set of diagnostics to qualify and optimize the neutral beam for ITER. In particular, the visible tomography diagnostic will estimate the uniformity of the beam using a set of linear charge-coupled device cameras, while emission spectroscopy will measure the spatial variation of plasma parameters in the RF plasma source and evaluate the divergence of the extracted beam, analyzing the spectrum collected over multiple lines of sight (LOS). This paper presents the optical layout of the LOS and detection heads of tomography and spectroscopy and the required accuracy in their alignment, proposing also alignment criteria.

Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS

We derive the physical properties at the base of the jet from DG Tau both along and across the flow and as a function of velocity. We analysed seven optical spectra of the DG Tau jet, taken with the Hubble Space Telescope Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel to the jet axis and stepping it across the jet width. The resulting position-velocity diagrams in optical forbidden emission lines allowed access to plasma conditions via calculation of emission line ratios. We find at the base of the jet high electron density, $n_e \sim $ 10$^5$, and very low ionisation, $x_e \sim 0.02-0.05$, which combine to give a total density up to $n_H \sim $ 3 10$^6$. This analysis confirms previous reports of variations in plasma parameters along the jet, (i.e. decrease in density by several orders of magnitude, increase of $x_e$ from 0.05 to a plateau at 0.7 downstream at 2$''$ from the star). Furthermore, a spatial coincidence is revealed between sharp gradients in the total density and supersonic velocity jumps. This strongly suggests that the emission is caused by shock excitation. The position-velocity diagrams indicate the presence of both fast accelerating gas and slower, less collimated material. We derive the mass outflow rate, $\dot{M}_j$, in the blue-shifted lobe in different velocity channels, that contribute to a total of $\dot{M}_j \sim$ 8 $\pm$ 4 10$^{-9}$ M$_\odot$ yr$^{-1}$. We estimate that a symmetric bipolar jet would transport at the low and intermediate velocities probed by rotation measurements, an angular momentum flux of $\dot{L}_j \sim$ 2.9 $\pm$ 1.5 10$^{-6}$ M$_\odot$ yr$^{-1}$ AU km s$^{-1}$. The derived properties of the DG Tau jet are demonstrated to be consistent with magneto-centrifugal theory. However, non-stationary modelling is required in order to explain all of the features revealed at high resolution.

Irregularities of the ionospheric plasma and the ULF electric components obtained from DEMETER satellite experiments above Chile earthquake (27 February 2010)

The detection of ionospheric disturbances associated with seismic activity is one of the main purposes of the Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER) micro-satellite. In this view, this study attempts to analyze the problem of detecting these disturbances before an earthquake occurrence and to discuss the Chile earthquake case study. Using ultralow frequency (ULF) measurements of Instrument Champ Electrique (ICE) experiments on board the DEMETER satellite, possible irregularities in this electric component have been investigated in the vicinity of Chile (27 February 2010) earthquake region. The data used in this paper cover the period from 10 to 26 February 2010. The anomalous variations in the calculated ULF electric component in the direction Z of satellite were clearly observed on the 17th, 11th, and 1st days preceding the earthquake. It is noted that the electric component obviously strongly decreased prior to the earthquake. These unusual variations have also been observed in the variation of several plasma parameters recorded by DEMETER directly above the epicenter during the whole period. Hence, the detected anomalies resulting from the ICE ULF waveforms in quiet geomagnetic conditions could be regarded as seismo-ionospheric precursors.

Study on formation process of honeycomb pattern in dielectric barrier discharge by optical emission spectrum

The authors report on the first investigation of the variations in the plasma parameters in the formation process of the honeycomb pattern in a dielectric barrier discharge by optical emission spectrum in argon and air mixture. The discharge undergoes hexagonal lattice, concentric spot-ring pattern and honeycomb pattern with the applied voltage increasing. The molecular vibration temperature, electron excitation temperature and electronic density of the three kinds of patterns were investigated by the emission spectra of nitrogen band of second positive system (C3pi(u) --> B3 pi(g)), the relative intensity ratio method of spectral lines of Ar I 763.51 nm (2P(6) --> 1S(5)) and Ar I 772.42 nm (2P(2) -->1S(3)) and the broadening of spectral line 696.5 nm respectively. It was found that the molecular vibration temperature and electron excitation temperature of the honeycomb pattern are higher than those of the hexagonal lattice, but the electron density of the former is lower than that of the latter. The discharge powers of the patterns were also measured with the capacitance method. The discharge power of the honeycomb pattern is much higher than that of the hexagonal lattice. These results are of great importance to the formation mechanism of the patterns in dielectric barrier discharge.

KP Burgers shocks in a warm electronegative plasma with q-nonextensive distributed electrons

Nonlinear properties of ion-acoustic (IA) shock are studied by incorporating the effects of electron nonextensivity in a warm electronegative plasma, whose constituents are the inertial positive/negative ions and q-distributed electrons. For this purpose, the evolution equations are solved to obtain Kadomtsev-Petviashvili (KP) Burgers equation by using the reductive perturbation technique and its solution by the tanh method. Furthermore, the conditions for the existence of oscillatory and monotonic shocks are discussed. Numerically, it is found that IA shock propagation characteristics are significantly modified by the variation of plasma parameters, such as, the effects of electron nonextensivity, the positive and negative ion-to-electron temperature ratios (θi,θn), respectively. The former also affect the dispersion, dissipation and nonlinearity coefficients of the KPB equation involving the IA shocks. The present analyses could be useful for understanding the nonlinear shock wave excitations in space and laboratory plasmas, where two distinct groups of ions are present.

Temperature effect on the optical emission intensity in laser induced breakdown spectroscopy of super alloys

In this paper, the influence of heating and cooling samples on the optical emission spectra and plasma parameters of laser-induced breakdown spectroscopy for Titanium 64, Inconel 718 super alloys, and Aluminum 6061 alloy is investigated. Samples are uniformly heated up to approximately 200°C and cooled down to -78°C by an external heater and liquid nitrogen, respectively. Variations of plasma parameters like electron temperature and electron density with sample temperature are determined by using Boltzmann plot and Stark broadening methods, respectively. Heating the samples improves LIBS signal strength and broadens the width of the spectrum. On the other hand, cooling alloys causes fluctuations in the LIBS signal and decrease it to some extent, and some of the spectral peaks diminish. In addition, our results show that electron temperature and electron density depend on the sample temperature variations.