Region Of Plasma Density Research Articles

Argon metastables in HiPIMS: time-resolved tunable diode-laser diagnostics

Time-resolved tunable diode-laser absorption spectroscopy measurements were performed on the argon metastable (Arm) level 3s23p54s excited at 801.478 nm, in the dense plasma region in front of the magnetron target in a high power impulse magnetron sputtering (HiPIMS) discharge. From the Doppler profile the evolution of the temperature and density was derived during the pulse as well as during the plasma decay, i.e. in the afterglow. It is shown that the Arm density sharply increases at the beginning of the discharge pulse, followed by a severe Arm depletion along with increasing gas temperature around the peak of the HiPIMS discharge current. The strong dynamics of these parameters involve many elementary processes such as electron-impact excitation, electron-impact de-excitation and ionization of Arm, gas rarefaction, electron temperature increase at the end of the pulse and gas diffusion. These phenomena are discussed with respect to several parameters: distance from the target, peak discharge current during the pulse, pulse length, and gas pressure.

Global observations of E region plasma density morphology and variability

The global morphology and variability of the ionospheric E region plasma density are estimated from satellite‐based radio occultation total electron content (ROTEC) measurements. Vertical profiles of E region electron density are estimated using the inversion technique recently proposed by Nicolls et al. (2009). In this technique, the F‐region contribution to each ROTEC measurement is removed using an assimilative model of the ionosphere in order to mitigate the effects of F‐region gradients in the estimation of E region profiles. The technique is applied to occultation observations made by GPS receivers onboard COSMIC satellites aided by F‐region electron density specification provided by the Ionospheric Data Assimilation Four‐Dimensional (IDA4D) algorithm. Global estimates of hmE, NmE, and E region total electron content (TEC) are presented for two different months: April 2007 and January 2008. Results of our analysis show that ROTEC measurements such as those provided by the COSMIC constellation can produce reasonable and valuable estimates of E region parameters on a global scale when properly treated for the effect of F‐region density gradients. The agreement between the α‐Chapman theory of ionization and recombination and estimated profiles is demonstrated. Reasonable estimates of E region variability can also be specified by the global measurements. The dependence of the E region variability on latitude is quantified and presented.

Magnetic bubbles and magnetic towers - I. General properties and simple analytical models

We consider magnetostatic equilibria in which a bounded region D containing a magnetized plasma is either fully confined by a field-free external medium – magnetic bubble equilibria (MBEqs) – or is confined by both such a medium and line-tying in a dense plasma region – magnetic tower equilibria (MTEqs). We first establish some of their general properties. In particular, we derive a series of useful integral equalities relating the magnetic field and the thermal pressures inside and outside D, respectively. We use them to prove the non-existence of an axisymmetric MBEq with a purely poloidal field, and to discuss some recent results of Braithwaite on MBEq formation by relaxation from an initial non-equilibrium state. We next present two families of exact analytical axisymmetric MBEqs with, respectively, spherical and toroidal shapes. The first family is extracted from Prendergast’s model of a self-gravitating magnetized body, while the second one is constructed by using Palumbo’s theory of isodynamic equilibria, for which both magnetic and thermal pressures take constant values on any flux surface. MTEqs with a large variety of structures are thus obtained in a simple way: we start from an arbitrary MBEq and just consider the part of it above a given plane cutting the bubble D. For MBEqs and MTEqs in either family, we compute in closed form most of the interesting physical quantities (such as energy, magnetic helicity and twist). Our results are expected to be useful for building up simple models of several astrophysical objects (such as X-ray cavities in the intracluster medium, jets emitted by disc accreting compact objects, eruptive events in stellar coronae and their ejecta).

Measurements and simulations of shock wave generated plasma-vacuum interface

A controlled gradient gas jet was designed, constructed, and tested at the Naval Research Laboratory for the generation of high density and sharp gradient plasma regions. The gas jet uses a laser-generated shock wave to control the density gradient at the vacuum and neutral gas interface. The length scale of the laser produced plasma density gradient is fully controlled by the strength of the shock wave and can be varied continuously from100 μm for a weak shock to under 20 μm in case of strong shock wave as verified by the experimental results and simulations.

Equinoctial asymmetry of ionospheric vertical plasma drifts and its effect onF-region plasma density

[1] The equinoctial asymmetry of the ionospheric vertical E × B plasma drift velocity (V⊥) in the equatorial F region is investigated based on observations from ROCSAT-1 during 1999 to 2004. It is found that the observed asymmetry exhibits obvious local time dependence with three noticeable features. First, in the Eastern Hemisphere during the interval between 0900 and 1300 LT, V⊥ is obviously higher at the northern spring equinox (March–April) than at the autumn equinox (September–October). Second, there is a pronounced asymmetry for wave number 4 longitudinal structures of V⊥, which exist almost throughout the daytime and even extend into the evening sector at autumn equinox, while they appear mainly at noon and prenoon at spring equinox. Third, around sunset we find an obvious asymmetry of the prereversal enhancement (PRE); the maximum PRE velocity is higher at autumn than at spring equinox in the longitude range from 320° to 150°, and the opposite situation occurs at other longitudes. On the basis of the drift observation mentioned, we also simulate the effect of the V⊥ asymmetry on the ionospheric plasma density by running the Theoretical Ionospheric Model of the Earth, Institute of Geology and Geophysics, Chinese Academy of Sciences (TIME-IGGCAS). It is found that the daytime V⊥ asymmetry can partly explain the equinoctial asymmetry in daytime low-latitude ionospheric plasma density observed by L. Liu et al. (J. Geophys. Res., 115, A09307).

Characteristics of emission spectroscopyof radiatively heated Al plasma

The X-ray spectrum of radiatively heated tracer Al in the doped foam was recorded by using a flat crystal spectrometer. The radiation thermal bath with a peak temperature of 150 eV is created inside the hohlraum irradiated by 8 laser beams, each delivering 250J. From the measured spectrum, we have observed that the intensities of jkl Li-like satellite were smaller than those of qr Li-like satellites, which is not a case usually encountered in electron collision dominated low density plasmas. The analyses of various generation and loss channels of doubly excited states indicate that in the radiatively heated plasma considered in the present work, the photo excitation and the photo ionization processes are fully responsible for the generation of doubly excited states instead of dielectronic recombination. Besides the satellite features, intercombination lines are also observed which imply the existence of a low density plasma region. Finally, we compare the calculated spectrum with the measured spectrum, and find that they are in fairly good agreement was achiered.

Survey and prediction of the ionospheric scintillation using data mining techniques

[1] Irregularly structured ionospheric regions may cause amplitude and phase fluctuations of radio signals. Such distortion is called ionospheric scintillation. These ionospheric irregularities occur as part of depleted plasma density regions that are generated at the magnetic equator after sunset by equatorial ionospheric plasma instability mechanism. Also known as ionospheric bubbles, they drift upward to high altitudes at the equator and extend/expand to low latitudes along the Earth magnetic field lines. Ionospheric irregularities affect the space weather since they present large variations with the solar cycle and during solar flares and coronal mass ejections. In general, navigation systems such as the Global Positioning System and telecommunications systems are also affected by the scintillation. The aim of this work is to apply data mining for the prediction of ionospheric scintillation. Data mining can be divided into two categories: descriptive or predictive. The first one describes a data set in a concise and summarized way, while the second one, used in this work, analyzes the data to build a model and tries to predict the behavior of a new data set. In this study we employed data series of ionospheric scintillation and other parameters such as the level of solar activity, vertical drift velocity of the plasma at the magnetic equator, and magnetic activity. The results show that prediction of the ionospheric scintillation occurrence during the analyzed period was possible regardless of the high variability of the ionospheric parameters that affect the generation of such irregularities.

High-latitude ionospheric response to co-rotating interaction region- and coronal mass ejection-driven geomagnetic storms revealed by GPS tomography and ionosondes

Positive ionospheric anomalies induced in the polar cap region by co-rotating interaction region (CIR)- and coronal mass ejection (CME)-driven geomagnetic storms are analysed using four-dimensional tomographic reconstructions of the ionospheric plasma density based on measurements of the total electron content along ray paths of GPS signals. The results of GPS tomography are compared with ground-based observations of F region plasma density by digital ionosondes located in the Canadian Arctic. It is demonstrated that CIR- and CME-driven storms can produce large-scale polar cap anomalies of similar morphology in the form of the tongue of ionization (TOI) that appears on the poleward edge of the mid-latitude dayside storm-enhanced densities in positive ionospheric storms. The CIR-driven event of 14–16 October 2002 was able to produce ionospheric anomalies (TOI) comparable to those produced by the CME-driven storms of greater Dst magnitude. From the comparison of tomographic reconstructions and ionosonde data with solar wind measurements, it appears that the formation of large-scale polar cap anomalies is controlled by the orientation of the interplanetary magnetic field (IMF) with the TOI forming during the periods of extended southward IMF under conditions of high solar wind velocity.

Nonlinear studies of fast electron current pulse propagation in a two dimensional inhomogeneous plasma

The evolution of fast current and magnetic field pulse structures through an inhomogeneous plasma medium was reported in recent publications [Yadav et al., Phys. Plasmas 15, 062308 (2008); Yadav et al., Phys. Plasmas 16, 040701 (2009)]. The evolution characteristics have been investigated here in further detail. Following specific issues have been addressed; (i) characterization of the phenomena of trapping versus transmission of the current pulse structures through a high density plasma region, (ii) interaction of the current pulse with plasma density inhomogeneity at various incidence angles, and (iii) destabilization of sharp sheared current layers resulting from the interaction with an elongated plasma density inhomogeneity. It is also illustrated that the destabilization of the current pulse trapped in an elongated high density plasma region forms a novel stable coherent nonlinear pattern of alternating signed vortices arranged as beads along the density profile.

Influence of midlatitude sporadic E layer patches upon the F region plasma density

Polarization electric fields of the Es patches on scales above about 10 km can map efficiently between the E and F regions. We propose a simple quantitative model in which the electrical coupling between the dense band‐like Es patch and the F region depends on the presence of field‐aligned closure currents and the orientation of the ambient effective electric field relative to the band and results in corresponding F region plasma density redistribution caused by E region polarization processes. Specifically, we show that for the representative ratio of Pedersen conductances inside and above the sporadic E patch and typical values of the ambient effective electric field, strong depletions in F layer plasma density can be produced by the patch's polarization electric field that maps up to the F region.

Non-LTE and gradient effects in K-shell oxygen emission laser-produced plasma

A recent effort [1] to develop a high-resolution spectroscopy capability for He-like and H-like ions to study far wing shapes from a laser-produced plasma experiment at the Lawrence Livermore National Laboratory found unexpected structure in the observed emission from a Mylar target. In this paper we propose that the observed features are due to exotic inner-shell transitions in mid-ionized oxygen, suggesting that the observed emission arises from two plasma regions: a hotter, less dense outer region gives rise to the observed He-like and H-like resonance lines, while a cooler, more dense plasma region closer to the target surface and in a region surrounding the laser spot generates the dielectronic satellites and the exotic inner-shell lines. Calculations using the Los Alamos suite of atomic physics codes and the plasma kinetics code ATOMIC are used to support this assertion.

A survey of optimal filament shape in a Japan Proton Accelerator Research Complex H− ion source

Dependence of various shapes of lanthanum hexaboride (LaB(6)) filaments on H(-) ion currents (I_H(-)'s) was examined by using the first Japan Proton Accelerator Research Complex (J-PARC) test ion source. It is almost the same with the J-PARC H(-) ion source (J-PARC-IS) except for the maximum arc current (290 A instead of 400 A). An I_H(-) of 35.2 mA was extracted by using a cylindrical double-spiral LaB(6) filament with a diameter of 29.5 mm and a length of 35.5 mm, which is the same one used in J-PARC-IS. It increased to 43.4 mA with a flat triple-hairpin LaB(6) filament. The I_H(-) is considered to be increased by the enlargement of the high density plasma region near the plasma electrode aperture and the reduction of the LaB(6) filament unemission area located in the high density plasma region.

Formation, Spatiotemporal Dynamics, and Control of Structures in Weakly Developed Drift Wave Turbulence

Experimental investigations of the evolution of the dynamics of drift wave turbulence in a linear high-density helicon plasma device are presented. The turbulent density fluctuations in the plasma edge are characterized by large intermittent events caused by radially propagating turbulent structures, which are formed in the radial plasma density region due to increased cross-field transport by a quasi-coherent drift wave mode. Similar to coherent drift wave modes the turbulent structures are correlated with fluctuating parallel currents. The role of fluctuating currents parallel to the ambient magnetic field in the evolution of coherent drift wave modes and drift turbulence is highlighted by investigations of the interaction of drift wave fluctuations with externally driven currents. Frequency pulling of coherent drift wave modes over a frequency range of up to 30% of the natural drift wave frequency is demonstrated. Furthermore, the drive of mode-selective current patterns allow for complete synchronization of drift wave turbulence and consequently leads to a strong reduction of the associated fluctuation-induced transport. c

Causal link of the wave‐4 structures in plasma density and vertical plasma drift in the low‐latitude ionosphere

We investigate the annual and local time variations of the wave‐4 structures in the plasma density and vertical drift in the low‐latitude F region by analyzing the measurements from the first Republic of China satellite (ROCSAT‐1) and conducting simulations with the Global Ionosphere and Plasmasphere (GIP) model. The GIP model uses apex magnetic coordinates with International Geomagnetic Reference Field (IGRF) for magnetic field, neutral wind from HWM‐07, and thermospheric parameters from the NRLMSISE‐00 model. In order to understand how the vertical drifts relate to the longitudinal structure of the topside ionosphere, we apply the equatorial vertical drifts observed from ROCSAT‐1 to drive the GIP model. The model well reproduces the longitudinal structure in electron density, and the magnitudes of electron density are comparable with ROCSAT‐1 measurement at 600 km. The ROCSAT‐1 observations of the vertical drift and plasma density show maximum amplitudes of their wave‐4 components in July–September and minimum amplitudes in December–February. An eastward shift of the wave‐4 components with increasing local time is observed in both the density and the vertical drift. The GIP model density showed similar annual and local time variations of the wave‐4 component. Since the model uses the observed equatorial vertical E × B drift as an input, the results indicate the vertical drifts are essential in the formation and evolution of the longitudinal wave‐4 density structure. The amplitude of the eastward propagating diurnal tide (DE3) at 110 km shows similar annual and local time variations as the F region parameters, supporting the link between the DE3 tide, vertical E × B drift, and F region plasma density on a global scale.

Evidence of anomalous resistivity for hot electron propagation through a dense fusion core in fast ignition experiments

Anomalous resistivity for hot electrons passing through a dense core plasma is studied for fast ignition laser fusion. The hot electrons generated via the ultra-intense laser pulse and guiding cone interactions are measured after they pass through a dense plasma with a density of 50–100 g cm−3 in a radius of 15–25 μm. When significant neutron enhancements are achieved by the ultra-intense laser pulse injection, the energy reduction of fast electrons is observed. Also, a reduction in the number of electrons with energy up to 15 MeV can be seen. We offer a new physical mechanism for the stopping of electrons, involving electron magnetohydrodynamic shock formation in the inhomogeneous plasma density region. The dissipation in the shock region can explain electron stopping with energies of the order of 15 MeV.

Mass- and energy-analyses of ions from plasma by means of a miniature Thomson spectrometer

The paper presents an improved version of a miniature mass-spectrometer of the Thomson-type, which has been adopted for ion analysis near the dense plasma region inside a vacuum chamber. Problems connected with the separation of ions from plasma streams are considered. Input diaphragms and pumping systems, needed to ensure good vacuum inside the analyzing region, are described. The application of the miniature Thomson-type analyzer is illustrated by ion parabolas recorded in plasma-focus facility and rod plasma injector experiment. A quantitative analysis of the recorded ion parabolas is presented. Factors influencing accuracy of the ion analysis are discussed and methods of the spectrometer calibration are described.

Self focusing of a laser pulse in plasma with periodic density ripple

AbstractPropagation of an intense laser pulse in plasma with a periodically modulated density is considered using envelope equations. The laser induces modifications of the plasma refractive indexviarelativistic and ponderomotive nonlinearities. In the region of high plasma density, the self focusing effect of nonlinearity suppresses the diffraction divergence, and the laser converges. As the beam enters into the low density region, the diffraction tends to diverge it offsetting the convergence due to the curvature it has acquired. For a given set of plasma parameters, there is a critical power of the laser above which it propagates in a periodically focused manner. Below this power the laser undergoes overall divergence. At substantially higher powers, the laser beam continues to converge until the saturation effect of nonlinearity suppresses the self focusing and diffraction predominates. The effect of density ripple is to cause overall increase in the self focusing length. The minimum spot size decreases with the wave number of the ripple.

RESONANT TRANSPARENCY OF A THREE-LAYER STRUCTURE CONTAINING THE DENSE PLASMA REGION

A study of electromagnetic wave propagation in dense plasmas when the wave frequency is below the cut-ofi frequency is presented. A three-layer symmetric structure consisting of dense plasma nested between two boundary layers is studied analytically and numerically. The permittivity of the dense plasma is negative, while the permittivity of each boundary layer is greater than 1. It is shown that total transmission of an electromagnetic wave can be achieved if an adequate incidence angle, dielectric permittivity of the boundary layers and corresponding boundary layer widths are chosen. It is found that plasma transparency is due to resonance between the evanescent waves in the dense plasma region and the standing waves in the boundary layers. Resonance conditions are derived analytically and the relationship between the corresponding parameters of the problem are studied numerically.

Nonlinear Propagation of a Short-Pulse Laser in a Plasma with Density Ripple

In recent experiments, density ripples have been used to achieve phase-matched harmonic generation from plasmas. We report a numerical study of nonlinear propagation of an intense short-pulse laser in a plasma containing a large-amplitude density ripple. The laser with a Gaussian distribution of the intensity along its wavefront undergoes periodic self focusing due to relativistic and ponderomotive nonlinearity. In the region of high plasma density, the self-focusing effect dominates over the diffraction effect and the laser converges. As the beam enters the low-density region, the diffraction effect supercedes the self-focusing effect and diverges the beam. For ripple periods shorter than the Rayleigh diffraction length, the beam width parameter has a minor wiggle as a function of the distance of propagation. When the ripple period is long, the wiggle in the beam width parameter becomes large.

Multipoint observations of magnetospheric compression‐related EMIC Pc1 waves by THEMIS and CARISMA

Following a long interval (many days) of sustained very quiet geomagnetic conditions, electromagnetic ion cyclotron (EMIC) wave activity was seen by the CARISMA array (www.carisma.ca) on the ground for several hours simultaneously with enhanced solar wind density and related magnetic compression seen at GOES 12 on 29th June 2007. The THEMIS C, D, and E satellites were outbound in a “string‐of‐pearls” configuration and each observed EMIC waves on L‐shells ranging from 5 to 6.5. THEMIS resolved some of the spatial‐temporal ambiguity and defined the radial extent of EMIC activity to be ∼1.3 Re. The band‐limited EMIC waves were seen slightly further out in radial distance by each subsequent THEMIS satellite, but in each case were bounded at high‐L by a decrease in density as monitored by spacecraft potential. The EMIC wave activity appears to be confined to a region of higher plasma density in the vicinity of the plasmapause, as verified by ground‐based cross‐phase analysis. The structured EMIC waves seen at THEMIS E and on the ground have the same repetition period, in contradiction to expectations from the bouncing wave packet hypothesis. Compression‐related EMIC waves are usually thought to be preferentially confined to higher L's than observed here. Our observations suggest solar wind density enhancements may also play a role in the excitation of radially localised EMIC waves near the plasmapause.