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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
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DOE OSTI2016

Nonlinear coherent structures of Alfvén wave in a collisional plasma

The Alfvén wave dynamics is investigated in the framework of two-fluid approach in a compressible collisional magnetized plasma. In the finite amplitude limit, the dynamics of the nonlinear Alfvén wave is found to be governed by a modified Korteweg-de Vries Burgers equation (mKdVB). In this mKdVB equation, the electron inertia is found to act as a source of dispersion, and the electron-ion collision serves as a dissipation. The collisional dissipation is eventually responsible for the Burgers term in mKdVB equation. In the long wavelength limit, this weakly nonlinear Alfvén wave is shown to be governed by a damped nonlinear Schrödinger equation. Furthermore, these nonlinear equations are analyzed by means of analytical calculation and numerical simulation to elucidate the various aspects of the phase-space dynamics of the nonlinear wave. Results reveal that nonlinear Alfvén wave exhibits the dissipation mediated shock, envelope, and breather like structures. Numerical simulations also predict the formation of dissipative Alfvénic rogue wave, giant breathers, and rogue wave holes. These results are discussed in the context of the space plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Perpendicular Subcritical Shock Structure in a Collisional Plasma Experiment

We present a study of perpendicular subcritical shocks in a collisional laboratory plasma. Shocks are produced by placing obstacles into the supermagnetosonic outflow from an inverse wire array z pinch. We demonstrate the existence of subcritical shocks in this regime and find that secondary shocks form in the downstream. Detailed measurements of the subcritical shock structure confirm the absence of a hydrodynamic jump. We calculate the classical (Spitzer) resistive diffusion length and show that it is approximately equal to the shock width. As a result, we measure little heating across the shock (<10 % of the ion kinetic energy) which is consistent with an absence of viscous dissipation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Charging of a conducting sphere in a weakly ionized collisional plasma: Temporal dynamics and stationary state

We consider the interaction of a isolated conducting sphere with a collisional weakly ionized plasma in an external field. We assume that the plasma consists of two species of ions neglecting of electrons. We take into account charging of the sphere due to sedimentation of plasma ions on it, the field of the sphere charge and the space charge, as well as recombination and molecular diffusion. The nonstationary problem of interaction of the sphere with the surrounding plasma is solved numerically. The temporal dynamics of the sphere charge and plasma perturbations is analyzed, as well as the properties of the stationary state. It is shown that the duration of transient period is determined by the recombination time and by the reverse conductivity of ions. The temporal dynamics of the sphere charge and plasma perturbations is determined by the intensity of recombination processes relative to the influence of the space charge field and diffusion. The stationary absolute value of the sphere charge increases linearly with the external electric field, decreases with the relative intensity of recombination processes and increases in the presence of substantial diffusion. The scales of the perturbed region in the plasma are determined by the radius of the sphere, the external field, the effect of diffusion, and the relative intensity of recombination processes. In the limiting case of the absence of molecular diffusion and a strong external field, the properties of the stationary state coincide with those obtained earlier as a result of approximate solution.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

On the Alfvén wave cut-off in partly ionized collisional plasmas

The cut-off of the Alfvén wave, caused by plasma collisions with neutrals in multi-component partially ionized plasmas, is discussed. Full multi-component theory is used, and similarities and differences regarding the classic magnetohydrodynamic theory are presented. It is shown that the cut-off in partially ionized plasma, in principle, may remain the same as predicted in classic magnetohydrodynamic works, although multi-component theory also yields some essential differences. Due to electric field, the ion motion is intrinsically two-dimensional and this results in additional forced oscillations of neutrals. One new small parameter, containing the ion inertial length, appears in the multi-component theory. This new small parameter is missing in the magnetohydrodynamic description, and it turns out that for some parameters it may be greater than the ions-to-neutrals density ratio which is the only small parameter in the magnetohydrodynamic description. Due to this the Alfvén wave behavior can become much different as compared to classic magnetohydrodynamic results. It is shown also that in plasmas with unmagnetized ions, Alfvén waves cannot be excited. This by all means applies to the solar photosphere where the ion collision frequency may be far above the ion gyro-frequency.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

THE STABILITY OF WEAKLY COLLISIONAL PLASMAS WITH THERMAL AND COMPOSITION GRADIENTS

Over the last decade, substantial efforts have been devoted to understanding the stability properties, transport phenomena, and long-term evolution of weakly collisional, magnetized plasmas which are stratified in temperature. The insights gained via these studies have led to a significant improvement of our understanding of the processes that determine the physical evolution and observational properties of the intracluster medium (ICM) permeating galaxy clusters. These studies have been carried out under the assumption that the ICM is a homogeneous medium. This, however, might not be a good approximation if heavy elements are able to sediment in the inner region of the galaxy cluster. Motivated by the need to obtain a more complete picture of the dynamical properties of the ICM, we analyze the stability of a weakly collisional, magnetized plane-parallel atmosphere which is stratified in both temperature and composition. This allows us to discuss for the first time the dynamics of weakly collisional environments where heat conduction, momentum transport, and ion-diffusion are anisotropic with respect to the direction of the magnetic field. We show that depending on the relative signs and magnitudes of the gradients in the temperature and the mean molecular weight, the plasma can be subject to a wide variety of unstable modes which include modifications to the magnetothermal instability (MTI), the heat-flux-driven buoyancy instability (HBI), and overstable gravity modes previously studied in homogeneous media. We also find that there are new modes which are driven by heat conduction and particle diffusion. We discuss the astrophysical implications of our findings for a representative galaxy cluster where helium has sedimented. Our findings suggest that the core insulation that results from the magnetic field configurations that arise as a natural consequence of the HBI, which would be MTI stable in a homogeneous medium, could be alleviated if the mean molecular weight gradient is steep enough, i.e., ({nabla}{mu})/{mu} > ({nabla}T)/T. This study constitutes a first step toward understanding the interaction between magnetic turbulence and the diffusion of heavy elements, and its consequences for the long-term evolution and observational signatures of the ICM in galaxy clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Drift wave dispersion relation for arbitrarily collisional plasma

The standard local linear analysis of drift waves in a plasma slab is generalized to be valid for arbitrarily collisional electrons by considering the electrons to be governed by the drift-kinetic equation with a BGK-like (Bhatnagar-Gross-Krook) collision operator. The obtained dispersion relation reduces to that found from collisionless kinetic theory when the collision frequency is zero. Electron temperature fluctuations must be retained in the standard fluid analysis in order to obtain good quantitative agreement with our general solution in the highly collisional limit. Any discrepancies between the fluid solution and our general solution in this limit are attributed to the limitations of the BGK collision operator. The maximum growth rates in both the collisional and collisionless limits are comparable and are both on the order of the fundamental drift wave frequency. The main role of the destabilizing mechanism is found to be in determining the parallel wave number at which the maximum growth rate will occur. The parallel wave number corresponding to the maximum growth rate is set by the wave-particle resonance condition in the collisionless limit and transitions to being set by the real frequency being on the order of the rate for electrons to diffuse a parallel wavelength in the collisional limit.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Fluctuations in collisional plasma in the presence of an external electric field

The theory of large-scale fluctuations in a plasma is used to calculate the correlations functions of electron and ion density with regard to particle collisions described within the Bhatnagar-Gross-Krook (BGK) model and the presence of a constant external electric field. The changes of plasma particle distribution functions due to an external electric field and their influence on the plasma dielectric response are taken into account. The dispersion relations for longitudinal waves in such a plasma are studied in details. It is shown that external electric field can lead to the ion-acoustic wave instability and anomalous growth of the fluctuation level. Detailed numerical studies of the general relations for electron number density fluctuations are performed and the effect of external electric field on the fluctuation spectra is studied.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Propagation of surface waves on a semi-bounded quantum magnetized collisional plasma

The propagation of surface waves on a semi-bounded quantum plasma in the presence of the external magnetic field and collisional effects is investigated by using quantum magnetohydrodynamics model. A general analytical expression for the dispersion relation of surface waves is obtained by considering the boundary conditions. It is shown that, in some special cases, the obtained dispersion relation reduces to the results reported in previous works. It is also indicated that the quantum, external magnetic field and collisional effects can facilitate the propagation of surface waves on a semi-bounded plasma. In addition, it is found that the growth rate of the surface wave instability is enhanced by increasing the collision frequency and plasmonic parameter.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Terahertz generation by mixing of two super-Gaussian laser beams in collisional plasma

Considering a realistic situation, where electron-neutral collisions persist in plasma, analytical calculations are carried out for the Terahertz (THz) radiation generation by beating of two Super-Gaussian (SG) lasers of index p. The competency of these lasers over Gaussian lasers is discussed in detail with respect to the effects of collision and beam width on the THz field amplitude and efficiency of the mechanism. A critical transverse distance of the peak of the THz field is defined that shows a dependence on the index of SG lasers. Although electron-neutral collisions and larger beam width lead to the drastic reduction in the THz field when the SG lasers are used in the plasma, the efficiency of the mechanism remains much larger than the case of Gaussian lasers. Moreover, the higher index SG lasers produce stronger and focused THz radiation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Highly Resolved Measurements of a Developing Strong Collisional Plasma Shock

In this paper, the structure of a strong collisional shock front forming in a plasma is directly probed for the first time in laser-driven gas-jet experiments. Thomson scattering of a 526.5 nm probe beam was used to diagnose temperature and ion velocity distribution in a strong shock ($M{\sim}11$) propagating through a low-density (${\rho}{\sim}0.01\text{ }\text{ }\mathrm{mg}/\mathrm{cc}$) plasma composed of hydrogen. A forward-streaming population of ions traveling in excess of the shock velocity was observed to heat and slow down on an unmoving, unshocked population of cold protons, until ultimately the populations merge and begin to thermalize. Finally, instabilities are observed during the merging, indicating a uniquely plasma-phase process in shock front formation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Kinetic description of linear wave propagation in inhomogeneous, nonstationary, anisotropic, weakly magnetized, and collisional plasma

This paper addresses the linear propagation of an electron wave in a plasma whose distribution function, at zero order in the wave amplitude, may be chosen arbitrarily, provided that it is not strongly peaked at the wave phase velocity, and that it varies very little over one wave period and one wavelength. Then, from first principles is derived an equation for the wave action density that allows for Landau damping, whose rate is calculated at first order in the variations of the wave number and frequency. Moreover, the effect of collisions is accounted for in a way that adapts to any choice for the collision operator in Boltzmann equation. The wave may also be externally driven, so that the results presented here apply to stimulated Raman scattering.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

On RF heating of inhomogeneous collisional plasma under ion-cyclotron resonance conditions

During ion-cyclotron resonance (ICR) heating of plasma by the magnetic beach method, as well as in some other versions of ICR heating, it is necessary to excite Alfvén oscillations. In this case, it is difficult to avoid the phenomenon of the Alfvén resonance, in which Alfvén oscillations transform into lower hybrid oscillations. The latter efficiently interact with electrons, due to which most of the deposited RF energy is spent on electron (rather than ion) heating. The Alfvén resonance takes place due to plasma inhomogeneity across the external magnetic field. Therefore, it could be expected that variations in the plasma density profile would substantially affect the efficiency of the interaction of RF fields with charged particles. However, the results obtained for different plasma density profiles proved to be nearly the same. In the present work, a plasma is considered the parameters of which correspond to those planned in future ICR plasma heating experiments on the PS-1 facility at the Kurchatov Institute. When analyzing the interaction of RF fields with charged particles, both the collisionless resonance interaction and the interaction caused by Coulomb collisions are taken into account, because, in those experiments, the Coulomb collision frequency will be comparable with the frequency of the heating field. Antennas used for ICR heating excite RF oscillations with a wide spectrum of wavenumbers along the magnetic field. After averaging over the spectrum, the absorbed RF energy calculated with allowance for collisions turns out to be close to that absorbed in collisionless plasma, the energy fraction absorbed by electrons being substantially larger than that absorbed by ions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Current-conserving relativistic linear response for collisional plasmas

Here we investigate the response of a relativistic plasma to electromagnetic fields in the framework of the Boltzmann equation incorporating a collision term in the relaxation rate approximation selected in a form assuring current conservation. We obtain an explicit solution for the linearized perturbation of the Fermi–Dirac equilibrium distribution in terms of the average relaxation rate k. We study the resulting covariant, gauge invariant, and current conserving form of the polarization tensor in the ultrarelativistic and non-relativistic limits. We evaluate the susceptibility in the ultrarelativistic limit and explore their dependence on k. Finally, we study the dispersion relations for the longitudinal and transverse poles of the propagator. We show that for k >2ω p , where ω p is the plasma frequency, the plasma wave modes are overdamped. In the opposite case, k$\ll$ ω p , the propagating plasma modes are weakly damped.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Self-focusing of pseudo-nondiffracting laser beams with circular symmetry in collisional plasma under two ramp density profiles

The self-focusing of zeroth-order Bessel–Gaussian beams through plasma under exponential and tangential ramp density profiles is investigated theoretically by using Wentzel–Kramers–Brillouin approximation. To examine the effect of plasma density ramp on self-focusing of the studied beams, the evolution of beam-width parameter with normalized propagation distance is reported for diverse laser beams and plasma parameters. The results reveal that stronger self-focusing is obtained with exponential ramp density profile as compared to the tangential density ramp one.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Connecting Collisionless Landau Fluid Closures to Collisional Plasma Physics Models

In this work, collisionless Landau fluid models are combined with collisional fluid models using a Padé approximation that is accurate in both collisionality limits. The collisionless models capture Landau damping through a nonlocal integro-differential sub-diffusion operator with ballistic characteristics. Collisional extensions of Landau fluid models are derived by analyzing the higher order moment equations which combine a Landau closure with collisional friction forces. The model derived here evolves fluid moments for density, parallel velocity, and anisotropic pressure and includes the frictional heat flux, the parallel thermal force and anisotropic electrical conductivity. Since anisotropies must vanish in the collisional limit, a simple closure can be derived if the friction force neglects pitch-angle scattering for the closure moments themselves. The resulting plasma physics model is potentially quite useful for applications in magnetic fusion and astrophysics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Ion Collection by a Sphere in a Drifting Collisional Plasma

SCEPTIC3D now includes charge-exchange collisions, and is used to examine ion collection by a floating conducting sphere (small compared to the electron debye length) in the presence of a background neutral drift. The enhancement of ion collection at moderate collisionalities seen for the stationary case is weakened as the drift speed increases from the ion thermal speed to the ion sound speed, above which no enhancement is seen.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Nonextensive statistics and the sheath criterion in collisional plasmas

The Bohm criterion in an electropositive plasma containing nonextensively distributed electrons and warm ions is investigated by using a steady state two-fluid model. Taking into account the ion-neutral collisions and finite temperature of ions, a modified Bohm criterion is derived which limits both maximum and minimum allowable velocity of ions at the sheath edge (u{sub 0i}). It is found that the degree of nonextensivity of electrons (q) and temperature of positive ions (T{sub i}) affect only the lower limit of the entrance velocity of ions into the sheath while the degree of ion collisionality (α) influences both lower and upper limits of the ion velocities at the sheath edge. In addition, depending on the value of q, it is shown that the minimum velocity of positive ions at the sheath edge can be greater or smaller than its Maxwellian counterpart. Moreover, it is shown that, depending on the values of α and T{sub i}, the positive ions with subsonic velocity may enter the sheath for either q > 1 or −1 < q < 1. Finally, as a practical application, the density distribution of charged particles in the sheath region is studied for different values of u{sub 0i}, and it is shown that monotonical reduction of the positive ion density distribution occurs only when the velocity of positive ions at the sheath edge lies between two above mentioned limits.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗