No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Dynamics of Laser-Plasma Expansion Across Strong Magnetic Field
Abstract
Explosive plasma expansion in an external magnetic field occurs in a variety of space and astrophysical events that include supernova explosions in the interstellar magnetic field and the interaction of solar coronal mass ejections with planetary magnetospheres. Effects of the interaction of plasma winds with magnetic obstacles were investigated in the laboratory using a laser produced plasma plume and an external magnetic field created by a current driven through a metal rod by a pulsed power generator. Instabilities develop at the plasma-field boundary. In the frontal direction, along the normal to the target, flute-like instabilities grow and eventually lead to plasma penetration across the magnetic field with constant velocity. Along the lateral boundary, the plasma deceleration by the transverse magnetic field initiated a Kelvin-Helmholtz instability.
The processes of the direct energy conversions of the ICF-microexplosion in magnetic fields are discussed and investigated by both the methods of their simulations in the experiments with usual Laser-Produced Plasma Clouds and via their numerical PIC-modelling in 3D-schemes. The problems of additional energy losses of exploding plasmas which could prevent to achieve more than 50% efficiency of such electrical conversion in uniform magnetic field are studied by comparison of experimental and numerical data. The opportunities of such common approach to simulation future NIF experiment with dipole field's thrust are shown.
/SOVIET JOURNAL of PLASMA PHYSICS/ A study is made of the effect of field diffusion on the deceleration, by a homogeneous magnetic field, of spherical clouds of a laser plasma expanding in vacuum. It is found that the observed level of lower-hybrid turbulence in the skin layer corresponds to the experimentally determined electron collision frequency for the known cases of current instability at the lower-hybrid frequency. It is shown experimentally that, in the case of such turbulence, plasma deceleration in the direction normal to the magnetic field occurs only when a pressure balance between the plasma and the field is achieved at cloud expansion scales that exceed the Larmor radius of the ions.
The basic physical equations as well as a computer code for the simulation of one-dimensional radiation hydrodynamics are described. The hydrodynamic equations are combined with a multigroup method for the radiation transport. The code, written in standard FORTRAN-77, is characterized by one-dimensional planar geometry with multilayer structure. A time-splitting schema has been adopted with implicit finite-differencing formulation (including the hydrodynamics), and Lagrangian coordinates. Tabulated equations of state and opacities are used.
Establishing the mechanisms by which the solar wind enters Earth's magnetosphere is one of the biggest goals of magnetospheric physics, as it forms the basis of space weather phenomena such as magnetic storms and aurorae. It is generally believed that magnetic reconnection is the dominant process, especially during southward solar-wind magnetic field conditions when the solar-wind and geomagnetic fields are antiparallel at the low-latitude magnetopause. But the plasma content in the outer magnetosphere increases during northward solar-wind magnetic field conditions, contrary to expectation if reconnection is dominant. Here we show that during northward solar-wind magnetic field conditions-in the absence of active reconnection at low latitudes-there is a solar-wind transport mechanism associated with the nonlinear phase of the Kelvin-Helmholtz instability. This can supply plasma sources for various space weather phenomena.
A comprehensive theoretical treatment of the linear stability of a sub‐Alfvénic plasma expansion is developed. The analysis is similar to those performed for the lower‐hybrid‐drift instability and the drift cyclotron instability. In addition to the diamagnetic drift (Vdi) that drives these instabilities, the gravitational drift (Vg) caused by the deceleration of the plasma shell, and the Pedersen drift (VP) caused by ion–neutral collisions and neutral gas flow, are included. The emphasis of the paper is on the instability driven by the gravitational drift. The theory is fully kinetic and includes finite‐beta effects (i.e., electromagnetic coupling and electron ∇B drift‐wave resonances), collisional effects (electron–ion, electron–neutral, and ion–neutral collisions), and neutral gas flow, effects that have not been considered to date. The analysis is carried out in a slab geometry although the applications are to spherical expansions. The main conclusions are as follows. In the strong drift limit (Vg>vi and Vdi∼vi, where vi is the ion thermal velocity) it is found that (1) finite‐beta effects are stabilizing and reduce the wavelength of the maximum growth rate; (2) ion–neutral collisions are stabilizing and do not affect the wavelength of the maximum growth rate; (3) electron–neutral collisions are stabilizing and increase the wavelength of the maximum growth rate; (4) the gravitational drift driven mode maximizes the growth rate at longer wavelengths than the diamagnetic drift driven mode; (5) the Pedersen drift effectively reduces the gravitational drift, and is therefore a stabilizing influence; and (6) the instability splits into two modes for Te≫Ti in finite‐beta plasmas: the lower‐hybrid‐drift instability at high frequencies and short wavelengths, and a gravitational mode at lower frequencies and longer wavelengths.In the weak drift regime (Vg<vi and Vdi<vi) it is found that (1) finite‐beta effects are stabilizing and increase the wavelength of the maximum growth rate; (2) ion–neutral collisions are destabilizing and decrease the wavelength of the maximum growth rate; and (3) electron–ion and electron–neutral collisions are stabilizing, and increase the wavelength of the maximum growth rate. When the growth rate becomes less than the ion cyclotron instability (γ<Ωi), the growth rate as a function of wave number ‘‘breaks up’’ into a discrete set of modes which is associated with the coupling of the drift waves to ion cyclotron waves. These results are applied to the AMPTE magnetotail release [J. Geophys. Res. 92, 5777 (1987)], the Naval Research Laboratory laser experiment [Phys. Rev. Lett. 59, 2299 (1987)], and the upcoming CRRES GTO releases [D. Reasoner (private communication, 1989)].
Hybrid simulations with kinetic ions and massless fluid electrons are used to investigate the linear and nonlinear behavior of the magnetized Rayleigh–Taylor instability in slab geometry with the plasma subject to a constant gravity. Three regimes are found, which are determined by the magnitude of the complex frequency ω=ωr+iΓ. For |ω|≪Ωi(Ωi= ion gyrofrequency), one finds the typical behavior of the usual fluid regime, namely the development of ‘‘mushroom-head’’ spikes and bubbles in the density and a strongly convoluted boundary between the plasma and magnetic field, where the initial gradient is not relaxed much. A second regime, where |ω|∼0.1Ωi, is characterized by the importance of the Hall term. Linearly, the developing flute modes are more finger-like and tilted along the interface; nonlinearly, clump-like structures form, leading to a significant broadening of the interface. The third regime is characterized by unmagnetized ion behavior, with |ω|∼Ωi. Density clumps, rather than flutes, form in the linear stage, while nonlinearly, longer-wavelength modes that resemble those in fluid regime dominate. Finite Larmor radius stabilization of short-wavelength modes is observed in each regime.
Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies
Plasma interaction at the interface between the magnetosheath and magnetosphere has been extensively studied during recent years. As a consequence various theoretical models have emerged. The impulsive penetration mechanism initially proposed by Lemaire and Roth as an alternative approach to the steady state reconnection, is a non-stationary model describing the processes which take place when a 3-D solar wind plasma irregularity interacts with the outer regions of the Earth's magnetosphere. In this paper we are reviewing the main features of the impulsive penetration mechanism and the role of the electric field in driving impulsive events. An alternative point of view and the controversy it has raised are discussed. We also review the numerical codes developed to simulate the impulsive transport of plasma across the magnetopause. They have illustrated the relationship between the magnetic field distribution and the convection of solar-wind plasma inside the magnetosphere and brought into perspective non-stationary phenomena (like instabilities and waves) which were not explicitly integrated in the early models of impulsive penetration. Numerical simulations devoted to these processes cover a broad range of approximations, from ideal MHD to hybrid and kinetic codes. The results show the limitation of these theories in describing the full range of phenomena observed at the magnetopause and magnetospheric boundary layers.
Laboratory experiments on the interaction of a plasma flow, produced by laser ablation of a solid target with the inhomogeneous
magnetic field from the Zebra pulsed power generator demonstrated the presence of strong wave activity in the region of the
flow deceleration. The deceleration of the plasma flow can be interpreted as the appearance of a gravity-like force. The drift
due to this force can lead to the excitation of flute modes. In this paper a linear dispersion equation for the excitation
of electromagnetic flute-type modes with plasma and magnetic field parameters, corresponding to the ongoing experiments is
examined. Results indicate that the wavelength of the excited flute modes strongly depends on the strength of the external
magnetic field. For magnetic field strengths ∼0.1 MG the excited wavelengths are larger than the width of the laser ablated
plasma plume and cannot be observed during the experiment. But for magnetic field strengths ∼1 MG the excited wavelengths
are much smaller and can then be detected.
Assuming that the solar wind is unsteady and non-uniform, it is suggested that field aligned plasma elements dent the magnetopause surface. This indentation makes the magnetopause boundary convex and therefore locally unstable with respect to flute instabilities. The intruding element is slowed down and stopped within 1 or 2 Earth radii from the magnetopause. Its excess convection kinetic energy is dissipated in the lower polar cusp ionosphere after ~ 50–500 s, depending on the value of the integrated Pedersen conductivity. Once the plasma element has been engulfed, keeping its identity, the warm plasma content is dissipated by precipitation and by drifting. The magnetosheath particles with large pitch angles are mirrored, and feed the plasma mantle flow.Several consequences of this penetration mechanism are pointed out:Ionospheric heating beneath the polar cusp;Birkeland currents on the eastward and westward edges of the plasma element;Diamagnetic field fluctuations within 1–2 RE from the magnetopause (multiple magnetopause crossings);Oscillation of the magnetopause surface after a new element has penetrated;Exit of energetic particles out of the magnetosphere, and entry of energetic solar wind particles into the magnetosphere along the magnetic field lines of the intruding element;Subtraction of magnetic flux from the dayside magnetosphere and its addition to the geomagnetic tail when the magnetic field of the element has a southward component.
Preliminary theoretical and computational analyses of the Combined Release and Radiation Effects Satellite (CRRES) magnetospheric barium releases are presented. The focus of the studies is on the evolution of the diamagnetic cavity which is formed by the barium ions as they expand outward, and on the structuring of the density and magnetic field during the expansion phase of the releases. Two sets of simulation studies are discussed. The first set is based upon a 2D ideal MHD code and provides estimates of the time and length scales associated with the formation and collapse of the diamagnetic cavity. The second set uses a nonideal MHD code; specifically, the Hall term is included. This additional term is critical to the dynamics of sub-Alfvenic plasma expansions, such as the CRRES barium releases, because it leads to instability of the expanding plasma. Detailed simulations of the G4 and G10 releases were performed. In both cases the expanding plasma rapidly structured: the G4 release structured at time t less than about 3 s and developed scale sizes of about 1-2 km, while the G10 release structured at time t less than about 22 s and developed scale sizes of about 10-15 km. It is also found that the diamagnetic cavity size is reduced from those obtained from the ideal MHD results because of the structure. On the other hand, the structuring allows the formation of plasma blobs which appear to free stream across the magnetic field; thus, the barium plasma can propagate to larger distances traverse to the magnetic field than the case where no structuring occurs. Finally, a new normal mode of the system was discovered which may be excited at the leading edge of the expanding barium plasma.
We observe linear and nonlinear features of a strong plasma/magnetic field interchange Rayleigh-Taylor instability in the limit of large ion Larmor radius. The instability undergoes rapid linear growth culminating in free-streaming flute tips.
We investigate through high resolution 3D simulations the nonlinear evolution of compressible magnetohydrodynamic flows subject to the Kelvin-Helmholtz instability. We confirm in 3D flows the conclusion from our 2D work that even apparently weak magnetic fields embedded in Kelvin-Helmholtz unstable plasma flows can be fundamentally important to nonlinear evolution of the instability. In fact, that statement is strengthened in 3D by this work, because it shows how field line bundles can be stretched and twisted in 3D as the quasi-2D Cat's Eye vortex forms out of the hydrodynamical motions. In our simulations twisting of the field may increase the maximum field strength by more than a factor of two over the 2D effect. If, by these developments, the Alfv\'en Mach number of flows around the Cat's Eye drops to unity or less, our simulations suggest magnetic stresses will eventually destroy the Cat's Eye and cause the plasma flow to self-organize into a relatively smooth and apparently stable flow that retains memory of the original shear. For our flow configurations the regime in 3D for such reorganization is $4\lesssim M_{Ax} \lesssim 50$, expressed in terms of the Alfv\'en Mach number of the original velocity transition and the initial Alfv\'en speed projected to the flow plan. For weaker fields the instability remains essentially hydrodynamic in early stages, and the Cat's Eye is destroyed by the hydrodynamic secondary instabilities of a 3D nature. Then, the flows evolve into chaotic structures that approach decaying isotropic turbulence. In this stage, there is considerable enhancement to the magnetic energy due to stretching, twisting, and turbulent amplification, which is retained long afterwards. The magnetic energy eventually catches up to the kinetic energy, and the nature of flows become magnetohydrodynamic.
Strong Heating Of Collisionless Plasma Flow Stopped By An External Magnetic Field
- Y Sentoku