Probing off-Hugoniot pressure and density in Ta Cu and Al to 1000 GPa compression with magnetically driven liner implosions.
Abstract not provided.
Lemke, Raymond W.↗
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Results for “MAGNETIC COMPRESSION”
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Exploring extreme magnetization phenomena in directly driven imploding cylindrical targets
Abstract This paper uses extended-magnetohydrodynamics (MHD) simulations to explore an extreme magnetized plasma regime realizable by cylindrical implosions on the OMEGA laser facility. This regime is characterized by highly compressed magnetic fields (greater than 10 kT across the fuel), which contain a significant proportion of the implosion energy and induce large electrical currents in the plasma. Parameters governing the different magnetization processes such as Ohmic dissipation and suppression of instabilities by magnetic tension are presented, allowing for optimization of experiments to study specific phenomena. For instance, a dopant added to the target gas-fill can enhance magnetic flux compression while enabling spectroscopic diagnosis of the imploding core. In particular, the use of Ar K-shell spectroscopy is investigated by performing detailed non-LTE atomic kinetics and radiative transfer calculations on the MHD data. Direct measurement of the core electron density and temperature would be possible, allowing for both the impact of magnetization on the final temperature and thermal pressure to be obtained. By assuming the magnetic field is frozen into the plasma motion, which is shown to be a good approximation for highly magnetized implosions, spectroscopic diagnosis could be used to estimate which magnetization processes are ruling the implosion dynamics; for example, a relation is given for inferring whether thermally driven or current-driven transport is dominating.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Kelvin–Helmholtz instability under stabilizing parallel magnetic field in nonhomogeneous compressible MHD flows
We study the Kelvin–Helmholtz instability (KHI) for the general case of a compressible, nonhomogeneous, magnetized plasma flow. The study is limited to a vortex sheet interface with an imposed parallel magnetic field. We introduce a new formalism based on a convective Mach number M c , a convective Alfvénic Mach number M Ac , and a total convective Mach number that combines the two. We derive an analytic expression of the KHI growth rate for a homogeneous flow (i.e., zero Atwood number, A=0) that converges toward both the expression for unmagnetized compressible flow and Chandrasekhar's expression for magnetized incompressible flow. Otherwise, the dispersion relation is solved numerically and allows deriving general stability diagrams of magnetized KHI for the triplet (A, M c , β −plasma) parameters. We show these parameters uniquely define all configurations for a parallel magnetic field. We also construct diagrams with respect to the convective Alfvénic Mach number, the β − plasma parameter, or the magnetic field showing which magnetic field strength is required for stabilizing a given shear flow. The theoretical growth rates are compared with 18 simulations made with the GAMERA code, currently used for 3D magnetospheric simulations. Finally, we apply our results to the analysis of a past KHI experiment performed at the OMEGA laser facility, showing linear theory succeeds to provide accurate estimates of the growth rate at early times. We further discuss how our results can inform future experiments in the high-Mach magnetized regime at the National Ignition Facility. Possible limitations of the study due to resistive, mixing, or turbulence effects are discussed.
compressible flows↗
On the Existence of the Kolmogorov Inertial Range in the Terrestrial Magnetosheath Turbulence
In the solar wind, power spectral density (PSD) of the magnetic field fluctuations generally follow the so-called Kolmogorov spectrum f {sup −5/3} in the inertial range, where the dynamics is thought to be dominated by nonlinear interactions between counter-propagating incompressible Alfvén wave parquets. These features are thought to be ubiquitous in space plasmas. The present study gives a new and more complex picture of magnetohydrodynamic (MHD) turbulence as observed in the terrestrial magnetosheath. The study uses three years of in situ data from the Cluster mission to explore the nature of the magnetic fluctuations at MHD scales in different locations within the magnetosheath, including flanks and subsolar regions. It is found that the magnetic field fluctuations at MHD scales generally have a PSD close to f {sup −1} (shallower than the Kolmogorov one f {sup −5/3}) down to the ion characteristic scale, which recalls the energy-containing scales of solar wind turbulence. The Kolmogorov spectrum is observed only away from the bow shock toward the flank and the magnetopause regions in 17% of the analyzed time intervals. Measuring the magnetic compressibility, it is shown that only a fraction (35%) of the observed Kolmogorov spectra was populated by shear Alfvénic fluctuations, whereas the majority of the events (65%) was found to be dominated by compressible magnetosonic-like fluctuations, which contrasts with well-known turbulence properties in the solar wind. This study gives a first comprehensive view of the origin of the f {sup −1} and the transition to the Kolmogorov inertial range; both questions remain controversial in solar wind turbulence.
79 ASTRONOMY AND ASTROPHYSICS↗
The magnetic field of the magnetosphere and tail
Solar wind compressed magnetic field in sunward magnetosphere and extended geomagnetic tail observation by Pioneer 7 spacecraft
Fairfield, D. H.↗
Note on lightning temperature
In this paper, some features of the dynamics of a lightning channel that emerges after the leader-streamer process, are theoretically studied. It is shown that the dynamic pinch effect in the channel becomes possible if a discharge current before the main (quasi-steady) stage of a lightning discharge increases rapidly. The ensuing magnetic compression of the channel increases plasma temperature to several million degrees leading to a soft x-ray flash within the highly ionized plasma. The relation between the plasma temperature and the channel radius during the main stage of a lightning discharge is derived.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
MAGO-IX Experiment
The results of the last (in the thermonuclear program “Magnetic compression”) MAGO-IX experiment with a plasma chamber including a third compartment designed to compress plasma with a converging liner are presented. An X-ray pulse consisting of an intense peak of 1-μs duration, followed by a low-intensity tail with a duration of more than 10 μs, was recorded. In the MAGO-IX experiment, the neutrons were generated mainly in the third compartment. A neutron yield of 2 × 10{sup 12} was obtained. The results demonstrate that the expected compression of preheated plasma in chambers similar to MAGO-IX is promising for achieving thermonuclear ignition.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Compressive coherent structures at ion scales in the slow solar wind
We present a study of magnetic field fluctuations in a slow solar wind stream, close to ion scales, where an increase of the level of magnetic compressibility is observed. Here, the nature of these compressive fluctuations is found to be characterized by coherent structures. Although previous studies have shown that current sheets can be considered the principal cause of intermittency at ion scales, here we show for the first time that, in the case of the slow solar wind, a large variety of coherent structures contributes to intermittency at proton scales, and current sheets are not the most common. Specifically, we find compressive (δb{sub ∥}≫δb{sub ⊥}), linearly polarized structures in the form of magnetic holes, solitons, and shock waves. Examples of Alfvénic structures (δb{sub ⊥}>δb{sub ∥}) are identified as current sheets and vortex-like structures. Some of these vortices have δb{sub ⊥}≫δb{sub ∥}, as in the case of Alfvén vortices, but the majority of them are characterized by δb{sub ⊥}≳δb{sub ∥}. Thanks to multi-point measurements by the Cluster spacecraft, for about 100 structures we could determine the normal, the propagation velocity, and the spatial scale along this normal. Independently of the nature of the structures, the normal is always perpendicular to the local magnetic field, meaning that k {sub ⊥} ≫ k {sub ∥}. The spatial scales of the studied structures are found to be between two and eight times the proton gyroradius. Most of them are simply convected by the wind, but 25% propagate in the plasma frame. Possible interpretations of the observed structures and the connection with plasma heating are discussed.
79 ASTRONOMY AND ASTROPHYSICS↗
Frequency and energy characteristics of a Cu – Ne laser at different durations of the leading edge of the excitation pulse
The lasing characteristics of a copper vapour laser in a tube with forced heating, having a length of 50 cm and a diameter of 2 cm, excited by a train of pulses are investigated. Comparative studies of the frequency and energy characteristics of the laser are performed with a leading edge duration of the excitation pulse of ∼25 ns (when the capacitance discharges through a thyratron and a magnetic compression line) and ∼3 and 1 ns (using circuits with a high-speed switch – kivotron). It is shown that a decrease in the leading edge duration gives rise to an increase in the optimal pulse repetition rate up to ∼30 kHz, the generation efficiency up to 3.2 % and the generation power per unit length over 100 W m{sup −1}. The results obtained confirm the concept of limiting the frequency and energy characteristics of a copper vapour laser due to the insufficient rate of energy input into the plasma at high prepulse electron concentrations. (paper)
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Anisotropies of the Magnetic Field Fluctuations at Kinetic Scales in the Solar Wind: Cluster Observations
We present the first statistical study of the anisotropy of the magnetic field turbulence in the solar wind between 1 and 200 Hz, i.e., from proton to sub-electron scales. We consider 93 ten-minute intervals of the Cluster/STAFF measurements. We find that the fluctuations δB{sub ⊥}{sup 2} are not gyrotropic at a given frequency f, a property already observed at larger scales (∥/⊥ means parallel/perpendicular to the average magnetic B{sub 0}). This non-gyrotropy gives indications of the angular distribution of the wave vectors k: at f< 10 Hz, we find that k{sub ⊥}≫k{sub ∥}, mainly in the fast wind; at f > 10 Hz, fluctuations with a non-negligible k {sub ∥} are also present. We then consider the anisotropy ratio δB{sub ∥}{sup 2}/δB{sub ⊥}{sup 2}, which is a measure of the magnetic compressibility of the fluctuations. This ratio, always smaller than 1, increases with f. It reaches a value showing that the fluctuations are more or less isotropic at electron scales, for f⩾50 Hz. From 1 to 15–20 Hz, there is a strong correlation between the observed compressibility and the one expected for the kinetic Alfvén waves (KAWs), which only depends on the total plasma β. For f>15--20 Hz, the observed compressibility is larger than expected for KAWs, and it is stronger in the slow wind: this could be an indication of the presence of a slow-ion acoustic mode of fluctuations, which is more compressive and is favored by the larger values of the electron to proton temperature ratio generally observed in the slow wind.
79 ASTRONOMY AND ASTROPHYSICS↗
Current sheets in the solar corona
Coronal magnetic fields are twisted up by motion of their footpoints in the photosphere. When the twist exceeds a critical amount, kink-ballooning instabilities occur. These instabilities are studied numerically, in long, thin, axially bounded magnetic fields. Nonlinearly, the three-dimensional kinking motion compresses magnetic flux, forming a current sheet. Magnetic energy can be dissipated at a rate orders of magnitude greater than without the current sheets. The energy of footpoint motion can then go into coronal heating, via Ohmic dissipation in the current sheets.
Strauss, H. R.↗
Kinetic simulations of laser parametric amplification in magnetized plasmas
Laser pulse compression using magnetized resonance near the upper-hybrid frequency is promising for achieving higher output intensity in regimes previously thought impossible using unmagnetized plasmas. Using one dimensional particle-in-cell simulations, we verify that, by partially replacing plasma with an external transverse magnetic field of megagauss scale, the output pulse can be intensified by a factor of a few, due to the increased allowable amplification time despite a decreased growth rate. Further improvement is impeded by the generation of an electromagnetic wakefield, to which the amplified pulse loses more energy than it does in the unmagnetized case. This limitation can however be circumvented by the use of a stronger pump. In contrast to unmagnetized compression, the magnetized amplification remains efficient when the pump intensity is well above the wavebreaking threshold, until a higher phase-mixing threshold is exceeded. Here, this surprising resilience to wavebreaking in magnetized plasma is of great benefit for magnetized compression.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Enhancing cylindrical compression by reducing plasma ablation in pulsed-power drivers
Warm dense matter, which can be found in planetary cores, is too dense to be described by plasma theory and too hot to be considered condensed matter. With no theory describing perfectly how such large quantum systems evolve at macroscopic scales, modeling planetary evolution is simply out of reach. While recent experiments using high power lasers and heavy ion beams have produced warm dense matter samples, they do not confine matter long enough to allow for bulk material properties to take hold, precluding the validation of any theories beyond electron-ion equilibration time. To this end, pulsed-power drivers are required. This approach allows experimentalists to probe macroscopic states of matter where bulk material properties are at equilibrium. High resolution numerical simulations show that a mega-ampere pulsed-power driver can generate macroscopic samples of warm dense matter, using direct magnetic compression, without any pusher. A thin coating, deposited onto the material just before the experiment, softens the density gradients responsible for plasma ablation. Starved of plasma outside the conductors, electrical currents are forced to flow along material surfaces, resulting in a very stable magnetic topology that yields homogeneous compression above 1 Mbar. Another key aspect is as follows: mega-ampere pulsed power systems are compact enough to be located next to existing high brilliance x-ray sources, which can probe best the properties of matter under extreme pressure.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Nature of Kinetic Scale Turbulence in the Earth's Magnetosheath
We present a combined observational and theoretical analysis to investigate the nature of plasma turbulence at kinetic scales in the Earth’s magnetosheath. In the first decade of the kinetic range, just below the ion gyroscale, the turbulence was found to be similar to that in the upstream solar wind: predominantly anisotropic, low-frequency and kinetic Alfvén in nature. A key difference, however, is that the magnetosheath ions are typically much hotter than the electrons, T{sub i}≫T{sub e}, which, together with β{sub i}∼1, leads to a change in behavior in the second decade, close to electron scales. The turbulence here is characterized by an increased magnetic compressibility, following a mode we term the inertial kinetic Alfvén wave, and a steeper spectrum of magnetic fluctuations, consistent with the prediction E{sub B}(k{sub ⊥})∝k{sub ⊥}{sup −11/3} that we obtain from a set of nonlinear equations. This regime of plasma turbulence may also be relevant for other astrophysical environments with T{sub i}≫T{sub e}, such as the solar corona, hot accretion flows, and regions downstream of collisionless shocks.
79 ASTRONOMY AND ASTROPHYSICS↗
Investigations of plasma evolution in Laser Ablation Z-Pinch Experiments using Time-Resolved Optical Spectroscopy
In this dissertation, investigations of the temporal evolution of magnetically driven plasmas are presented. These studies were carried at the Nevada Terawatt Facility using the Zebra pulsed-power accelerator to magnetically compress and confine a cylindrical plasma column. Important parameters needed to characterize these laboratory plasmas include magnetic field strength and orientation, as well as electron number density and temperature. To fully understand the characteristics of a plasma, accurate experimental measurement of these parameters is essential. To determine the electron temperature, the Boltzmann plot method was used. Simulated spectra from PrismSPECT were compared to experimental measurements in order to measure electron number density and temperature early in the pinched plasma formation. Simultaneously, Mach-Zehnder laser interferometry was used to provide complementary measurements of the electron number density. A novel method, streaked Zeeman-induced Magnetic Splitting optical spectroscopy, was developed to measure time- and space-resolved magnetic field strengths in these hot, dense plasmas. This technique has also been applied to diagnose the magnetic field strengths in laser ablation Z-pinch experiments, wherein a pulsed power driver pinches a laser ablated plume.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Three-dimensional Simulations and Spacecraft Observations of Sub-ion Scale Turbulence in the Solar Wind: Influence of Landau Damping
Three-dimensional nonlinear finite Larmor radius (FLR)–Landau fluid simulations, which include some small-scale (k{sub ⊥}ρ{sub i}≳1) kinetic effects, are performed to explore the nature of the sub-ion scale turbulence in the solar wind and to investigate the role of Landau damping and FLR corrections. The resulting steady-state magnetic power spectrum in the dispersive range display exponents that vary within a range of values compatible with statistical results reported from in situ spacecraft measurements of solar wind turbulence as well as from gyrokinetic simulations. The spectral slopes are shown to depend on the strength of the nonlinear effects and on the scale at which turbulent fluctuations are driven in the simulations. The influence of Landau damping is addressed by comparison with simulations where the double-adiabatic closure is imposed. The role of FLR corrections is also analyzed. Comparison with in situ observations in the solar wind are performed to enlighten the influence of the fluctuations power at different scales on the spectral slopes in the sub-ion range. Using diagnosis of both magnetic compressibility and frequency-wavenumber spectra, it is shown that in spite of the evidence of the presence of fast-magnetosonic modes, the magnetic energy is mostly distributed around the kinetic Alfvén waves and the slow modes, in agreement with solar wind measurements. The observed large broadening about the linear dispersion relations may reflect the presence of coherent structures.
79 ASTRONOMY AND ASTROPHYSICS↗
Maser radiation from collisionless shocks: application to astrophysical jets
This paper describes a model of electron energization and cyclotron-maser emission applicable to astrophysical magnetized collisionless shocks. It is motivated by the work of Begelman, Ergun and Rees [Astrophys. J. 625 , 51 (2005)] who argued that the cyclotron-maser instability occurs in localized magnetized collisionless shocks such as those expected in blazar jets. We report on recent research carried out to investigate electron acceleration at collisionless shocks and maser radiation associated with the accelerated electrons. We describe how electrons accelerated by lower-hybrid waves at collisionless shocks generate cyclotron-maser radiation when the accelerated electrons move into regions of stronger magnetic fields. The electrons are accelerated along the magnetic field and magnetically compressed leading to the formation of an electron velocity distribution having a horseshoe shape due to conservation of the electron magnetic moment. Under certain conditions the horseshoe electron velocity distribution function is unstable to the cyclotron-maser instability [Bingham and Cairns, Phys. Plasmas 7 , 3089 (2000); Melrose, Rev. Mod. Plasma Phys. 1 , 5 (2017)].
Optics↗
Substorm associated traveling compression regions in the distant tail - ISEE-3 geotail observations
While in the lobes of the distant magnetotail, ISEE-3 encountered regions of compressed magnetic field at a rate of several per day. The duration of these events was 5 to 20 minutes and they were observed 10 to 30 minutes following the onset of substorm activity near the earth. During each event, the lobe magnetic field tilted first northward and then southward with the inflection point near the time of peak field strength. Following the compression events, the lobe field weakened and retained a southward component for 20 to 40 minutes. It is suggested that these traveling compression regions are the lobe signatures of plasmoids moving rapidly down the tail in the plasma sheet. Comparison of ISEE-3 compression event times with substorm onset times yielded propagation speeds of 350 to 750 km/s.
Slavin, J. A.↗