Plasma Shell Research Articles

Reduction of the deceleration phase to mitigate the negative effect of hydrodynamic instabilities in direct-drive ICF implosions

In the deceleration phase of an Inertial Confinement Fusion capsule implosion Rayleigh–Taylor hydrodynamic instability can affect or even quench the ignition and thermonuclear burn wave propagation. This instability tends to mix the inner hot plasma with the cold and dense plasma shell providing a mixing layer where nuclear fusion reactions are inhibited. The 1D hydrodynamics code Multi-IFE has been used to simulate the implosion of a direct-drive high-gain laser-capsule design and the temporal evolution of the average radius and thickness of the mixing layer have been estimated. To mimic the effect of the reduced reaction rate, the fuel reactivity in the mixing layer is artificially set to zero thus inhibiting the burn wave propagation throughout it nullifying the energy gain. In order to overcome this negative effect, secondary short and powerful laser pulse is added, shortening this way the deceleration phase, which in turn reduces the thickness of the mixing layer. A study has been carried out to identify the optimal secondary laser pulse that recovers the high energy gain.

Exploration of microscopic physical processes of Z-pinch by a modified electrostatic direct implicit particle-in-cell algorithm

Abstract For investigating efficiently the stagnation kinetic-process of Z-pinch, we develop a novel modified electrostatic implicit particle-in-cell algorithm in radial one-dimension for Z-pinch simulation in which a small-angle cumulative binary collision algorithm is used. In our algorithm, the electric field in z-direction is solved by a parallel electrode-plate model, the azimuthal magnetic field is obtained by Ampere’s law, and the term for charged particle gyromotion is approximated by the cross product of the averaged velocity and magnetic field. In simulation results of 2 MA deuterium plasma shell Z-pinch, the mass-center implosion trajectory agrees generally with that obtained by one-dimensional MHD simulation, and the plasma current also closely aligns with the external current. The phase space diagrams and radial-velocity probability distributions of ions and electrons are obtained. The main kinetic characteristic of electron motion is thermal equilibrium and oscillation, which should be oscillated around the ions, while that of ion motion is implosion inwards. In the region of stagnation radius, the radial-velocity probability distribution of ions transits from the non-equilibrium to equilibrium state with the current increasing, while of electrons is basically the equilibrium state. When the initial ion density and current peak are not high enough, the ions may not reach their thermal equilibrium state through collisions even in its stagnation phase.

Integrative implementation of scattering reduction and radiation enhancement for an electrically small antenna by subwavelength plasmas

The integrative design of scattering and radiation characteristics of antennas is of great practical significance for modern wireless communication. In this work, from the perspective of separate and cooperative modes, we comprehensively discussed the possibility of simultaneously and harmoniously implementing scattering suppression and radiation enhancement for an electrically small antenna by subwavelength plasmas. For the separate mode where the two functions are decoupled based on a two-layer structure, it is shown that an overdense–underdense core–shell density profile is preferred to achieve the optimal synergism between radiation enhancement and plasmonic-cloaking-induced invisibility, where the angular frequency of detecting waves (ωd) is supposed to be lower than that of communication signals (ωc). For the cooperative mode where the two functions are coupled within one plasma shell, the collaborative strategies between plasmonic-cloaking/Fano-resonance-induced invisibility and radiation enhancement are analyzed. The results show that the plasmonic-cloaking type requires ωd > ωc, while for the Fano-resonance type, ωd is larger/less than ωc when radiation enhancement is dominated by the symmetrically/asymmetrically coupled plasmon modes. Also, we provided clearer perspectives to distinguish the physical differences between plasmonic-cloaking and Fano-resonance-induced invisibility and between radiation enhancement underlying the two modes. Our results provide promising solutions for designing next-generation plasma-based tunable and intelligent stealth antennas.

Formation of hot spots at end-on pre-compressed isochoric fuels for fast ignition

It is crucial to form a hot spot that can drive a self-heating nuclear burn propagation into the surrounding cold fuel in ignition process of laser fusion. In this paper, we discuss the formation of a hemispherical hot spot located at the edge of an end-on precompressed isochoric fuel instead of a central spherical hot spot surrounded by cold fuel in a corona plasma shell. This configuration leads to a new energy loss mechanism named hot spot collapse, which originates from mass loss of the hot spot through the interface with the vacuum. A semi-analytical model is proposed including α particle induced burning, hot spot collapse and other energy loss processes. Then a modified ignition criterion is derived. The results show that the competition between the hot spot collapse and the burn propagation is crucial in the formation of the hot spot. The formation of such a hot spot at end-on precompressed isochoric fuels would require a somewhat higher initial temperature of Th=9 keV for a hot spot with an initial areal density ρd=0.6gcm−2 . Simulations are performed with a 3D hybrid particle-in-cell/fluid code for the heating process by fast electrons and a 3D radiation hydrodynamics code for the burning process. Simulation results agree with the model. Our optimized result shows a 10 ps heating laser pulse with an energy as low as 39 kJ can lead to a fast ignition in the ideal case. This study provides an effective reference for design and evaluation of experiments planned for fast ignition schemes.

Investigation on the near-field cutoff effect in a subwavelength plasma shell with near-zero permittivity.

Although the plasma-induced receiving and radiating near-field cutoff phenomena in the subwavelength regime are found of crucial importance in electromagnetic (EM) signal transmissions and plasma property studies, their mechanisms to a large extent remain unclear and undistinguished. In this paper, in the perspective of field and energy transfer, it is demonstrated that the cutoff in the near-field regime is completely different from that in the geometric optical regime. Results show that, for the receiving mode, epsilon-near-zero (ENZ) plasmas can be treated as a nearly ideal EM fluid, and thus, EM waves are restricted into the plasma channel. For the radiating mode, on the other hand, it is the destructive interference between the electric dipole fields of the antenna and the ENZ plasma that results in vanishing far-field radiation. As an important supplement to the existing cutoff theories, our results not only offer clearer physical insights into the near-field cutoff effect but also provide a helpful reference for cutoff-related practical applications in various frequency bands.

Interhemispheric ionosphere-plasmasphere system shows a high sensitivity to the exospheric neutral hydrogen density: a caution of the global reference atmospheric model hydrogen density

This study explores the impact of the exosphere hydrogen (H) density on the ionosphere-plasmasphere system using a model whose key inputs are constrained by ionosphere observations at both ends of the magnetic field line with an L-value of 1.75 in the American longitudinal sector during a period with low solar and magnetic activities. This study is the first to be validated by ground-based and satellite data in the plasmasphere and both hemispheres. The main finding is that the entire ionosphere-plasmasphere system is very sensitive to the neutral hydrogen density in the lower exosphere. It was found that an increase in the H density by a factor of 2.75 from the commonly accepted values was necessary to bring the simulated plasma density into satisfactory agreement with Arase satellite measurements in the plasmasphere and also with DMSP satellite measurements in the topside ionospheres of the northern and southern hemispheres. A factor of 2.75 increase in the H density increases the simulated plasma density in the afternoon plasmasphere up to ∼80% and in the nighttime topside ionosphere up to ∼100%. These results indicate prominently that using the commonly accepted empirical model of the H density causes unacceptable errors in the simulated plasma density of the near-Earth plasma shells. We alert the space science community of this problem.

Axion-sourced fireballs from supernovae

New feebly interacting particles would emerge from a supernova core with 100-MeV-range energies and produce $\gamma$-rays by subsequent decays. These would contribute to the diffuse cosmic $\gamma$-ray background or would have shown up in the Solar Maximum Mission (SMM) satellite from SN~1987A. However, we show for the example of axion-like particles (ALPs) that, even at distances beyond the progenitor star, the decay photons may not escape, and can instead form a fireball, a plasma shell with $T\lesssim 1$ MeV. Thus, existing arguments do not exclude ALPs with few 10 MeV masses and a two-photon coupling of a few $10^{-10}~{\rm GeV}^{-1}$. However, the energy would have showed up in sub-MeV photons, which were not seen from SN 1987A in the Pioneer Venus Orbiter (PVO), closing again this new window. A careful re-assessment is required for other particles that were constrained in similar ways.

Impact of the radial density profile on the Z-pinch stability at a microsecond rise time of the driving current

This paper considers the instabilities of imploding aluminum metal-puff Z-pinches with an outer plasma shell. An experiment was performed on the GIT-12 generator (3.2–3.6 MA, ∼1 μs implosion times, and ∼15 cm initial Z-pinch radius). It was shown that the density profile of the Z-pinch material had the dominant effect on the growth and suppression of instabilities. Two Z-pinch load configurations were used. The first configuration provided a tailored density profile (TDP) [A. L. Velikovich et al., Phys. Rev. Lett. 77, 853 (1996)], which ensured the suppression of the magneto-Rayleigh–Taylor (MRT) instability in the Z-pinch. For the second configuration, the density profile was changed in such a way that a density notch from 10 to 0.5 μg/cm3 occurred at a radius of about 3 cm from the Z-pinch axis. The notch in the density profile and the nonmonotonic increase in density resulted in a completely unstable compression of the Z-pinch. This gave rise to large-scale instabilities, which were detected by optical diagnostics. The instabilities grew and were not suppressed even in the stagnation phase, despite a sharp increase in the density of the Z-pinch material near the axis. The results were interpreted using the model proposed by Curzon et al. [Proc. R. Soc. London A 257, 386 (1960)]. The total instability amplitude is the sum of the amplitudes of MRT and magneto-hydrodynamic (MHD) instabilities. The growth of the total instability in the density notch region is due to the development of MRT instability. Thus, if the density profile has a notch, the Z-pinch compression in the stagnation phase occurs under strong perturbations at the magnetic field/plasma interface. This results in a dramatic growth of MHD instabilities. Hence, a stable implosion of a Z-pinch with TDP is possible only if the density increases monotonically toward the axis.

Shock Corrugation to the Rescue of the Internal Shock Model in Microquasars: The Single-scale Magnetohydrodynamic View

Questions regarding the energy dissipation in astrophysical jets remain open to date, despite numerous attempts to limit the diversity of the models. Some of the most popular models assume that energy is transferred to particles via internal shocks, which develop as a consequence of the nonuniform velocity of the jet matter. In this context, we study the structure and energy deposition of colliding plasma shells, focusing our attention on the case of initially inhomogeneous shells. This leads to the formation of distorted (corrugated) shock fronts—a setup that has recently been shown to revive particle acceleration in relativistic magnetized perpendicular shocks. Our study shows that the radiative power of the far downstream of nonrelativistic magnetized perpendicular shocks is moderately enhanced with respect to the flat-shock cases. Based on the decay rate of the downstream magnetic field, we make predictions for multiwavelength polarization properties.

Metrological aspects of determining the electron concentration and the conductivity of a melt using a conductivity sensor

For radiophysics, research into plasma physics is of great interest. During the movement of a hypersonic aircraft, the main physical phenomenon, which radically affects the conditions for the propagation of electromagnetic waves in its vicinity is the formation of a plasma shell around the apparatus due to the ionization of neutral particles. The resulting plasma around a hypersonic aircraft has a very high concentration of electrons and prevents the propagation of electromagnetic waves in space, partially absorbing them and reflecting them from its surface. When measuring the electron concentration, it is necessary to take into account the measurement error. In this article, the error is measured on the basis of expressions obtained for a transmission line whose longitudinal dimension exceeds the wavelength propagating in it. Mathematical model for finding the error in determining the electron concentration and melt conductivity is considered, and an analysis of the optimal combination of sensors is carried out.

Error of determination of radio-brightness temperature and temperature of plasma electrons

In our time, plasma physics is a hot topic of research. During the movement of a hypersonic aircraft, the main physical phenomenon, which radically affects the conditions for the propagation of electromagnetic waves in its vicinity is the formation of a plasma shell around the apparatus due to the ionization of neutral particles. When measuring the radio-brightness temperature and the temperature of plasma electrons, it is necessary to take into account the measurement error. In this article, these measurements are carried out, and the dependence of the root-mean-square relative error of temperature measurement on the power division factor is also given.

The moving mirror model for fast radio bursts

ABSTRACT Recent observations of coherent radiation from the Crab pulsar suggest the emission is driven by an ultrarelativistic (γ ∼ 104), cold plasma flow. A relativistically expanding plasma shell can compress the ambient magnetic field, like a moving mirror, and thus produce coherent radiation whose wavelength is shorter than that of the ambient medium by γ2. This mechanism has been previously studied in the context of radio loud supernova explosions. In this work, we propose that a similar mechanism drives the coherent emission in fast radio bursts. The high Lorenz factors dramatically lower the implied energy and magnetic field requirements, allowing the spin-down energy of regular (or even recycled), fast spinning pulsars, rather than slow spinning magnetars, to explain FRBs. We show that this model can explain the frequency and the time evolution of observed FRBs, as well as their duration, energetics, and absence of panchromatic counterparts. We also predict that the peak frequency of subpulses decline with observation time as $\omega _{\rm obs} \propto t_{\rm obs}^{-1/2}$. Unfortunately, with current capabilities it is not possible to constrain the shape of the curve ωobs(tobs). Finally, we find that a variation of this model can explain weaker radio transients, such as the one observed from a galactic magnetar. In this variant, the shock wave produces low-frequency photons that are then Compton scattered to the GHz range.

Radiation pattern in a tunable plasma window antenna

The work aims to theoretically and experimentally investigate the radiation characteristics of the plasma window antenna for beam-steering applications. The antenna system consists of a wire antenna in the center, surrounded by a circular array of 22 cylindrical plasma columns. The research reveals that the radiation pattern of the antenna system can be simply controlled by exploiting the variable parameters, such as working frequency, driving current, and plasma configurations. It implies that the beam narrows as the plasma antenna aperture decreases, implying a greater directivity. By electrically tuning the plasma, a maximum directivity of 9.09 dBi and a minimum half-power beam width of 35.86∘ emerged for a specific configuration. The results show that higher currents prevent radiation from escaping from the plasma shell, while higher frequency microwaves are more likely to penetrate the plasma blanket.

Measurements of the imploding plasma sheath in triple-nozzle gas-puff z pinches

Gas-puff z-pinch implosions are characterized by the formation of a dense annular plasma shell, the sheath, that is driven to the axis by magnetic forces and therefore subject to the magneto-Rayleigh–Taylor instability. Here, the conditions within these sheaths are measured on the 1-MA COBRA generator at Cornell University [Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)] for various gas species and initial fill densities. The gas-puff loads are initialized by a 7 cm diameter triple-nozzle gas valve assembly with concentric outer and inner annular nozzles and a central gas jet. Thomson scattering and laser interferometry provide spatially resolved flow, temperature, and electron density profiles midway through the implosion, while extreme ultraviolet pinhole cameras record the evolution of the plasma column and photoconducting diodes measure x-ray emission. Analysis of the scattering spectra includes a means of discriminating between thermal and non-thermal broadening to test for the presence of hydrodynamic turbulence. Two types of sheath profiles are observed, those with sharp discontinuities at the leading edge and those with smooth gradients. In both cases, non-thermal broadening is generally peaked at the front of the sheath and exhibits a characteristic decay length that roughly scales with the sheath ion mean free path. We demonstrate that this non-thermal broadening term is inconsistent with laminar velocity gradients and is more consistent with dissipative turbulence driven by unstable plasma waves in a collisionless shock. The resulting differences in sheath profile are then set by the sheath ion collisionality in a manner consistent with recent 1D kinetic simulations [Angus et al., Phys. Plasmas 28, 010701 (2021)].

Effect of tailored density profiles on the stability of imploding Z-pinches at microsecond rise time megaampere currents

To study the effect of the radial density profile of the material of a metal-plasma Z-pinch load on the development of magneto-Rayleigh–Taylor (MRT) instabilities, experiments have been performed at the Institute of High Current Electronics with the GIT-12 generator which produces microsecond rise time megaampere currents. The Z-pinch load was an aluminum plasma jet (PJ) with an outer plasma shell. This configuration leads to the formation of a uniform current sheath in a Z-pinch load upon application of a high-voltage pulse. It was successfully used in experiments with hybrid deuterium gas-puffs (Klir et al 2020 New J. Phys. 22 103036). The initial density profiles of the Z-pinch loads were estimated from the pinch current and voltage waveforms using the zero-dimensional ‘snowplow’ model, and they were verified by simulating the expansion of the PJ formed by a vacuum arc using a two-dimensional quasi-neutral hybrid model (Shmelev et al 2020 Phys. Plasmas 27 092708). Two Z-pinch load configurations were used in the experiments. The first configuration provided tailored load density profiles, which could be described as ρ(r) ≈ 1/r^s for s > 2. In this case, MRT instabilities were suppressed and thus a K-shell radiation yield of 11 kJ cm−1 and a peak power of 0.67 TW cm−1 could be attained at a current of about 3 MA. For the second configuration, the radial density profiles were intentionally changed using a reflector. This led to the appearance of a notch in the density profiles at radii of 1–3 cm from the pinch axis and to magnetohydrodynamic instabilities at the final implosion stage. As a result, the K-shell radiation yield more than halved and the power decreased to 0.15 TW cm−1 at a current of about 3.5 MA.

Visualizing magnetically driven converging radiative shock generated in Z-pinch foil liner implosion

A study of the evolution and structure of magnetically driven converging radiative shock waves generated in Z-pinch foil liner implosion at an 8-MA pulsed-power facility is presented. End-on extreme ultraviolet images show an inward propagating shock that is circular to <±5% as a function of azimuthal angle, with a standard deviation in the emission intensity of <±30%, implying good cylindrical symmetry. The launch time and shock trajectory are determined by linear fitting of the measured data, giving a shock speed of Mach 6. One-dimensional radiation hydrodynamics MULTI-IFE-Z simulations agree with the experimental observations qualitatively and confirm the existence of a radiative precursor. It is demonstrated with experiment and simulation that the radiative shock wave is generated by magnetic piston compression of dense plasma shell. Analytic estimates of the post-shock plasma conditions suggest that these Z-pinch magnetically-driven high-Mach shocks are strongly radiatively cooled. It is applicable to the optically thick downstream, optically thin upstream radiative shock regime; thus, it can be described by three-layer model, which potentially could be applied to scale studies of astrophysical shocks in the laboratory.

Effect of the plasma self-radiation on the growth of thermal filamentation instabilities in imploding Z pinches

The development of thermal filamentation (TF) instabilities in a current-carrying plasma shell under the action of the plasma self-radiation was analyzed in terms of a small perturbation theory. A stationary collisional radiative model was used to calculate the parameters of the bremsstrahlung, recombination radiation, and spectral line radiation. It has been shown that radiative losses can either enhance or weaken the growth of TF instabilities. The pattern of the effect is governed by the dependence of the energy lost by the plasma due to radiation, Q Rad, on the plasma temperature T. If Q Rad increases slower than ∼T, the radiative losses enhance TF instabilities. In the opposite case, that is when Q Rad increases faster than ∼T, the radiative losses lead to suppression of TF instabilities. When the energy lost due to radiation is greater than the Joule energy input, TF instabilities can be completely stabilized due to radiation. The plasma parameter ranges for which stabilization of TF instabilities may occur due to radiation have been found for aluminum and argon.

Fermi acceleration in relativistic collisionless plasma shocks correlates with anisotropic energy gains

Collisionless shocks generated by two colliding relativistic electron-positron plasma shells are studied using particle-in-cell simulations. Shocks are mediated by the Weibel instability (WI), and the kinetic energy of the fastest accelerated particles is found to be anisotropically modified by WI-induced electric fields. Specifically, we show that all particles interacting with the shock bifurcate into two groups based on their final relativistic Lorentz factor γ: slow (γ<γbf) and fast (γ>γbf), where γbf is the bifurcation Lorentz factor that was found to be approximately twice the initial (upstream) Lorentz factor γ0. We have found that the kinetic energies of the slow particles are equally affected by the longitudinal and transverse components of the shock electric field, whereas the fast particles are primarily accelerated by the transverse field component.

Stable filamentary structures in atmospheric pressure microwave plasma torch

This paper experimentally investigates the processes governing the single- and multi-filament regimes in an atmospheric pressure microwave (MW) torch operated in argon. Optical emission spectroscopy and spectral imaging are the principal diagnostics techniques which are employed. MW power is found to be the main parameter controlling the number of filaments. The single-filament regime exhibits many properties typical for surface wave discharges, e.g. a linear decrease in electron density along the axis or the existence of a central dip in the radial/lateral emission profiles. Simple geometric quantities, such as the length or thickness of the filament(s), vary almost linearly with the input MW power, and exhibit discontinuities at successive filament splitting events. These take place at similar values of filament maximum thickness, and may be due to skin-depth limited power transfer. The presence and chemistry of a low-emission intensity plasma shell surrounding the filament(s) is also investigated. The gas temperature is estimated from the OH band and complemented by Schlieren imaging, which revealed that a much larger cone of gas is being heated by filaments than is their diameter.

K-shell radiation and neutron emission from z-pinch plasmas generated by hybrid gas-puff implosions onto on-axis wires

Z-pinches have been explored as efficient soft x-ray sources for many years. To optimize x-ray emission, various z-pinch configurations were tested. This paper presents data obtained with a hybrid gas-puff z-pinch imploding onto on-axis wires on a microsecond, multi-megaampere GIT-12 generator. In our previous experiments, the hybrid gas puff, i.e., an inner deuterium gas puff surrounded by an outer hollow cylindrical plasma shell, was used to produce energetic protons, deuterons, and neutrons up to 60 MeV [Klir et al., New J. Phys. 22, 103036 (2020)]. The behavior of the hybrid gas-puff z-pinch on GIT-12 was interpreted as a high-density plasma opening switch with a microsecond conduction time, 3 MA conduction current, nanosecond opening, and up to 60 MV stand-off voltage. These properties can be employed to transfer the current into an on-axis load with a high rise rate. In the recent experiments on GIT-12, we therefore placed single or multiple aluminum wires on the axis of the hybrid gas-puff z-pinch. Before a current sheath arrived at the axis, a coronal plasma was seen around the wire. A rapid increase in x-ray radiation was observed when the coronal plasma imploded onto the axis. The coronal plasma implosion resulted in a long (2 cm), narrow (∼mm) column radiating in the Al K-shell lines. With the single Al wire of 80 μm diameter, the K-shell x-ray output reached 5.5 ± 0.8 kJ in a 0.6 ± 0.1 TW peak power and 7 ± 1 ns pulse. The higher K-shell yield of 12 ± 2 kJ and peak K-shell power of 0.7 ± 0.1 TW were achieved with four 38 μm diameter Al wires. (Their cross section formed the corners of a square with 1 mm side.) The presence of the wires on the axis significantly suppressed ion acceleration and neutron production. Deuterium-deuterium (DD) neutron yields of about 1.2 × 1011 were 20 times smaller than the yields produced in shots without any wire. The DD neutron yield was increased up to 4.5 × 1011 when the Al wire was replaced by a fiber from deuterated polyethylene. A characteristic feature of the experiments with the (CD2)n fiber was a rapid expansion with the velocity approaching 900 km/s.