Cylindrical liner z-pinch experiments on the magpie generator

Journal of Quantitative Spectroscopy and Radiative Transfer, 2000

Z-pinches, created using the Z accelerator, generate &220 TW, 1.7 MJ radiation pulses that can heat large (&10 cm) hohlraums to 100}150 eV temperatures for times of order 10 ns. We are performing experiments exploiting this intense radiation to drive shock waves for equation of state studies. The shock pressures are typically 1}10 Mbar with 10 ns duration in 6-mm-diameter samples. We demonstrate the ability to perform optical spectroscopy measurements on shocked samples located in close proximity to the z-pinch. These experiments are particularly well suited to optical spectroscopy because of the relatively large sample size and long duration. The optical emission is collected using "ber optics and recorded with a streaked spectrograph. Other diagnostics include VISAR and active shock breakout measurements of the shocked sample and a suite of diagnostics that characterize the radiation drive. Our near term goal is to use the spectral emission to obtain the temperature of the shocked material. Longer term objectives include the examination of deviations of the spectrum from blackbody, line emission from lower density regions, determination of kinetic processes in molecular systems, evaluation of phase transitions such as the onset of metallization in transparent materials, and characterization of the plasma formed when the shock exits the rear surface. An initial set of data illustrating both the potential and the challenge of these measurements is described.

Plasma Physics and Controlled Fusion

Contemporary lasers allow us to create shocks in the laboratory that propagate at a speed that matches that of energetic astrophysical shocks like those that ensheath supernova blast shells. The rapid growth time of the shocks and the spatio-temporal resolution, with which they can be sampled, allow us to identify the processes that are involved in their formation and evolution. Some laser-generated unmagnetized shocks are mediated by collective electrostatic forces and effects caused by binary collisions between particles can be neglected. Hydrodynamic models, which are valid for many large-scale astrophysical shocks, assume that collisions enforce a local thermodynamic equilibrium in the medium; laser-generated shocks are thus not always representative for astrophysical shocks. Laboratory studies of shocks can improve the understanding of their astrophysical counterparts if we can identify processes that affect electrostatic shocks and hydrodynamic shocks alike. An example is the nonlinear thin-shell instability (NTSI). We show that the NTSI destabilizes collisionless and collisional shocks by the same physical mechanism.

Astronomy & Astrophysics, 2009

Radiative shocks are found in various astrophysical objects and particularly at different stages of stellar evolution. Studying radiative shocks, their topology, and thermodynamical properties is therefore a starting point to understanding their physical properties. This study has become possible with the development of large laser facilities, which has provided fresh impulse to laboratory astrophysics. We present the main characteristics of radiative shocks modeled using cylindrical simulations. We focus our discussion on the importance of multi-dimensional radiative-transfer effects on the shock topology and dynamics. We present results obtained with our code HERACLES for conditions corresponding to experiments already performed on laser installations. The multi-dimensional hydrodynamic code HERACLES is specially adapted to laboratory astrophysics experiments and to astrophysical situations where radiation and hydrodynamics are coupled. The importance of the ratio of the photon mean free path to the transverse extension of the shock is emphasized. We present how it is possible to achieve the stationary limit of these shocks in the laboratory and analyze the angular distribution of the radiative flux that may emerge from the walls of the shock tube. Implications of these studies for stellar accretion shocks are presented.

Plasma Physics Reports, 2002

It is shown that the development of instabilities in a Z-pinch plasma formed by loading a relatively thick Al wire (an initial diameter of 120 µ m and a maximum discharge current of 2-3 MA) is slowed down due to the high plasma density in the wire corona. A cylindrically symmetric, regular, and stable corona surrounding the wire contains a helical formation with a dense, cold, and magnetized plasma. X-ray pulses with a photon energy of several keV and an FWHM duration of 10-20 ns are generated by a few imploded neck structures in the pinch phase of the corona evolution (70-100 ns after the current onset). The main part of X radiation emitted by individual bright spots in the photon energy range 1.5-2.4 keV (up to 40 J at a peak power of 4 GW) consists of the continuum and the bound-bound transition radiation from Hand He-like Al ions. A possible scenario for the axial magnetic field evolution during an X-ray pulse is outlined.

Physics of Plasmas, 2013

The development and use of a single-fluid two-temperature approximated 2-D Magneto-Hydrodynamics code is reported. Z-pinch dynamics and the evolution of Magneto-Rayleigh-Taylor (MRT) instabilities in a gas jet type Extreme Ultraviolet (EUV) source are investigated with this code. The implosion and stagnation processes of the Z-pinch dynamics and the influence of initial perturbations (single mode, multi- mode, and random seeds) on MRT instability are discussed in detail. In the case of single mode seeds, the simulation shows that the growth rates for mm-scale wavelengths up to 4 mm are between 0.05 and 0.065 ns−1. For multi-mode seeds, the mode coupling effect leads to a series of other harmonics, and complicates MRT instability evolution. For perturbation by random seeds, the modes evolve to longer wavelengths and finally converge to a mm-scale wavelength approximately 1 mm. MRT instabilities can also alter the pinch stagnation state and lead to temperature and density fluctuations...

Physical Review E, 2021

An experimental study of the magnetic field distribution in gas-puff Z pinches with and without a preembedded axial magnetic field (B z0) is presented. Spatially resolved, time-gated spectroscopic measurements were made at the Weizmann Institute of Science on a 300 kA, 1.6 μs rise time pulsed-power driver. The radial distribution of the azimuthal magnetic field, B θ , during the implosion, with and without a preembedded axial magnetic field of B z0 = 0.26 T, was measured using Zeeman polarization spectroscopy. The spectroscopic measurements of B θ were consistent with the corresponding values of B θ inferred from current measurements made with a B-dot probe. One-dimensional magnetohydrodynamic simulations, performed with the code TRAC-II, showed agreement with the experimentally measured implosion trajectory, and qualitatively reproduced the experimentally measured radial B θ profiles during the implosion when B z0 = 0.26 T was applied. Simulation results of the radial profile of B θ without a preembedded axial magnetic field did not qualitatively match experimental results due to magneto-Rayleigh-Taylor (MRT) instabilities. Our analysis emphasizes the importance of MRT instability mitigation when studying the magnetic field and current distributions in Z pinches. Discrepancies of the simulation results with experiment are discussed.

New Journal of Physics, 2018

Acceleration of high energy ions was observed in z-pinches and dense plasma foci as early as the 1950s. Even though many theories have been suggested, the ion acceleration mechanism remains a source of controversy. Recently, the experiments on the GIT-12 generator demonstrated acceleration of ions up to 30 MeV from a deuterium gas-puff z-pinch. High deuteron energies enable us to obtain unique information about spatial, spectral and temporal properties of accelerated ions. In particular, the offaxis ion emission from concentric circles of a ∼1 cm diameter and the radial lines in an ion beam profile are germane for the discussion of acceleration mechanisms. The acceleration of 30 MeV deuterons can be explained by the fast increase of an impedance with a sub-nanosecond e-folding time. The high (>10 Ω) impedance is attributed to a space-charge limited flow after the effective ejection of plasmas from m=0 constrictions. Detailed knowledge of the ion acceleration mechanism is used with a neutron-producing catcher to increase neutron yields above 10 13 at a currentof2.7 MA.

The Astrophysical Journal, 2000

We report on the initial results of experiments being developed on the Falcon laser to simulate radiative astrophysical shocks. Cylindrically diverging blast waves were produced in low-density (∼10 18 cm ), high-Z gas Ϫ3 by laser-irradiating Xe gas jets containing atomic clusters. The blast-wave trajectory was measured by Michelson interferometry. The velocity for the blast wave is slightly less than the adiabatic Sedov-Taylor prediction, and an ionization precursor is observed ahead of the shock front. This suggests energy loss through radiative cooling and reduced compression due to preheat deposited ahead of the shock, both consistent with one-dimensional radiation hydrodynamics simulations.

2000

The properties of laser-produced plasma are similar in many features to the ones of various space and astrophysical plasma releases of explosive nature. It allows to investigate the processes of an interaction of those natural plasmas with background media by the means of laboratory simulation. In the given paper such simulation experiments with quasi-spherical plasma clouds (from small pellet target) and with plasma streams (from solid ones) are described. This experiments are related to the dynamics of collisionless expansion of exploding space plasmas into surrounding magnetized media at various Alfven-Mach numbers. A non-uniform media properties as well has been simulated. Results are analysed in connection with dynamics of Supernova remnants in interstellar medium, Barium or others releases in geoplasma and possible disturbances of the latter caused by high-energy explosions.

Physical Review E, 2009

It is known from experiments that the radiated x-ray energy appears to exceed the calculated implosion kinetic energy and Spitzer resistive heating ͓C. Deeney et al., Phys. Rev. A 44, 6762 ͑1991͔͒ but possible mechanisms of the enhanced x-ray production are still being discussed. Enhanced plasma heating in smalldiameter wire arrays with decreased calculated kinetic energy was investigated, and a review of experiments with cylindrical arrays of 1-16 mm in diameter on the 1 MA Zebra generator is presented in this paper. The implosion and x-ray generation in cylindrical wire arrays with different diameters were compared to find a transition from a regime where thermalization of the kinetic energy is the prevailing heating mechanism to regimes with other dominant mechanisms of plasma heating. Loads of 3-8 mm in diameter generate the highest x-ray power at the Zebra generator. The x-ray power falls in 1-2 mm loads which can be linked to the lower efficiency of plasma heating with the lack of kinetic energy. The electron temperature and density of the pinches also depend on the array diameter. In small-diameter arrays, 1-3 mm in diameter, ablating plasma accumulates in the inner volume much faster than in loads of 12-16 mm in diameter. Correlated bubblelike implosions were observed with multiframe shadowgraphy. Investigation of energy balance provides evidence for mechanisms of nonkinetic plasma heating in Z pinches. Formation and evolution of bright spots in Z pinches were studied with a time-gated pinhole camera. A comparison of x-ray images with shadowgrams shows that implosion bubbles can initiate bright spots in the pinch. Features of the implosions in smalldiameter wire arrays are discussed to identify mechanisms of energy dissipation.