B.E. Blue

Measurements of magneto-Rayleigh–Taylor instability growth during the implosion of initially solid metal liners a)

A recent publication [D. B. Sinars et al., Phys. Rev. Lett. 105, 185001 (2010)] describes the first controlled experiments measuring the growth of the magneto-Rayleigh–Taylor instability in fast (∼100 ns) Z-pinch plasmas formed from initially solid aluminum tubes (liners). Sinusoidal perturbations on the surface of these liners with wavelengths of 25–400 μm were used to seed single-mode instabilities. The evolution of the outer liner surface was captured using multiframe 6.151 keV radiography. The initial paper shows that there is good agreement between the data and 2-D radiation magneto-hydrodynamic simulations down to 50 μm wavelengths. This paper extends the previous one by providing more detailed radiography images, detailed target characterization data, a more accurate comparison to analytic models for the amplitude growth, the first data from a beryllium liner, and comparisons between the data and 3D simulations.

FLUID DYNAMICS OF STELLAR JETS IN REAL TIME: THIRD EPOCH HUBBLE SPACE TELESCOPE IMAGES OF HH 1, HH 34, AND HH 47

We present new, third-epoch Hubble Space Telescope H? and [S II] images of three Herbig-Haro (HH) jets (HH?1&2, HH?34, and HH?47) and compare the new images with those from previous epochs. The high spatial resolution, coupled with a time series whose cadence is of order both the hydrodynamic and radiative cooling timescales of the flow, allows us to follow the hydrodynamic/magnetohydrodynamic evolution of an astrophysical plasma system in which ionization and radiative cooling play significant roles. Cooling zones behind the shocks are resolved, so it is possible to identify which way material flows through a given shock wave. The images show that heterogeneity is paramount in these jets, with clumps dominating the morphologies of both bow shocks and their Mach disks. This clumpiness exists on scales smaller than the jet widths and determines the behavior of many of the features in the jets. Evidence also exists for considerable shear as jets interact with their surrounding molecular clouds, and in several cases we observe shock waves as they form and fade where material emerges from the source and as it proceeds along the beam of the jet. Fine structure within two extended bow shocks may result from Mach stems that form at the intersection points of oblique shocks within these clumpy objects. Taken together, these observations represent the most significant foray thus far into the time domain for stellar jets, and comprise one of the richest data sets in existence for comparing the behavior of a complex astrophysical plasma flow with numerical simulations and laboratory experiments.

Beryllium liner implosion experiments on the Z accelerator in preparation for magnetized liner inertial fusion

Multiple experimental campaigns have been executed to study the implosions of initially solid beryllium (Be) liners (tubes) on the Z pulsed-power accelerator. The implosions were driven by current pulses that rose from 0 to 20 MA in either 100 or 200u2009ns (200u2009ns for pulse shaping experiments). These studies were conducted in support of the recently proposed Magnetized Liner Inertial Fusion concept [Slutz et al., Phys. Plasmas 17, 056303 (2010)], as well as for exploring novel equation-of-state measurement techniques. The experiments used thick-walled liners that had an aspect ratio (initial outer radius divided by initial wall thickness) of either 3.2, 4, or 6. From these studies, we present three new primary results. First, we present radiographic images of imploding Be liners, where each liner contained a thin aluminum sleeve for enhancing the contrast and visibility of the liners inner surface in the images. These images allow us to assess the stability of the liners inner surface more accurately and more directly than was previously possible. Second, we present radiographic images taken early in the implosion (prior to any motion of the liners inner surface) of a shockwave propagating radially inward through the liner wall. Radial mass density profiles from these shock compression experiments are contrasted with profiles from experiments where the Z accelerators pulse shaping capabilities were used to achieve shockless (“quasi-isentropic”) liner compression. Third, we present “micro-B” measurements of azimuthal magnetic field penetration into the initially vacuum-filled interior of a shocked liner. Our measurements and simulations reveal that the penetration commences shortly after the shockwave breaks out from the liners inner surface. The field then accelerates this low-density “precursor” plasma to the axis of symmetry.

Laboratory Experiments, Numerical Simulations, and Astronomical Observations of Deflected Supersonic Jets: Application to HH 110

Collimated supersonic flows in laboratory experiments behave in a similar manner to astrophysical jets provided that radiation, viscosity, and thermal conductivity are unimportant in the laboratory jets and that the experimental and astrophysical jets share similar dimensionless parameters such as the Mach number and the ratio of the density between the jet and the ambient medium. When these conditions apply, laboratory jets provide a means to study their astrophysical counterparts for a variety of initial conditions, arbitrary viewing angles, and different times, attributes especially helpful for interpreting astronomical images where the viewing angle and initial conditions are fixed and the time domain is limited. Experiments are also a powerful way to test numerical fluid codes in a parameter range in which the codes must perform well. In this paper, we combine images from a series of laboratory experiments of deflected supersonic jets with numerical simulations and new spectral observations of an astrophysical example, the young stellar jet HH 110. The experiments provide key insights into how deflected jets evolve in three dimensions, particularly within working surfaces where multiple subsonic shells and filaments form, and along the interface where shocked jet material penetrates into and destroys the obstacle along its path. The experiments also underscore the importance of the viewing angle in determining what an observer will see. The simulations match the experiments so well that we can use the simulated velocity maps to compare the dynamics in the experiment with those implied by the astronomical spectra. The experiments support a model where the observed shock structures in HH 110 form as a result of a pulsed driving source rather than from weak shocks that may arise in the supersonic shear layer between the Mach disk and bow shock of the jets working surface.

TWO-DIMENSIONAL BLAST-WAVE-DRIVEN RAYLEIGH-TAYLOR INSTABILITY: EXPERIMENT AND SIMULATION

This paper shows results from experiments diagnosing the development of the Rayleigh-Taylor instability with two-dimensional initial conditions at an embedded, decelerating interface. Experiments are performed at the Omega Laser and use ~5 kJ of energy to create a planar blast wave in a dense, plastic layer that is followed by a lower density foam layer. The single-mode interface has a wavelength of 50 μm and amplitude of 2.5 μm. Some targets are supplemented with additional modes. The interface is shocked then decelerated by the foam layer. This initially produces the Richtmyer-Meshkov instability followed and then dominated by Rayleigh-Taylor growth that quickly evolves into the nonlinear regime. The experimental conditions are scaled to be hydrodynamically similar to SN1987A in order to study the instabilities that are believed to occur at the He/H interface during the blast-wave-driven explosion phase of the star. Simulations of the experiment were performed using the FLASH hydrodynamics code.

Diagnosing laser-preheated magnetized plasmas relevant to magnetized liner inertial fusion

We present a platform on the OMEGA EP Laser Facility that creates and diagnoses the conditions present during the preheat stage of the MAGnetized Liner Inertial Fusion (MagLIF) concept. Experiments were conducted using 9u2009kJ of 3ω (355u2009nm) light to heat an underdense deuterium gas (electron density: 2.5×1020u2009cm−3=0.025 of critical density) magnetized with a 10u2009T axial field. Results show that the deuterium plasma reached a peak electron temperature of 670u2009±u2009140u2009eV, diagnosed using streaked spectroscopy of an argon dopant. The results demonstrate that plasmas relevant to the preheat stage of MagLIF can be produced at multiple laser facilities, thereby enabling more rapid progress in understanding magnetized preheat. Results are compared with magneto-radiation-hydrodynamics simulations, and plans for future experiments are described.

WHEN SHOCK WAVES COLLIDE

Supersonic outflows from objects as varied as stellar jets, massive stars and novae often exhibit multiple shock waves that overlap one another. When the intersection angle between two shock waves exceeds a critical value, the system reconfigures its geometry to create a normal shock known as a Mach stem where the shocks meet. Mach stems are important for interpreting emission-line images of shocked gas because a normal shock produces higher postshock temperatures and therefore a higher-excitation spectrum than an oblique one does. In this paper we summarize the results of a series of numerical simulations and laboratory experiments designed to quantify how Mach stems behave in supersonic plasmas that are the norm in astrophysical flows. The experiments test analytical predictions for critical angles where Mach stems should form, and quantify how Mach stems grow and decay as intersection angles between the incident shock and a surface change. While small Mach stems are destroyed by surface irregularities and subcritical angles, larger ones persist in these situations, and can regrow if the intersection angle changes to become more favorable. The experimental and numerical results show that although Mach stems occur only over a limited range of intersection angles and size scales, within these ranges they are relatively robust, and hence are a viable explanation for variable bright knots observed in HST images at the intersections of some bow shocks in stellar jets.

Ablation dynamics, precursor formation, and instability studies on thin foil copper liners

Summary form only given. Solid liners are an attractive load design, for use with magnetized liner inertial fusion (MagLIF) and for possible replacement of wire-array Z-pinches on larger pulsed-power drivers. Comparisons between equally massed and dimensioned wire-arrays and thin foil liners will be presented. Using an axial X pinch backlighter1, the ablation characteristics and precursor formation are studied. The liners show very little ablation and precursor as compared to wire-arrays.Using laser shadowgraphy and comparisons with the extended magnetohydrodynamics (XMHD) PERSEUS code2, instability growth on the outside of the liners is examined. The instability is found not to be magnetic Rayleigh-Taylor (MRT), but a velocity shear flow instability.

Dramatic reduction of Magneto-Rayleigh Taylor instability growth in magnetically driven Z-pinch liners

Summary form only given. In this paper, we will present new Z-pinch liner experiments on Sandias Z facility (20 MA, 100ns current pulse) that demonstrate the integral Magneto-Rayleigh-Taylor (MRT) instability growth can be dramatically limited by controlling the growth of electrothermal instabilities that form early in the electrical current pulse as Joule heating melts and vaporizes the liner surface[1]. In these experiments, we implode Al and Be solid liners to inertial confinement fusion relevant velocities [2] and large convergence ratios and show that thick (~70 μm) dielectric coatings are very effective in controlling the deleterious effects of the electrothermal instability, limiting the seed for subsequent MRT growth, and ultimately lead to a more stable implosion. These experiments extend the previously reported work on the Z facility which also showed dramatic reduction of instability growth in non-imploding solid metallic rods[3].

The effect of surface roughness and structure on subsequent magneto-Rayleigh-Taylor instability growth in beryllium liner implosions on Z

Summary form only given. Sandia is investigating a new magnetized liner inertial fusion (MagLIF) concept that uses cylindrical Be or Al liners to compress magnetized and preheated fusion fuel. As part of this work, we have been studying the growth of instabilities in initially solid liners driven with 20-24 MA, 100-ns current pulses on the Z pulsed power facility. The magneto-Rayleigh-Taylor instability in particular can disrupt the plasma liner during its implosion. A remarkable degree of azimuthal symmetry is observed near stagnation in beryllium liner implosions. This symmetry is captured in 3D calculations only when some azimuthally correlated perturbations are seeded initially. One possibility is that the MRT instability is directly seeded by azimuthally correlated, fine-scale structures on the surface that result from diamond turning the liner on a lathe. A second possibility is that the electro-thermal instability is seeding the MRT instability. Simulations suggest that the level of instability growth seeded by the ET instability is not strongly dependent on the surface roughness. Understanding the surface finish requirements for liner implosions is an important practical question. We will discuss the results of experiments in which the surface of Be liners was altered by polishing the liners along the axial direction after machining. This removes the azimuthally correlated structure, leaving only axially correlated grooves. The latter should noticeably affect the growth of MRT if the surface structure is directly seeding the instability, but may not have any impact if the ET instability is the dominant seed.