We observe linear and nonlinear features of a strong plasma-magnetic-field interchange RayleighTaylor instability in the limit of large ion Larmor radius. The instability undergoes rapid linear growth culminating in free-streaming flute tips.PACS numbers: 52.35. Gz, 52.35.Py, 52.50.Lp, 52.55.Lf Plasma expanding into a magnetic field can undergo Rayleigh-Taylor or interchange instability as the heavy fluid (plasma) is decelerated by the light fluid (magnetic field). 1>2 Direct observations of this instability have been made in the limit of small ion Larmor radius (compared to density gradients and wavelengths), 3 where conventional MHD theory applies. When the ion Larmor radius becomes finite the instability is predicted to stabilize. 4 However, when the ion Larmor radius becomes large compared to other characteristic plasma dimensions, i.e., when the ions are effectively unmagnetized but the electrons are effectively magnetized, a related instability is predicted with an even higher growth rate than that of the original MHD instability. 5 The recent barium-release space experiment with the Active Magnetospheric Particle Tracer Explorer satellite, which showed substantial structure, was in such a regime. 6 A previous laser-plasma experiment in a regime of moderate-sized ion Larmor radius also measured instability growth. 7 In this paper, we observe a robust interchangelike instability in the limit of very large ion Larmor radius. The instability exhibits a rapid linear phase with subsequent nonlinear free-streaming flutes and examples of density clumping, flute-tip bifurcation, and interesting late-time spirallike structures.Our experiment is comprised of an energetic laserproduced plasma expanding radially outward into a uniform magnetic field B formed by a pair of Helmholtz coils, 8 as depicted in Fig. 1. Steady-state (on the time scale of the experiment) vacuum B fields from 0 to 1 T are used. Plasma bursts are created by our focusing a beam of the Pharos III neodymium laser onto small Al (2 jum thick, 1 mm diam) disk targets. Unless noted otherwise, the nominal laser pulse has an irradiance of about 10 13 W/cm 2 , 30 J of energy, and 3-ns duration (FWHM). The principal diagnostic used to measure the plasma and instability development is a Grant Applied Physics fast-gated microchannel-plate optical camera focused onto the target midplane antiparallel (usually) to the magnetic field lines. Shutter speeds of 1 or 2 ns are used. In addition to the gated camera, we also used ion time-of-flight detectors to measure the plasma ion velocity distribution, several small (230 jj.m diam, two turn) magnetic induction probes to obtain magnetic field dynamics, small Langmuir and capacitive probes to measure density gradients and fluctuations, open-shutter photography and witness plates to see persistent structure, and fiber-optic spectroscopy to estimate density profiles during the plasma/magnetic field interaction.The velocity distribution of the expanding plasma, measured for B =0 with an ion time-of-flight detector, pe...

Krypton-fluoride (KrF) lasers are of interest to laser fusion because they have both the large bandwidth capability (≳THz) desired for rapid beam smoothing and the short laser wavelength (1/4 μm) needed for good laser–target coupling. Nike is a recently completed 56-beam KrF laser and target facility at the Naval Research Laboratory. Because of its bandwidth of 1 THz FWHM (full width at half-maximum), Nike produces more uniform focal distributions than any other high-energy ultraviolet laser. Nike was designed to study the hydrodynamic instability of ablatively accelerated planar targets. First results show that Nike has spatially uniform ablation pressures (Δp/p<2%). Targets have been accelerated for distances sufficient to study hydrodynamic instability while maintaining good planarity. In this review we present the performance of the Nike laser in producing uniform illumination, and its performance in correspondingly uniform acceleration of targets.

We have developed an improved x-ray imaging system based on spherically curved crystals. It is designed and used for diagnostics of targets ablatively accelerated by the Nike KrF laser. A spherically curved quartz crystal (d = .?, R = mm) has been used to produce monochromatic backlit images with the He-like Si resonance line (1865 eV) as the source of radiation. The spatial resolution of the x-ray optical system is 1.7 mum in selected places and 2-3 mum over a larger area. Time-resolved backlit monochromatic images of polystyrene planar targets driven by the Nike facility have been obtained with a spatial resolution of 2.5 mum in selected places and 5 mum over the focal spot of the Nike laser.

We present experimental results and simulations that study the effects of thin metallic layers with high atomic number (high-Z) on the hydrodynamics of laser accelerated plastic targets. These experiments employ a laser pulse with a low-intensity foot that rises into a high-intensity main pulse. This pulse shape simulates the generic Report Documentation PageForm Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. We present experimental results and simulations that study the effects of thin metallic layers with high atomic number (high-Z) on the hydrodynamics of laser accelerated plastic targets. These experiments employ a laser pulse with a low-intensity foot that rises into a high-intensity main pulse. This pulse shape simulates the generic shape needed for high-gain fusion implosions. Imprint of laser nonuniformity during start up of the low intensity foot is a well-known seed for hydrodynamic instability. We observe large reductions in hydrodynamic instability seeded by laser imprint when certain minimum thickness gold or palladium layers are applied to the laser-illuminated surface of the targets. The experiment indicates that the reduction in imprint is at least as large as that obtained by a 6 times improvement in the laser uniformity. We present simulations supported by experiments showing that during the low intensity foot the laser light can be nearly completely absorbed by the high-Z layer. X-rays originating from the high-Z layer heat the underlying lower-Z plastic target material and cause large buffering plasma to form between the layer and the accelerated target. This long-scale plasma apparently isolates the target from laser nonuniformity and accounts for the observed large reduction in laser imprint. With onset of the higher intensity main pulse, the high-Z layer expands and the laser light is transmitted. This technique will be useful in reducing laser imprint in pellet implosions and thereby allow the design of more robust targets for high-gain laser fusion. Prescribed by ANSI Std Z39-18 2 shape needed for high-gain fusion implosions. Imprint of laser nonuniformity during start up of the low intensity foot is a well-known seed for hydrodynamic instability. We observe large reductions in hydrodynamic instability seeded by laser...

Perturbations that seed Rayleigh-Taylor (RT) instability in laser-driven targets form during the early-time period. This time includes a shock wave transit from the front to the rear surface of the target, and a rarefaction wave transit in the opposite direction. During this time interval, areal mass perturbations caused by all sources of nonuniformity (laser imprint, surface ripple) are expected to oscillate. The first direct experimental observations of the areal mass oscillations due to ablative Richtmyer-Meshkov (RM) instability and feedout followed by the RT growth of areal mass modulation are discussed. The experiments were made with 40 to 99 µm thick planar plastic targets rippled either on the front or on the rear with a sine wave ripple with either Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

Feedout means the transfer of mass perturbations from the rear to the front surface of a driven target. When a planar shock wave breaks out at a rippled rear surface of the target, a lateral pressure gradient drives sonic waves in a rippled rarefaction wave propagating back to the front surface. This process redistributes mass in the volume of the target, forming the feedout-generated seed for ablative Rayleigh-Taylor (RT) instability. We report the first direct experimental observation of areal mass oscillation associated with feedout, followed by the onset of exponential RT growth.

We report the first direct experimental observation of the ablative RichtmyerMeshkov instability. It manifests itself in oscillations of areal mass that occur during the shock transit time, which are caused by the rocket effect or dynamic overpressure characteristic of interaction between the laser absorption zone and the ablation front.With the 4 ns long Nike KrF laser pulse and our novel diagnostic technique (monochromatic x-ray imaging coupled to a streak camera) we were able to register a peak and a valley of the areal mass variation before the observed onset of the RayleighTaylor growth.PACS numbers: 52.57. Fg, 52.70.La, Report Documentation PageForm Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. ABSTRACTWe report the first direct experimental observation of the ablative Richtmyer-Meshkov instability. It manifests itself in oscillations of areal mass that occur during the shock transit time, which are caused by the rocket effect or dynamic overpressure characteristic of interaction between the laser absorption zone and the ablation front. With the 4 ns long Nike KrF laser pulse and our novel diagnostic technique (monochromatic x-ray imaging coupled to a streak camera) we were able to register a peak and a valley of the areal mass variation before the observed onset of the Rayleigh-Taylor growth. Most of our knowledge in this field still comes from simulations and theory. The seed amplitudes are known to form during the early-time period that includes a shock wave transit from the front to the rear surface of the target, and a rarefaction wave transit in the opposite direction. During this time interval, the areal mass perturbations caused by all sources of non-uniformity are expected to oscillate [4][5][6][7][8][9]. The physical mechanisms driving the oscillations depend on whether the perturbations are initially at the front surface of the target (laser imprint, front surface roughness) or at its rear surface (feedout, see [9]). Here, we limit ourselves to the former case, where the oscillations are caused by the rocket effect, or the dynamic overpressure [6,8,10,11]. The oscillatory behavior is consistent with the expression for the growth rate Γ of ablative RT instability, which has recently been established [10,11] for the case of large Froude number (low acceleration): perturbed, and a part of it gets closer to the hot laser absorption zone, the temperature at the ablation front does not increase, but the temperature gradient in its vicinity, T ∇ , does. This, in turn, increases the local heat flux to the ablation front, T ∇ − κ , hence, the rate of mass ablation from it, thereby increasing the ablative pressure and pushing this part of the ablation front back. The physics of this rocket effect is explained in detail inRefs. 6,8,10. The rocket effect rat...

FAST is a radiation transport hydrodynamics code that simulates laser matter interactions of relevance to direct-drive laser fusion target design. FAST solves the Euler equations of compressible flow using the Flux-Corrected Transport finite volume method. The advection algorithm provides accurate computation of flows from nearly incompressible vortical flows to those that are highly compressible and dominated by strong pressure and density gradients. In this paper we describe the numerical techniques and physics packages. FAST has also been benchmarked with Nike laser facility experiments in which linearly perturbed, low adiabat planar plastic targets are ablatively accelerated to velocities approaching 107 cm/s. Over a range of perturbation wavelengths, the code results agree with the measured Rayleigh–Taylor growth from the linear through the deeply nonlinear regimes. FAST has been applied to the two-dimensional spherical simulation design to provide surface finish and laser bandwidth tolerances for a promising new direct-drive pellet that uses a foam ablator.