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...

Highly collimated plasmas jets are produced with laser irradiation of solid barium targets. The plasma streams many Larmor radii across a strong transverse magnetic field (10 kG) with little inhibition. The plasma jet is observed to narrow or "focus" in the plane perpendicular to the field, while in the plane of the field the plasma expands along the field lines and displays flutelike striations. The narrowing of the plasma jet is understood in terms of the configuration of the plasma polarization fields, while the flute structure is identified as an electron-ion hybrid velocity-shear instability.

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.

New laser-driven shock experiments have been used to study the equation-of-state (EOS) properties of liquid deuterium. Reflected shocks are utilized to increase the shock pressure and to enhance the sensitivity to differences in compressibility. The results of these experiments differ substantially from the predictions of the Sesame EOS. EOS models showing large dissociation effects with much greater compressibility (up to a factor of 2) agree with the data. By use of independent techniques, this experiment offers the first confirmation of an earlier observation of enhanced compressibility in liquid deuterium.

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...

The first observations of the interaction of the Richtmyer-Meshkov (RM) instability with reflected shock and rarefaction waves in laser-driven targets are reported.The RM growth is started by a shock wave incident upon a rippled interface between low-density foam and solid plastic. Subsequent interaction of secondary rarefaction and/or shock waves arriving from the ablation front and the rear surface of the target with the RM-unstable interface stops the perturbation growth and reverses its direction. The ensuing exponential Rayleigh-Taylor growth thus can sometimes proceed with an inverted phase.