Nathan Metzler

Measurements of laser-imprinted perturbations and Rayleigh–Taylor growth with the Nike KrF laser

Nike is a 56 beam Krypton Fluoride (KrF) laser system using Induced Spatial Incoherence (ISI) beam smoothing with a measured focal nonuniformity 〈ΔI/I〉 of 1% rms in a single beam [S. Obenschain et al., Phys. Plasmas 3, 1996 (2098)]. When 37 of these beams are overlapped on the target, we estimate that the beam nonuniformity is reduced by 37, to (ΔI/I)≅0.15% (excluding short-wavelength beam-to-beam interference). The extraordinary uniformity of the laser drive, along with a newly developed x-ray framing diagnostic, has provided a unique facility for the accurate measurements of Rayleigh–Taylor amplified laser-imprinted mass perturbations under conditions relevant to direct-drive laser fusion. Data from targets with smooth surfaces as well as those with impressed sine wave perturbations agree with our two-dimensional (2-D) radiation hydrodynamics code that includes the time-dependent ISI beam modulations. A 2-D simulation of a target with a 100 A rms randomly rough surface finish driven by a completely unif...

Direct observation of mass oscillations due to ablative Richtmyer–Meshkov instability and feedout in planar plastic targets

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 40–99 μm thick planar plastic targets rippled either on the front or on the rear with a sine wave ripple with either 30 or 45 μm wavelength and with 0.5, 1, or 1.5 μm amplitude. Targets were irradiated with 4 ns long Nike KrF laser pulses at ∼50 TW/cm2. The oscillations were observed with our novel diagnostic technique, a monochromatic x-ray imager coupled to a streak c...

Richtmyer–Meshkov-like instabilities and early-time perturbation growth in laser targets and Z-pinch loads

The classical Richtmyer–Meshkov (RM) instability develops when a planar shock wave interacts with a corrugated interface between two different fluids. A larger family of so-called RM-like hydrodynamic interfacial instabilities is discussed. All of these feature a perturbation growth at an interface, which is driven mainly by vorticity, either initially deposited at the interface or supplied by external sources. The inertial confinement fusion relevant physical conditions that give rise to the RM-like instabilities range from the early-time phase of conventional ablative laser acceleration to collisions of plasma shells (like components of nested-wire-arrays, double-gas-puff Z-pinch loads, supernovae ejecta and interstellar gas). In the laser ablation case, numerous additional factors are involved: the mass flow through the front, thermal conduction in the corona, and an external perturbation drive (laser imprint), which leads to a full stabilization of perturbation growth. In contrast with the classical R...

Laser imprint reduction with a short shaping laser pulse incident upon a foam-plastic target

In the previous work [Metzler et al., Phys. Plasmas 6, 3283 (1999)] it was shown that a tailored density profile could be very effective in smoothing out the laser beam nonuniformities imprinted into a laser-accelerated target. However, a target with a smoothly graded density is difficult to manufacture. A method of dynamically producing a graded density profile with a short “shaping” laser pulse irradiating a foam layer on top of the payload prior to the drive pulse is proposed. It is demonstrated that the intensity and the duration of the shaping pulse, the time interval between the shaping pulse and the drive pulse, and the density ratio between the foam and the payload can be selected so that the laser imprint of the drive pulse is considerably suppressed without increasing the entropy of the payload. The use of the foam-plastic target and a shaping pulse reduces the imprinted mass perturbation amplitude by more than an order of magnitude compared to a solid plastic target. The requirements to the smo...

Reduction of Early-Time Perturbation Growth in Ablatively Driven Laser Targets Using Tailored Density Profiles

The effects of tailoring the density profile in a laser target in order to decrease imprinting of mass perturbations due to the long-wavelength modes are investigated analytically and numerically. Inverting the acceleration of the ablation front during the shock transit time could reduce the early-time mass perturbation amplitudes developed in the target after the shock transit. This principle was first suggested for mitigating the Rayleigh–Taylor (RT) instability of imploding Z-pinches [Velikovich et al., Phys. Rev. Lett. 77, 853 (1996); Phys. Plasmas 5, 3377 (1998)]. As the shock wave slows down propagating into higher density layers, the effective gravity near the ablation front has the same direction as the density gradient. This makes the mass perturbations near it oscillate at a higher frequency and at a lower amplitude than they normally would due to the “rocket effect” caused by mass ablation [Sanz, Phys. Rev. Lett. 73, 2700 (1994); Piriz et al., Phys. Plasmas 4, 1117 (1997)]. So, tailoring densit...

Large-scale high-resolution simulations of high gain direct-drive inertial confinement fusion targets

Targets have been designed that produce moderate to high gain when directly driven by lasers. The intrinsic sensitivity of these targets to hydro instabilities is found using the FAST(2D) multidimensional radiation hydrocode [J. H. Gardner, A. J. Schmitt, J. P. Dahlburg et al., Phys. Plasmas 5, 1935 (1998)], which simulates the simultaneous behavior of a large bandwidth (e.g., l=2–256) of perturbations from compression to acceleration, and then to stagnation and burn. The development of the structure in these multimode simulations is benchmarked to theoretical analysis and single-mode calculations, which reveals the need to “renormalize” the simulation after compression. The simulations predict that a direct drive point design is expected to degrade significantly from its one-dimensional clean yield, yet still ignite and give appreciable gain. Simulations of high-gain pellets using a spike prepulse to inhibit Richtmyer–Meshkov growth show a considerable robustness, with high (>100) gains possible even wit...

Feedout and Richtmyer–Meshkov instability at large density difference

The feedout process transfers mass perturbations from the rear to the front surface of a driven target, producing the seed for the Rayleigh–Taylor (RT) instability growth. The feedout mechanism is investigated analytically and numerically for the case of perturbation wavelength comparable to or less than the shock-compressed target thickness. The lateral mass flow in the target leads to oscillations of the initial mass nonuniformity before the reflected rippled rarefaction wave breaks out, which may result in RT bubbles produced at locations where the areal mass was initially higher. This process is determined by the evolution of hydrodynamic perturbations in the rippled rarefaction wave, which is not the same as the Richtmyer–Meshkov (RM) interfacial instability. An exact analytical formula is derived for the time-dependent mass variation in a rippled rarefaction wave, and explicit estimates are given for the time of first phase reversal and frequency of the oscillations. The limiting transition from the...

Laser imprint reduction with a shaping pulse, oscillatory Richtmyer–Meshkov to Rayleigh–Taylor transition and other coherent effects in plastic-foam targets

A substantial reduction of the laser imprint with a short, low-energy “shaping” laser pulse incident upon a foam–plastic sandwich target prior to the main laser pulse has been demonstrated to be possible [Metzler et al., Phys. Plasmas 9, 5050 (2002)]. Nonuniformity of this shaping pulse, however, produces standing sonic waves in the target. Laser-imprinted seeds for the Rayleigh–Taylor (RT) instability growth then emerge from the interaction of these waves with the strong shock wave launched by the drive laser pulse. Such coherent interaction between different waves and modes perturbed at the same wavelength is shown to be important in a variety of situations relevant to the inertial confinement fusion studies. As an example, an oscillatory transition from the classical Richtmyer–Meshkov shock-interface instability development to the RT growth exhibiting a characteristic phase reversal in a target of finite thickness is described. Another example refers to the feedout mechanism of seeding the perturbation...

Strong shock wave and areal mass oscillations associated with impulsive loading of planar laser targets

When a rippled surface of a planar target is irradiated with a short (subnanosecond) laser pulse, the shock wave launched into the target and the mass distribution of the shocked plasma will oscillate. These oscillations are found to be surprisingly strong compared, for example, to the case when the laser radiation is not turned off but rather keeps pushing the shock wave into the target. Being stronger than the areal mass oscillations due to ablative Richtmyer–Meshkov instability and feedout in planar targets, which have recently been observed at the Naval Research Laboratory (NRL) [Aglitskiy et al., Phys. Plasmas 9, 2264 (2002)], these oscillations should therefore be directly observable with the same diagnostic technique. Irradiation of a target with a short laser pulse represents a particular case of an impulsive loading, a fast release of finite energy in a thin layer near the surface of a target. Renewed interest to the impulsive loading in the area of direct-drive laser fusion is due to the recent ...

Observed transition from Richtmyer-Meshkov jet formation through feedout oscillations to Rayleigh-Taylor instability in a laser target

Experimental study of hydrodynamic perturbation evolution triggered by a laser-driven shock wave breakout at the free rippled rear surface of a plastic target is reported. At sub-megabar shock pressure, planar jets manifesting the development of the Richtmyer-Meshkov-type instability in a non-accelerated target are observed. As the shock pressure exceeds 1 Mbar, an oscillatory rippled expansion wave is observed, followed by the “feedout” of the rear-surface perturbations to the ablation front and the development of the Rayleigh-Taylor instability, which breaks up the accelerated target.