Lee Phillips

Direct-Drive Laser Fusion; Status and Prospects

Techniques have been developed to improve the uniformity of the laser focal profile, to reduce the ablative Rayleigh–Taylor instability, and to suppress the various laser–plasma instabilities. There are now three direct-drive ignition target designs that utilize these techniques. An evaluation of these designs is still ongoing. Some of them may achieve the gains above 100 that are necessary for a fusion reactor. Two laser systems have been proposed that may meet all of the requirements for a fusion reactor.

Effects of Thin High-z Layers on the Hydrodynamics of Laser-Accelerated Plastic Targets

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 are presented. 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. Large reductions are observed 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. Simulations supported by experiments are presented showing that during the low intensity foot the laser light can be nearly completely absorbed by the high-Z layer. X ra...

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...

Instability of a plane centered rarefaction wave

An analytic small‐amplitude theory of the instability of a plane centered rarefaction wave (which has recently been discovered numerically by Yang et al.) is presented. A finite‐difference (FCT) calculation is performed and compares well with the theory. The instability manifests itself as perturbation growth on the wave’s trailing edge. The asymptotic value approached by the perturbed velocity of the trailing edge is expressed as kδx0a0u∞(M,γ), where k is the perturbation wave number, δx0 is the constant perturbation amplitude of the leading edge, a0 is the sound speed in the unperturbed gas, and u∞(M,γ) is a dimensionless function that depends on the adiabatic exponent, γ, and the strength of the rarefaction wave, M, taken as the ratio of sound speeds behind and ahead of it. This function is essentially determined by the way the perturbed rarefaction wave is formed, e.g., by moving a corrugated piston from a gas‐filled space or by interaction of a plane shock wave with a rippled contact interface betwee...

The interaction of shocks and defects in Lennard-Jones crystals

The authors examine, using computational molecular dynamics, shocks launched in two-dimensional crystals by a flying plate. The interaction of the shock with various lattice defects is observed, and is seen to create sites of rapidly growing, thermalized, hot fluid-like phases included within the crystal lattice. They hypothesize that these fluid-like regions are the sites of the initial chemical reactions leading to detonation in energetic materials, and that crystallographic defects therefore control the sensitivity of single-crystal high explosives to shock-initiation. The computations are carried out on the massively parallel CM-200 using a parellelized version of the MLG algorithm.

Reflected shock experiments on the equation-of-state properties of liquid deuterium at 100–600 GPa (1–6 Mbar)

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.

Study of radiative plasma structures in laser driven ablating plasmas

The mechanism is analyzed that generates radiative plasma structures (RPS) [J. P. Dahlburg et al., J. Quant Spectros. Radiat. Transfer 54, 113 (1995)] in driven, ablating plasmas of subcritical density. A reduced set of radiation-hydrodynamics equations is derived which model the onset of RPS phenomenon.

Laser plasma instability experiments with KrF lasers

Deleterious effects of laser-plasma instability (LPI) may limit the maximum laser irradiation that can be used for inertial confinement fusion. The short wavelength (248nm), large bandwidth, and very uniform illumination available with krypton-fluoride (KrF) lasers should increase the maximum usable intensity by suppressing LPI. The concomitant increase in ablation pressure would allow implosion of low-aspect-ratio pellets to ignition with substantial gain (>20) at much reduced laser energy. The proposed KrF-laser-based Fusion Test Facility (FTF) would exploit this strategy to achieve significant fusion power (150MW) with a rep-rate system that has a per pulse laser energy well below 1 MJ. Measurements of LPI using the Nike KrF laser are presented at and above intensities needed for the FTF (I∼2×1015W∕cm2). The results to date indicate that LPI is indeed suppressed. With overlapped beam intensity above the planar, single beam intensity threshold for the two-plasmon decay instability, no evidence of instab...

Initiation of detonations in three-dimensional crystals with defects

We examine shock-induced detonation in a three-dimensional model of a nitromethane crystal. The crystal may contain a defect in the form of a vacancy cluster, or small void. Three regimes are identified: the shock can be weak enough that no chemical bonds are broken; the shock can be so strong that a detonation front is established in the perfect crystal; or the shock can be of intermediate strength, where chemical activity is dependent on the existence of the defect. In all regimes, the defect increases the reaction rate and causes a hot spot to appear.

Observation of parametric instabilities in the quarter critical density region driven by the Nike KrF laser

The krypton-fluoride (KrF) laser is an attractive choice for inertial confinement fusion due to its combination of short wavelength (λ=248 nm), large bandwidth (up to 3 THz), and superior beam smoothing by induced spatial incoherence. These qualities improve the overall hydrodynamics of directly driven pellet implosions and should allow use of increased laser intensity due to higher thresholds for laser plasma instabilities when compared to frequency tripled Nd:glass lasers (λ=351 nm). Here, we report the first observations of the two-plasmon decay instability using a KrF laser. The experiments utilized the Nike laser facility to irradiate solid plastic planar targets over a range of pulse lengths (0.35 ns≤τ≤1.25 ns) and intensities (up to 2×1015 W/cm2). Variation of the laser pulse created different combinations of electron temperature and electron density scale length. The observed onset of instability growth was consistent with the expected scaling that KrF lasers have a higher intensity threshold for ...