D. D. Meyerhofer

Point design targets, specifications, and requirements for the 2010 ignition campaign on the National Ignition Facility

Point design targets have been specified for the initial ignition campaign on the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 443, 2841 (2004)]. The targets contain D-T fusion fuel in an ablator of either CH with Ge doping, or Be with Cu. These shells are imploded in a U or Au hohlraum with a peak radiation temperature set between 270 and 300 eV. Considerations determining the point design include laser-plasma interactions, hydrodynamic instabilities, laser operations, and target fabrication. Simulations were used to evaluate choices, and to define requirements and specifications. Simulation techniques and their experimental validation are summarized. Simulations were used to estimate the sensitivity of target performance to uncertainties and variations in experimental conditions. A formalism is described that evaluates margin for ignition, summarized in a parameter the Ignition Threshold Factor (ITF). Uncertainty and shot-to-shot variability in ITF are evaluated, and...

Laser ionization of noble gases by Coulomb-barrier suppression

Laser ionization of noble gases was studied with a 1.053-μm, 1-psec Nd:glass laser. A systematic scan of intensities from mid-1013 W/cm2 to mid-1016 W/cm2 was performed, resulting in the production of charge states as high as Xe12+. Ionization occurs exclusively in the tunneling regime. We compare experimental ion production rates with those predicted by several different theories. Agreement between experimental ion-production curves and theoretical predictions is good for two theoretical models: (1) an elaboration of the Keldysh tunneling model, developed by Ammosov et al. [ Sov. Phys. JETP64, 1191 ( 1986)] and (2) a much more primitive model, based on Coulomb-barrier suppression, in which tunneling and other quantum-mechanical effects are ignored completely. The success of the more primitive model suggests that a new term, barrier-suppression ionization, rather than tunneling or multiphoton ionization, may be the most appropriate at this wavelength and in this range of intensities.

POSITRON PRODUCTION IN MULTIPHOTON LIGHT-BY-LIGHT SCATTERING

A signal of 106 14 positrons above background has been observed in collisions of a low-emittance 46.6-GeV electron beam with terawatt pulses from a Nd:glass laser at 527 nm wavelength in an experiment at the Final Focus Test Beam at SLAC. The positrons are interpreted as arising from a two-step process in which laser photons are backscattered to GeV energies by the electron beam followed by a collision between the high-energy photon and several laser photons to produce an electron-positron pair. These results are the rst laboratory evidence for inelastic light-by-light scattering involving only real photons. Submitted to Physical Review Letters Work supported by Department of Energy contract DE{AC03{76SF00515 and grants DE{FG02{ 91ER40671, DE{FG02{91ER40685 and DE{FG05{91ER40627. Present address: Hughes Leitz Optical Technologies Ltd., Midland, Ontario, Canada L4R 2H2. Present address: Lawrence Livermore National Laboratory, Livermore, CA 94551. also Department of Mechanical Engineering Present address: Panoramastrasse 8, 78589 Durbheim, Germany The production of an electron-positron pair in the collision of two real photons was rst considered by Breit and Wheeler [1] who calculated the cross section for the reaction !1 + !2 ! e e (1) to be of order r e , where re is the classical electron radius. While pair creation by real photons is believed to occur in astrophysical processes [2] it has not been observed in the laboratory up to the present. After the invention of the laser the prospect of intense laser beams led to reconsideration of the Breit-Wheeler process by Reiss [3] and others [4, 5]. Of course, for production of an electron-positron pair the center-of-mass (CM) energy of the scattering photons must be at least 2mc 1 MeV. While this precludes pair creation by a single electromagnetic wave, the necessary CM energy can be achieved by colliding a laser beam against a highenergy photon beam created, for example, by backscattering the laser beam o a high-energy electron beam. With laser light of wavelength 527 nm (energy 2.35 eV), a photon of energy 111 GeV would be required for reaction (1) to proceed. However, with an electron beam of energy 46.6 GeV as available at the Stanford Linear Accelerator Center (SLAC) the maximum Compton-backscattered photon energy from a 527-nm laser is only 29.2 GeV. In strong electromagnetic elds the interaction need not be limited to initial states with two photons [3], but rather the number of interacting photons becomes large as the dimensionless, invariant parameter = e q hA A i=mc 2 = eErms=m!0c = eErms 0=mc approaches or exceeds unity. Here the laser beam has laboratory frequency !0, reduced wavelength 0, root-mean-square electric eld Erms, and four-vector potential A ; e and m are the charge and mass of the electron, respectively, and c is the speed of light. For photons of wavelength 527 nm a value of = 1 corresponds to laboratory eld strength of Elab = 6 10 V/cm and intensity I = 10 W/cm. Such intensities are now practical in tabletop laser systems based on chirped-pulse ampli cation [6]. Then the multiphoton Breit-Wheeler reaction

Spectrometry of charged particles from inertial-confinement-fusion plasmas

High-resolution spectrometry of charged particles from inertial-confinement-fusion (ICF) experiments has become an important method of studying plasma conditions in laser-compressed capsules. In experiments at the 60-beam OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], utilizing capsules with D2, D3He, DT, or DTH fuel in a shell of plastic, glass, or D2 ice, we now routinely make spectral measurements of primary fusion products (p, D, T, 3He, α), secondary fusion products (p), “knock-on” particles (p, D, T) elastically scattered by primary neutrons, and ions from the shell. Use is made of several types of spectrometers that rely on detection and identification of particles with CR-39 nuclear track detectors in conjunction with magnets and/or special ranging filters. CR-39 is especially useful because of its insensitivity to electromagnetic noise and its ability to distinguish the types and energies of individual particles, as illustrated here by detailed calibrations of its respo...

Direct‐drive laser‐fusion experiments with the OMEGA, 60‐beam, >40 kJ, ultraviolet laser system

OMEGA, a 60‐beam, 351 nm, Nd:glass laser with an on‐target energy capability of more than 40 kJ, is a flexible facility that can be used for both direct‐ and indirect‐drive targets and is designed to ultimately achieve irradiation uniformity of 1% on direct‐drive capsules with shaped laser pulses (dynamic range ≳400:1). The OMEGA program for the next five years includes plasma physics experiments to investigate laser–matter interaction physics at temperatures, densities, and scale lengths approaching those of direct‐drive capsules designed for the 1.8 MJ National Ignition Facility (NIF); experiments to characterize and mitigate the deleterious effects of hydrodynamic instabilities; and implosion experiments with capsules that are hydrodynamically equivalent to high‐gain, direct‐drive capsules. Details are presented of the OMEGA direct‐drive experimental program and initial data from direct‐drive implosion experiments that have achieved the highest thermonuclear yield (1014 DT neutrons) and yield efficienc...

Reduction of laser imprinting using polarization smoothing on a solid-state fusion laser

We demonstrate a laser beam-smoothing technique known as polarization smoothing. A birefringent optical wedge splits the individual laser beams into two orthogonally polarized beams that, when coupled with a distributed phase plate, produce two speckle patterns shifted with respect to one another. This instantaneously reduces the on-target nonuniformity by a factor of √. We measured this reduction optically and its effect is demonstrated in laser-driven targets.

High-Energy Petawatt Capability for the Omega Laser

The 60-beam Omega laser system at the University of Rochesters Laboratory for Laser Energetics (LLE) has been a workhorse on the frontier of laser fusion and high-energy-density physics for more than a decade. LLE scientists are currently extending the performance of this unique, direct-drive laser system by adding high-energy petawatt capabilities.

Proton Radiography of Inertial Fusion Implosions

A distinctive way of quantitatively imaging inertial fusion implosions has resulted in the characterization of two different types of electromagnetic configurations and in the measurement of the temporal evolution of capsule size and areal density. Radiography with a pulsed, monoenergetic, isotropic proton source reveals field structures through deflection of proton trajectories, and areal densities are quantified through the energy lost by protons while traversing the plasma. The two field structures consist of (i) many radial filaments with complex striations and bifurcations, permeating the entire field of view, of magnetic field magnitude 60 tesla and (ii) a coherent, centrally directed electric field of order 109 volts per meter, seen in proximity to the capsule surface. Although the mechanism for generating these fields is unclear, their effect on implosion dynamics is potentially consequential.

Capsule implosion optimization during the indirect-drive National Ignition Campaign

Capsule performance optimization campaigns will be conducted at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)] to substantially increase the probability of ignition. The campaigns will experimentally correct for residual uncertainties in the implosion and hohlraum physics used in our radiation-hydrodynamic computational models using a variety of ignition capsule surrogates before proceeding to cryogenic-layered implosions and ignition experiments. The quantitative goals and technique options and down selections for the tuning campaigns are first explained. The computationally derived sensitivities to key laser and target parameters are compared to simple analytic models to gain further insight into the physics of the tuning techniques. The results of the validation of the tuning techniques at the OMEGA facility [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] under scaled hohlraum and capsule conditions relevant to the ignition design are shown ...

Streaked optical pyrometer system for laser-driven shock-wave experiments on OMEGA

The temperature of laser-driven shock waves is of interest to inertial confinement fusion and high-energy-density physics. We report on a streaked optical pyrometer that measures the self-emission of laser-driven shocks simultaneously with a velocity interferometer system for any reflector (VISAR). Together these diagnostics are used to obtain the temporally and spatially resolved temperatures of approximately megabar shocks driven by the OMEGA laser. We provide a brief description of the diagnostic and how it is used with VISAR. Key spectral calibration results are discussed and important characteristics of the recording system are presented.