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...

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

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 10 9 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.

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Proton Radiography of Inertial Fusion Implosions

Enhancement of the ion temperature and fusion yield has been observed in magnetized laser-driven inertial confinement fusion implosions on the OMEGA Laser Facility. A spherical CH target with a 10 atm ${\mathrm{D}}_{2}$ gas fill was imploded in a polar-drive configuration. A magnetic field of 80 kG was embedded in the target and was subsequently trapped and compressed by the imploding conductive plasma. As a result of the hot-spot magnetization, the electron radial heat losses were suppressed and the observed ion temperature and neutron yield were enhanced by 15% and 30%, respectively.

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Fusion yield enhancement in magnetized laser-driven implosions.

Understanding and designing inertial confinement fusion (ICF) implosions through radiation-hydrodynamics simulations relies on the accurate knowledge of the equation of state (EOS) of the deuterium and tritium fuels. To minimize the drive energy for ignition, the imploding shell of DT fuel must be kept as cold as possible. Such low-adiabat ICF implosions can access to coupled and degenerate plasma conditions, in which the analytical EOS models become inaccurate due to many-body effects. Using the path-integral Monte Carlo (PIMC) simulations we have derived a first-principles EOS (FPEOS) table of deuterium that covers typical ICF fuel conditions at densities ranging from 0.002 to 1596 g/cm${}^{3}$ and temperatures of 1.35 eV to 5.5 keV. We report the internal energy and the pressure and discuss the structure of the plasma in terms of pair-correlation functions. When compared with the widely used SESAME table and the revised Kerley03 table, discrepancies in the internal energy and in the pressure are identified for moderately coupled and degenerate plasma conditions. In contrast to the SESAME table, the revised Kerley03 table is in better agreement with our FPEOS results over a wide range of densities and temperatures. Although subtle differences still exist for lower temperatures ($T$ 10 eV) and moderate densities (1 to 10 g/cm${}^{3}$), hydrodynamics simulations of cryogenic ICF implosions using the FPEOS table and the Kerley03 table have resulted in similar results for the peak density, areal density (\ensuremath{\rho}$R$), and neutron yield, which differ significantly from the SESAME simulations.

First-principles equation-of-state table of deuterium for inertial confinement fusion applications

The physics of thermonuclear ignition in inertial confinement fusion (ICF) is presented in the familiar frame of a Lawson-type criterion. The product of the plasma pressure and confinement time Pτ for ICF is cast in terms of measurable parameters and its value is estimated for cryogenic implosions. An overall ignition parameter χ including pressure, confinement time, and temperature is derived to complement the product Pτ. A metric for performance assessment should include both χ and Pτ. The ignition parameter and the product Pτ are compared between inertial and magnetic-confinement fusion. It is found that cryogenic implosions on OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have achieved Pτ∼1.5 atm s comparable to large tokamaks such as the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] where Pτ∼1 atm s. Since OMEGA implosions are relatively cold (T∼2 keV), their overall ignition parameter χ∼0.02–0.03 is ∼5× lower than in JET (χ∼0.13), where the average temp...

Thermonuclear ignition in inertial confinement fusion and comparison with magnetic confinement

In this paper we present measurements of thermal transport in spherical geometry made with 24 uv (351-nm) beams from the OMEGA laser system of the Laboratory for Laser Energetics of the University of Rochester. The measurements, using time-resolved x-ray spectroscopy on solid glass targets coated with varying thicknesses of plastic, provide absolute measurements of the onset times of the resonance x-ray lines of silicon. From these measurements, the scaling of the instantaneous mass-ablation rate with absorbed intensity is obtained for times before and after the peak of the laser pulse. The observed large burnthrough depths and early onsets of the x-ray lines are explained by carrying out detailed hydrodynamic-code simulations for the range of the estimated laser-intensity distribution on target which is obtained from the superposition of the equivalent target-plane intensity distribution of a single beam. We find that neglecting the effects of laser illumination nonuniformities can lead to erroneous conclusions about the heat transport. We conclude from the analysis that the experimental results can be explained by the presence of significant energy at intensities three times the nominal intensity, contained in hot spots of size less than 20 ..mu..m. This is almost twice as much as the maximum intensitymore » obtained from the superposition of the single-beam distribution.« less

Effect of laser illumination nonuniformity on the analysis of time-resolved x-ray measurements in uv spherical transport experiments.

In this talk, measurements of thermal transport in spherical geometry using time-resolved x-ray spectroscopy are presented. The time dependence of the mass ablation rate (m) is determined by following the progress of the ablation surface through thin layers of material embedded at various depths below the surface of the target. These measurements made with 6, 12 and 24 uv (351 nm) beams from OMEGA are compared to previous thermal transport data and are in qualitative agreement with detailed LILAC hydrodynamic code simulations which predict a sharp decrease in m after the peak of the laser pulse. Viewgraphs of the talk comprise the report.

Temporal dependence of the mass-ablation rate in uv-laser-irradiated spherical targets

X-ray crystal spectroscopy of laser produced plasmas has long been considered an important diagnostic of plasma density and temperature.(1) Absorption spectroscopy of the dense transient plasmas existing in imploding laser fusion targets has the potential of being an important avenue of fuel and shell density and temperature diagnosis.(2,3) Meaningful data, however, will require sufficient space and/or time resolution to pinpoint the region of the plasma being diagnosed. In this paper we review recent progress made in this area in studying spherical fusion targets imploded by the 24 beam OMEGA laser system at LLE.(4) Space resolved spectra have been obtained with a multi-frame imaging planar crystal i)ectrometer,(5) and time-resolved data with the SPEAXS curved crystal streak spectrograph.(6'7) The evolution of the intensity of the spectral features as a function of space for different types of targets has been investigated in order to make estimates of the density and temperature of the shell during the final stages of compression of the imploding fusion target. Comparisons of this technique with other diagnostic approaches are also made.

Time And Space Resolved X-Ray Spectra Of Imploding Laser Fusion Targets

In this report we will review recent developments in our x -ray streak camera program. The characterization of thelinearity and dynamic range of these diagnostics is vital for quantitative analysis of the data, which can then be compared tohydrocode simulations. Examples of time -resolved x -ray imaging and x -ray spectroscopy are presented to illustrate our currentcapabilities. Introduction Time -resolving x -ray instrumentation provides an important research tool for studying laser fusion plasmas. These instruments use the ten -picosecond time resolution capabilities of x -ray streak cameras to record the output of an x -ray spectrometer or imaging device. Generally, several x -ray streak cameras are deployed simultaneously on a given experimentto provide a comprehensive data set of the x -ray emission spectrum and target hydrodynamics. Careful synchronization of thevarious streak records to each other and to the incident laser pulse through the use of optical timing fiducials allows us to

Application Of X-Ray Streak Cameras For Fusion Diagnostics

The analysis of soft x-ray emission from plasmas created by intense short-wavelength laser radiation can provide much useful information on the density, temperature and ionization distribution of the plasma. Until recently, limitations of sensitivity and the availability of suitable x-ray optical elements have restricted studies of soft x-ray emission from laser plasmas. In this paper we describe novel instrumentation which provides high sensitivity in the soft x-ray spectrum with spatial and temporal resolution in the micron and picosecond ranges respectively. These systems exploit advances made in soft x-ray optic and electro-optic technology. Their application in current studies of laser fusion, x-ray lasers, and high density atomic physics will be discussed.

B. L. Henke

Papers

Effect of laser illumination nonuniformity on the analysis of time-resolved x-ray measurements in uv spherical transport experiments.

Temporal dependence of the mass-ablation rate in uv-laser-irradiated spherical targets

Time And Space Resolved X-Ray Spectra Of Imploding Laser Fusion Targets

Application Of X-Ray Streak Cameras For Fusion Diagnostics

Space- And Time-Resolved Diagnostics Of Soft X-Ray Emission From Laser Plasmas