R. S. Craxton

Initial performance results of the OMEGA laser system

Abstract OMEGA is a 60-terawatt, 60-beam, frequency-tripled Nd:glass laser system designed to perform precision direct-drive inertial-confinement-fusion (ICF) experiments. The upgrade to the system, completed in April 1995, met or surpassed all technical requirements. The acceptance tests demonstrated exceptional performance throughout the system: high driver stability (

Improved laser‐beam uniformity using the angular dispersion of frequency‐modulated light

A new technique is presented for obtaining highly smooth focused laser beams. This approach is consistent with the constraints on frequency tripling the light, and it will not produce any significant high‐intensity spikes within the laser chain, making the technique attractive for the high‐power glass lasers used in current fusion experiments. Smoothing is obtained by imposing a frequency‐modulated bandwidth on the laser beam using an electro‐optic crystal. A pair of gratings is used to disperse the frequencies across the beam, without distorting the temporal pulse shape. The beam is broken up into beamlets, using a phase plate, such that the beamlet diffraction‐limited focal spot is the size of the target. The time‐averaged interference between beamlets is greatly reduced because of the frequency differences between the beamlets, and the result is a relatively smooth diffraction‐limited intensity pattern on target.

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.

Demonstration of high efficiency third harmonic conversion of high power Nd-glass laser radiation

Abstract We report on efficient conversion from 1.054 μm to 0.35 μm by third harmonic generation in two Type II KDP crystals. Energy conversion efficiencies of up to 80% have been measured under conditions applicable to large glass laser systems. A new tripling scheme used for these experiments requires a minimum of optical components and is insensitive to exact crystal alignment and laser beam divergence. A convenient scaling law allows tripling optimization for many different laser conditions.

Direct-drive inertial confinement fusion: A review

The direct-drive, laser-based approach to inertial confinement fusion (ICF) is reviewed from its inception following the demonstration of the first laser to its implementation on the present generation of high-power lasers. The review focuses on the evolution of scientific understanding gained from target-physics experiments in many areas, identifying problems that were demonstrated and the solutions implemented. The review starts with the basic understanding of laser–plasma interactions that was obtained before the declassification of laser-induced compression in the early 1970s and continues with the compression experiments using infrared lasers in the late 1970s that produced thermonuclear neutrons. The problem of suprathermal electrons and the target preheat that they caused, associated with the infrared laser wavelength, led to lasers being built after 1980 to operate at shorter wavelengths, especially 0.35 μm—the third harmonic of the Nd:glass laser—and 0.248 μm (the KrF gas laser). The main physics areas relevant to direct drive are reviewed. The primary absorption mechanism at short wavelengths is classical inverse bremsstrahlung. Nonuniformities imprinted on the target by laser irradiation have been addressed by the development of a number of beam-smoothing techniques and imprint-mitigation strategies. The effects of hydrodynamic instabilities are mitigated by a combination of imprint reduction and target designs that minimize the instability growth rates. Several coronal plasma physics processes are reviewed. The two-plasmon–decay instability, stimulated Brillouin scattering (together with cross-beam energy transfer), and (possibly) stimulated Raman scattering are identified as potential concerns, placing constraints on the laser intensities used in target designs, while other processes (self-focusing and filamentation, the parametric decay instability, and magnetic fields), once considered important, are now of lesser concern for mainline direct-drive target concepts. Filamentation is largely suppressed by beam smoothing. Thermal transport modeling, important to the interpretation of experiments and to target design, has been found to be nonlocal in nature. Advances in shock timing and equation-of-state measurements relevant to direct-drive ICF are reported. Room-temperature implosions have provided an increased understanding of the importance of stability and uniformity. The evolution of cryogenic implosion capabilities, leading to an extensive series carried out on the 60-beam OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)], is reviewed together with major advances in cryogenic target formation. A polar-drive concept has been developed that will enable direct-drive–ignition experiments to be performed on the National Ignition Facility [Haynam et al., Appl. Opt. 46(16), 3276 (2007)]. The advantages offered by the alternative approaches of fast ignition and shock ignition and the issues associated with these concepts are described. The lessons learned from target-physics and implosion experiments are taken into account in ignition and high-gain target designs for laser wavelengths of 1/3 μm and 1/4 μm. Substantial advances in direct-drive inertial fusion reactor concepts are reviewed. Overall, the progress in scientific understanding over the past five decades has been enormous, to the point that inertial fusion energy using direct drive shows significant promise as a future environmentally attractive energy source.

Irradiation uniformity for high-compression laser-fusion experiments

High-compression direct-drive laser-fusion experiments require the rms laser-irradiation nonuniformity to be below the 1%–2% level. The combination of two-dimensional smoothing by spectral dispersion (SSD), phase plates, polarization smoothing, and beam overlap are shown to be sufficient to reach this goal. Here is presented a discussion of the mathematical formalism of two-dimensional SSD with numerical calculations illustrating the factors that affect irradiation uniformity. The levels of uniformity that can be achieved on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the University of Rochester and at the National Ignition Facility (NIF) [Paisner et al., Laser Focus World 30, 75 (1994)] being built at the Lawrence Livermore National Laboratory are examined.

Laser-plasma interactions in long-scale-length plasmas under direct-drive National Ignition Facility conditions

Laser-plasma interaction experiments have been carried out on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] under plasma conditions representative of the peak of a 1.5 MJ direct-drive laser pulse proposed for the National Ignition Facility (NIF). Plasmas have been formed by exploding 18–20 μm thick CH foils and by irradiating solid CH targets from one side, using up to 20 kJ of laser energy with phase plates installed on all beams. These plasmas and the NIF plasmas are predicted to have electron temperatures of 4 keV and density scale lengths close to 0.75 mm at the peak of the laser pulse. The electron temperature and density of the exploding-foil plasmas have been diagnosed using time-resolved x-ray spectroscopy and stimulated Raman scattering, respectively, and are consistent with predictions of the two-dimensional Eulerian hydrodynamics code SAGE [R. S. Craxton and R. L. McCrory, J. Appl. Phys. 56, 108 (1984)]. When the solid-target or exploding-foil plasmas were irradiate...

Highly efficient second-harmonic generation of ultraintense Nd:glass laser pulses

Second-harmonic conversion efficiencies of 80% for type I KDP crystals and 70% for type II have been demonstrated with 500-fs, 1.053-μm laser pulses at intensities as high as 400 GW/cm2. The results are generally in good agreement with simulations and indicate that self-phase modulation and cross-phase modulation may be significant. For type II conversion with a predelay between the input o and e waves, some evidence is obtained of pulse shortening to ~100 fs.

Improving the hot-spot pressure and demonstrating ignition hydrodynamic equivalence in cryogenic deuterium–tritium implosions on OMEGAa)

Reaching ignition in direct-drive (DD) inertial confinement fusion implosions requires achieving central pressures in excess of 100 Gbar. The OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] is used to study the physics of implosions that are hydrodynamically equivalent to the ignition designs on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)]. It is shown that the highest hot-spot pressures (up to 40 Gbar) are achieved in target designs with a fuel adiabat of α ≃ 4, an implosion velocity of 3.8 × 107 cm/s, and a laser intensity of ∼1015 W/cm2. These moderate-adiabat implosions are well understood using two-dimensional hydrocode simulations. The performance of lower-adiabat implosions is significantly degraded relative to code predictions, a common feature between DD implosions on OMEGA and indirect-drive cryogenic implosions on the NIF. Simplified theoretical models are developed to gain physical understanding of the implosion dynamics th...