J. H. Kelly

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 (

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...

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.

Experimental investigation of smoothing by spectral dispersion

Measurements of smoothing rates for smoothing by spectral dispersion (SSD) of high-power, solid-state laser beams used for inertial confinement fusion (ICF) research are reported. Smoothing rates were obtained from the intensity distributions of equivalent target plane images for laser pulses of varying duration. Simulations of the experimental data with the known properties of the phase plates and the frequency modulators are in good agreement with the experimental data. These results inspire confidence in extrapolating to higher bandwidths and other SSD configurations that may be suitable for ICF experiments and ultimately for direct-drive laser-fusion ignition.

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...

Crossed-beam energy transfer in direct-drive implosions

Direct-drive-implosion experiments on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have showed discrepancies between simulations of the scattered (non-absorbed) light levels and measured ones that indicate the presence of a mechanism that reduces laser coupling efficiency by 10%-20%. This appears to be due to crossed-beam energy transfer (CBET) that involves electromagnetic-seeded, low-gain stimulated Brillouin scattering. CBET scatters energy from the central portion of the incoming light beam to outgoing light, reducing the laser absorption and hydrodynamic efficiency of implosions. One-dimensional hydrodynamic simulations including CBET show good agreement with all observables in implosion experiments on OMEGA. Three strategies to mitigate CBET and improve laser coupling are considered: the use of narrow beams, multicolor lasers, and higher-Z ablators. Experiments on OMEGA using narrow beams have demonstrated improvements in implosion performance.

Large-aperture grating tiling by interferometry for petawatt chirped-pulse-amplification systems

A tiled-grating assembly with three large-scale gratings is developed with real-time interferometric tiling control for the OMEGA EP Laser Facility. An automatic tiling method is achieved and used to tile a three-tile grating assembly with the overall wavefront reconstructed. Tiling-parameters sensitivity and focal-spot degradation from all combined tiling errors are analyzed for a pulse compressor composed of four such assemblies.

The omega high-power phosphate-glass system: Design and performance

A 24-beam phosphate-glass laser system has been built and tested. Major design features include Nd:phosphate glass, rod amplifiers up to 90 mm diameter, propagation of circularly polarized light, extensive image relaying and spatial filtering, utilization of large-aperture Pockels cells (PCs) hard-aperture (HA) input and polarizing beamsplitters for beam balance. The system has demonstrated focusable power in excess of 12 terawatts (TW) in 50 ps (FWHM) and over 1.75 kJ in 300 ps (FWHM). The pulse is near-diffraction limited with a peak-to-background energy contrast of 108. Shot rates have exceeded 2/h. A detailed description of the system design and performance is presented.

Performance of 1-THz-bandwidth, two-dimensional smoothing by spectral dispersion and polarization smoothing of high-power, solid-state laser beams

Laser beam smoothing achieved with 1-THz-bandwidth, two-dimensional smoothing by spectral dispersion and polarization smoothing on the 60-beam, 30-kJ, 351-nm OMEGA laser system is reported. These beam-smoothing techniques are directly applicable to direct-drive ignition target designs for the 192-beam, 1.8-MJ, 351-nm National Ignition Facility. Equivalent-target-plane images for constant-intensity laser pulses of varying duration were used to determine the smoothing. The properties of the phase plates, frequency modulators, and birefringent wedges were simulated and found to be in good agreement with the measurements.

Direct-drive-implosion experiments with enhanced fluence balance on OMEGA

Direct-drive experiments on OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are now regularly performed with enhanced single-beam uniformity (1-THz-bandwidth smoothing by spectral dispersion, including distributed phase plates and polarization rotators). The on-target-beam-energy balance is determined by a set of beam pickoffs that are cross-calibrated to full beam calorimeters and by independent measurements for each beam of the losses incurred in transporting the beams to the target chamber. Variations of the illumination due to beam-to-beam, on-target energy differences and beam-to-beam shape differences can then introduce variations of the illumination and hence drive symmetry that will affect implosions. A new technique has been implemented that determines variations of the beam peak fluences at target chamber center on a full-power target shot by simultaneously measuring the x-ray flux produced by all 60 beams separated on a 4-mm-diam, Au-coated spherical target. The inferred beam-to-beam,...