S. A. Letzring

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...

OMEGA Upgrade laser for direct-drive target experiments

Validation of the direct-drive approach to inertial confinement fusion requires the development of a 351-nm wavelength, 30-kJ, 50-TW laser system with flexible pulse shaping and irradiation uniformity approaching 1%. An upgrade of the existing OMEGA direct-drive facility at Rochester is planned to meet these objectives. In this article, we review the design rationale and specifications of the OMEGA Upgrade laser with particular emphasis on techniques planned to achieve the required degree of beam smoothing, temporal pulse shape, and beam-to-beam power balance.

Pulse-shaping system for the 60-beam, 30-kJ (UV) OMEGA laser

The optical pulse-shaping system for the 60-beam 30-kJ (UV) OMEGA fusion laser is capable of producing complex temporally shaped optical pulses for amplification and delivery to fusion targets. The pulse-shaping system consists of optical modulators driven by an optically activated electrical waveform generator. The electrical waveform generator consists of Si photoconductive switches, and variable impedance microstrip lines. Complex optical pulse shapes with 50 to 100 ps structure have been produced.

Laser‐beam pulse shaping using spectral beam deflection

Laser‐beam temporal pulse shaping is examined using a technique based on beam deflection. The deflection is induced by encoding the beam with phase‐modulated bandwidth and passing the beam through a diffraction grating. Pulse‐shaping examples are modeled numerically to determine the resolution of this technique. Other applications using this method of spectral beam deflection are discussed.

The upgrade to the OMEGA laser system

The authors report on fusion research at the University of Rochester`s Laboratory for Laser Energetics. They describe the configuration of the upgrade to the OMEGA laser system - a 30-kJ, 351-nm, 60-beam, Nd:glass, direct-drive laser-fusion system. The system utilizes rod and disk amplifiers and frequency-tripling to produce intense UV. Target irradiation uniformity is controlled using phase conversion and smoothing by spectral dispersion (SSD). Dual driver lines will feed the propagation of two coaxial beams that have different pulse widths and occupy different portions of the laser aperture. Operation of the laser will begin in November 1994, and the target area will be completed in March 1995.

Short-wavelength-laser requirements for direct-drive ignition and gain

Inertial confinement fusion (ICF) requires high compression of fusion fuel to densities approaching 1000 times liquid density of deuterium-tritium (D–T) at central temperatures in excess of 5 keV. The goal of ICF is to achieve high gain (of the order of 100 or greater) in the laboratory. To meet this objective with minimum driver energy, a number of central issues must be addressed. Research in ICF with laser drivers has shown the importance of using short wavelength (λ 10 14 W/cm 2 ) are required. The directdrive approach to ICF is more energy efficient than indirect drive if the stringent drive symmetry and hydrodynamic stability requirements can be met by a suitable laser irradiation scheme and target design. Experiments carried out at 351 nm on the 2-kJ, 24-beam OMEGA laser system at the Laboratory for Laser Energetics (LLE) at the University of Rochester, and future experiments to be performed on a 30-kJ upgrade of this laser, can resolve the remaining physics issues for direct drive: (1) energy coupling and transport scaling; (2) irradiation-uniformity requirements for high gain; (3) hydrodynamic stability constraints; and (4) hot-spot and main-fuel-layer physics. We review progress made on achieving uniform drive conditions with the OMEGA system and present results for direct-drive cryogenic-fuel-capsule and CD-shell, “surrogate” cryogenic-capsule implosion experiments that illustrate the constraints imposed by hydrodynamic instabilities and drive uniformity on the design of high-performance direct-drive targets. Target designs have been identified that will explore the ignition-scaling regime using the OMEGA Upgrade. Experiments on the OMEGA Upgrade will signal whether or not there is a high probability of achieving modest to high gain using direct drive on an upgrade of the NOVA facility.

High-density, direct-drive implosion experiments

A critical test of direct-drive laser-fusion has been conducted with the demonstration of DT compression to densities in the range of 100–200 times liquid density in experiments on the University of Rochesters OMEGA laser facility. The high-density cyrogenic experiments used 351-nm laser pulses with energies of 1500–1800 J and pulse widths in the range of 600–700 ps.

Laser compression and stability in inertial confinement fusion

Inertial confinement fusion (ICF) requires high compression of fusion fuel to densities approaching 1000 times liquid density of deuterium-tritium (DT), at central temperatures in excess of 5 keV. The direct-drive approach to ICF is more energy efficient than indirect drive if the stringent drive symmetry and hydrodynamic stability requirements can be met by a suitable laser irradiation and target design. Experiments using cryogenic fuel capsules in conjunction with distributed phase plates (DPPs) on the frequency-tripled OMEGA laser system have achieved compressed DT fuel densities in the 100-200 times liquid density regime, but the experiments exhibited deviations from one-dimensional performance. The deviations are believed to result from nonuniform implosion of fuel and shell material due to irradiation nonuniformities not removed by the DPPs. Improvements in irradiation uniformity through the use of a new technique, smoothing by spectral dispersion (SSD), may lead to reduced hydrodynamic instability growth and nearly one-dimensional capsule performance. SSD allows high-efficiency frequency tripling in a solid-state laser system.