Terrance J. Kessler

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...

Distributed phase plates for super-Gaussian focal-plane irradiance profiles

The design of phase plates based on a phase-retrieval algorithm has been shown to be successful in achieving high order super-Gaussian distributions in the far field; however, these phase plates exhibit wide-angle scattering losses due to phase discontinuities. We report that, by starting with a distributed phase plate that is a strictly continuous surface and using only a few cycles of a phase-retrieval algorithm, we have obtained good fourth-order super-Gaussian fit and lower scattering loss.

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.

Demonstration of coherent addition of multiple gratings for high-energy chirped-pulse-amplified lasers

Petawatt solid-state lasers require meter-sized gratings to reach multiple-kilojoule energy levels without laser-induced damage. As an alternative to large single gratings, we demonstrate that smaller, coherently added (tiled) gratings can be used for subpicosecond-pulse compression. A Fourier-transform-limited, 650-fs chirped-pulse-amplified laser pulse is maintained by replacing a single compression grating with a tiled-grating assembly. Grating tiling provides a means to scale the energy and irradiance of short-pulse lasers.

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.

Design of continuous surface-relief phase plates by surface-based simulated annealing to achieve control of focal-plane irradiance

High-performance phase plates are of vital concern for controlling the far-field irradiance of laser-fusion systems. Several designs for solving this difficult problem have been reported in Optics Letters [e. g., S. N. Dixit et al., Opt. Lett. 19, 417 (1994)]. We report a surface-based form of simulated annealing that significantly improves the irradiance control while eliminating the high-scatter problems that have plagued other methods.

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.

Phase conversion of lasers with low-loss distributed phase plates

Efficient phase conversion of laser light can be achieved using continuous distributed phase plates (DPPs) for applications in laser-beam shaping and laser-beam smoothing. These DPPs are deep, surface-relief, continuous phase plates that exhibit near-unity energy efficiency, envelope and power spectrum flexibility, and reduced near-field intensity modulation as compared to stepped diffractive optics. Specific DPP designs for inertial-confinement-fusion laser systems, used to irradiate fuel capsules, is presented.

High-energy petawatt project at the university of rochester's laboratory for laser energetics

Abstract A high-energy petawatt laser, OMEGA EP, is currently under construction at the University of Rochesters Laboratory for Laser Energetics. Integrated into the existing OMEGA laser, it will support three major areas of research: (a) backlighting of high-energy-density plasmas, (b) integrated fast ignition experiments, and (c) high-intensity physics. The laser will provide two beams combined collinearly and coaxially with short pulses (~1 to 100 ps) and high energy (2.6 kJ at 10 ps). Cone-in-shell fuel-assembly experiments and simulations of short-pulse heated cryogenic targets are being performed in preparation for cryogenic integrated fast ignitor experiments on OMEGA EP.