Robert L. Kremens

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...

D–3He proton spectra for diagnosing shell ρR and fuel Ti of imploded capsules at OMEGA

Recent work has resulted in the first high-resolution, spectroscopic measurements of energetic charged particles on OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 496 (1997)]. Energy spectra of charged fusion products have been obtained from two spectrometers, and have been used to deduce various physical quantities in imploded capsules. In this paper the first use of 14.7 MeV deuterium–helium3 (D–3He) proton spectra for diagnosing shell areal density (ρR) and fuel ion temperature (Ti) is discussed. For thick-plastic shell capsules, shell areal densities between 20 and 70 mg/cm2 and ion temperatures between 3 and 5 keV have been determined. The spectral linewidths associated with such capsules are found to be wider than the doppler widths. This effect, the focus of future study, is the result of ρR evolution during the burn; or is the result of an extended burn region; or results from nonuniformities in the shell. For thin-glass shell capsules, the spectral linewidths are dominated by the do...

Demonstration of time-dependent symmetry control in hohlraums by drive-beam staggering

Indirect-drive inertial confinement fusion makes use of cavities constructed of high atomic number materials to convert laser power into x-rays for ablatively driving an implosion capsule. Obtaining spatially uniform drive on the capsule requires a careful balancing between the laser absorption region (high drive) and the laser entrance holes (low drive). This balancing is made difficult because of plasma expansion, and the associated movement of the laser absorption region with time. This paper reports the first experimental demonstration of compensation for this motion by using different laser beams at different times, in agreement with modeling.

Design of an electronic charged particle spectrometer to measure 〈ρR〉 on inertial fusion experiments

The design and fabrication of a new diagnostic that measures the energy spectra of charged particles from targets on the Omega Upgrade are actively underway. Using seven 512×512 charge coupled devices (CCDs) and a 7.5 kG permanent magnet, this instrument will uniquely determine particle identities and measure particle energies from 1 MeV up to the maximum charged particle energies of interest for ρR measurements (10.6 MeV knock-on tritons, 12.5 MeV knock-on deuterons and 30.8 MeV tertiary protons). The resolution of the diagnostic will be better than 5%. We have tested the response of SITe back-illuminated CCDs to 1.2–13.6 MeV protons from our Cockcroft–Walton accelerator and to alpha particles from an Am241 source, and the results agree extremely well with predictions. With its high density picture elements, each CCD has 105 single-hit detectors. In the case of a low DT yield of 109 neutrons, about 100 knock-on charged particles will be detected when the spectrometer aperture is 60 cm from the implosion....

Inertial confinement fusion experiments with OMEGA-A 30-kJ, 60-beam UV laser

Abstract The Laboratory for Laser Energetics (LLE) experimental program supports the US inertial confinement fusion (ICF) effort by investigating the requirements for attaining ignition using direct drive targets. The primary tool for this research is OMEGA, a 60-beam, 351-nm, Nd:glass laser with an on-target energy capability in excess of 30 kJ. The laser is designed to ultimately achieve an irradiation uniformity of ∼1% on direct-drive capsules with shaped laser pulses (dynamic range>400:1). In addition, OMEGA provides unique capabilities for irradiating indirect-drive targets. This paper reports on a number of recent laser enhancements, including a new design for distributed phase plates (DPPs), two-dimensional smoothing by spectral dispersion (2-D SSD), distributed polarization rotators (DPRs) and laser pulse shaping. A variety of spherical-implosion, planar-target, and indirect-drive experiments attest to the versatility of the OMEGA laser. A key result is the highest thermonuclear yield (10 14 neutrons) and yield efficiency (1% of scientific breakeven) ever attained in laser fusion experiments.

Indirect drive experiments utilizing multiple beam cones in cylindrical hohlraums on OMEGA

Current plans for time-dependent control of flux asymmetry in the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, and M. Sorem, “The National Ignition Facility Project,” ICF Quart. 5, 110 (1995)] hohlraums rely on multiple beam cones with different laser power temporal profiles in each cone. Experiments with multiple beam cones have begun on the Omega laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the University of Rochester. In addition to allowing symmetry experiments similar to those performed on Nova [A. Hauer et al., Rev. Sci. Instrum. 66, 672 (1995)], the Omega facility allows multiple beam cones to be moved independently to confirm our ability to model the resulting implosion image shapes. Results indicate that hohlraum symmetry behaves similarly with multiple rings of beams as with a single ring, but with the weighted beam spot position used to parametrize the beam pointing.

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.

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.

The OMEGA laser electronic timing system

A high-precision, computer-controlled electronic timing system has been built for the OMEGA inertial confinement fusion laser. The timing system consists of five modular subcomponents and is easily expandable to produce any number of delay channels. The timing from the system is synchronized by a distributed clock, which is derived from the radio-frequency-signal that drives the mode-locked laser master oscillator. The system produces IEEE RS-170 television synchronization signals and several subfrequencies for operating different parts of the laser. Performance of the system and cost will be discussed.

Experiments on the OMEGA laser to validate high‐gain, direct‐drive performance on the National Ignition Facility

Within the US ICF program the direct‐drive method has been chosen using the OMEGA facility based on the Neodymium glass laser. Requirements for the future experiments are reviewed and the design is outlined.