C. Freeman

Core performance and mix in direct-drive spherical implosions with high uniformity

The performance of gas-filled, plastic-shell implosions has significantly improved with advances in on-target uniformity on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)]. Polarization smoothing (PS) with birefringent wedges and 1-THz-bandwidth smoothing by spectral dispersion (SSD) have been installed on OMEGA. The beam-to-beam power imbalance is ⩽5% rms. Implosions of 20-μm-thick CH shells (15 atm fill) using full beam smoothing (1-THz SSD and PS) have primary neutron yields and fuel areal densities that are ∼70% larger than those driven with 0.35-THz SSD without PS. They also produce ∼35% of the predicted one-dimensional neutron yield. The results described here suggest that individual-beam nonuniformity is no longer the primary cause of nonideal target performance. A highly constrained model of the core conditions and fuel–shell mix has been developed. It suggests that there is a “clean” fuel region, surrounded by a mixed region, that acc...

First results from cryogenic target implosions on OMEGA

Initial results from direct-drive spherical cryogenic target implosions on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] are presented. These experiments are part of the scientific base leading to direct-drive ignition implosions planned for the National Ignition Facility (NIF) [W. J. Hogan, E. I. Moses, B. E. Warner et al., Nucl. Fusion 41, 567 (2001)]. Polymer shells (1-mm diam with walls <3 μm) are filled with up to 1000 atm of D2 to provide 100-μm-thick ice layers. The ice layers are smoothed by IR heating with 3.16-μm laser light and are characterized using shadowgraphy. The targets are imploded by a 1-ns square pulse with up to ∼24 kJ of 351-nm laser light at a beam-to-beam rms energy balance of <3% and full-beam smoothing. Results shown include neutron yield, secondary neutron and proton yields, the time of peak neutron emission, and both time-integrated and time-resolved x-ray images of the imploding core. The experimental values are...

Inference of mix in direct-drive implosions on OMEGA

Direct-drive implosions on the OMEGA laser [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] have been diagnosed using a wide range of techniques based on neutrons, charged particles, and x rays. These implosions use full single-beam smoothing (distributed phase plates, 1-THz-bandwidth smoothing by spectral dispersion and polarization smoothing). The beam-to-beam power imbalance is ⩽5%. Fuel areal densities close to those in one-dimensional (1-D) simulations are inferred for implosions with calculated convergence ratios ∼15. The experimental neutron yields are ∼35% of 1-D yields. The complementary nature of the experimental observables is exploited to infer fuel shell mix in these implosions. Data suggest that this mix occurs at relatively small scales. Analysis of the experimental observables results in a picture of the core and mix region indicating that nearly 70% of the compressed fuel areal density is unmixed, and about 20% of the compressed shell areal density is in the...

Direct-drive inertial confinement fusion research at the Laboratory for Laser Energetics: charting the path to thermonuclear ignition

Significant theoretical and experimental progress continues to be made at the University of Rochesters Laboratory for Laser Energetics (LLE), charting the path to direct-drive inertial confinement fusion (ICF) ignition. Direct drive offers the potential for higher-gain implosions than x-ray drive and is a leading candidate for an inertial fusion energy power plant. LLEs direct-drive ICF ignition target designs for the National Ignition Facility (NIF) are based on hot-spot ignition. A cryogenic target with a spherical DT-ice layer, within or without a foam matrix, enclosed by a thin plastic shell, will be directly irradiated with ~1.5 MJ of laser energy. Cryogenic and plastic/foam (surrogate-cryogenic) targets that are hydrodynamically scaled from these ignition target designs are imploded on the 60-beam, 30 kJ, UV OMEGA laser system to validate the key target physics issues, including energy coupling, hydrodynamic instabilities and implosion symmetry. Prospects for direct-drive ignition on the NIF are extremely favourable, even while it is in its x-ray-drive irradiation configuration, with the development of the polar-direct-drive concept. A high-energy petawatt capability is being constructed at LLE next to the existing 60-beam OMEGA compression facility. This OMEGA EP (extended performance) laser will add two short-pulse, 2.6 kJ beams to the OMEGA laser system to backlight direct-drive ICF implosions and study fast-ignition physics with focused intensities up to 6 × 1020 W cm−2.

Impact of x-ray dose on track formation and data analysis for CR-39-based proton diagnostics.

The nuclear track detector CR-39 is used extensively for charged particle diagnosis, in particular proton spectroscopy, at inertial confinement fusion facilities. These detectors can absorb x-ray doses from the experiments in the order of 1-100 Gy, the effects of which are not accounted for in the previous detector calibrations. X-ray dose absorbed in the CR-39 has previously been shown to affect the track size of alpha particles in the detector, primarily due to a measured reduction in the material bulk etch rate [Rojas-Herrera et al., Rev. Sci. Instrum. 86, 033501 (2015)]. Similar to the previous findings for alpha particles, protons with energies in the range 0.5-9.1 MeV are shown to produce tracks that are systematically smaller as a function of the absorbed x-ray dose in the CR-39. The reduction of track size due to x-ray dose is found to diminish with time between exposure and etching if the CR-39 is stored at ambient temperature, and complete recovery is observed after two weeks. The impact of this effect on the analysis of data from existing CR-39-based proton diagnostics on OMEGA and the National Ignition Facility is evaluated and best practices are proposed for cases in which the effect of x rays is significant.

Core Performance and Mix in Direct-Drive Spherical Implosions on Omega

A wide variety of diagnostics have been deployed to study direct-drive spherical implosions of gas-filled, plastic shells on the 60-beam OMEGA laser system at the Laboratory for Laser Energetics. Neutron detectors using plastic scintillators measure primary yield, ion temperature, secondary neutron yield, and burn history. Charged- particle detectors record, on CR-39 samples, the secondary protons in pure D2 implosions and elastically scattered protons, deuterons, and tritons in DT implosions to measure the shell and fuel areal densities. Primary D3He protons from plastic shells with a CD layer at the shell-gas interface and a 3He fill provide a significant signal only if the shell and fuel regions are mixed. Shells with an Ar-doped fill are used to infer the emissivity- averaged core electron temperatures and densities from the measured time-dependent Ar K-shell spectra. The level of nonuniformity in the shell areal density at stagnation is measured using gated x-ray images of Ti-doped shells.