Laser Fusion Experiments Research Articles

A parametric study of microencapsulation approach to the preparation of polystyrene shells

Spherical polystyrene shells for laser fusion experiments were made in a density-matched microencapsulation approach. The yield, diameter, wall thickness, vacuole content, and surface finish were determined for different polystyrene concentrations varied from 5 to 13 wt% in an equivolume mixture of toluene and 1,2-dichloroethane, and an internal water phase containing surfactants, Tween 40 and Brij 30, at concentrations of 0.05 and 0.1 wt%, respectively. The main observations are: (1) The yield of shells is improved with added surfactants, and the effect is more pronounced at a higher polymer concentration; (2) The outer shell diameter increased with added surfactants at a constant polymer concentration. This is attributed to larger droplets being more stable in the first stage of the fabrication process; (3) Thicker shell walls are achieved with higher polymer concentrations; (4) The outer shell diameter increased with increasing polymer concentration regardless of added surfactants. This is attributed to a thicker wall providing greater mechanical stability; (5) The number of vacuoles over the cross-sectional area of the wall increases with increasing polymer concentration and with the addition of surfactants; and (6) The shells possess a smooth surface with a root-mean-square surface roughness less than 25 nm over a 30 μm × 30 μm area, and a sphericity greater than 99.6%.

Study of laser-imploded core plasmas with an advanced Kirkpatrick–Baez x-ray microscope

We developed an advanced Kirkpatrick–Baez (AKB) x-ray microscope which consisted of two hyperbolic mirrors and two elliptic mirrors. The spatial resolution of ≲3 μm was realized over ∼1 mm diam. This AKB microscope was used for x-ray imaging in laser fusion experiments. Laser absorption nonuniformity with a large wave number on a spherical solid target or a plane slab target was estimated by measurements of x-ray emission from the target surface with the microscope. The x-ray images of the imploded core plasmas were also obtained with the AKB microscope, changing the laser focus condition and the laser energy balance.

A framed monochromatic x-ray microscope for ICF (invited)

The Laser Fusion Experiments Groups from the Laboratory for Laser Energetics (LLE) and the Los Alamos National Laboratory (LANL) have jointly developed an instrument capable of simultaneously space-, time-, and spectrally resolving x-ray emission from inertial confinement fusion (ICF) targets. Uses of the instrument include framed imaging of line emission from fuel or shell dopants and monochromatic backlighting. The x-ray imaging is accomplished with a Kirkpatrick-Baez (KB)-type four-image microscope, which has a best spatial resolution of ∼5 μm and a sensitive energy range of ∼2–8 keV. Time-resolved x-ray images are obtained with a pair of custom framing cameras, each of which records two of the four images in two independent 80-ps time intervals. In addition, the energy range of the images can be restricted to a narrow (monochromatic) spectral range (∼10–100 eV) by the introduction of diffracting crystals. This technique has been demonstrated with an e-beam-generated dc x-ray source, and at the LANL Trident laser facility and the LLE OMEGA laser facility with x-rays from laser-produced plasmas.

Diagnosis of core-shell mixing using absorption and emission spectra of a doped layer

We present theoretical analysis of a diagnostic method for detecting core-shell mixing in laser-fusion experiments, based on measuring the emission and absorption spectrum of a Ti-doped layer embedded within the target shell. The doped target layer is placed away from the shell-fuel interface so that the Ti remains relatively cool throughout the implosion. As a result, only absorption lines are observed, of 1 s−2 l transitions in ions with an unfilled L shell. However, for a sufficiently strong mixing, some Ti material migrates to the interface region and emits He- and H-like lines. A particular target experiment is simulated and the expected experimental signature calculated and analyzed.

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 efficiency (1% of scientific breakeven) ever attained in laser-fusion experiments.

Ion impacts on moving emitters: A convergent theory of anisotropic broadening in high-temperature plasmas

This paper results from a cross-fertilization of two ideas. The first one is the idea by Seidel: for a radiator with constant velocity Stark broadening is effected by an anisotropic plasma so that in a frame of reference moving with the emitter the line shape depends not only on the radiator speed but on the direction of emission as well. However his calculations had a shortcoming typical for the standard semiclassical theories of Stark broadening: they were intrinsically divergent at small impact parameters. The second idea originates from our previous papers: the convergency of semiclassical theories may be achieved by treating one projection of the dynamic electric microfield more accurately than two other components. The results of the cross-fertilization are the following: (1) our theory of anisotropic broadening is convergent; (2) for a particular case of the L α line its profile calculated with an allowance for the effects of the anisotropy is significantly narrower than without this allowance. From the practical point of view our results can serve as an improved basis for a novel spectroscopic diagnostics of the effective charge Z eff in tokamak plasmas as well as for measurements of plasma parameters in some laser fusion experiments.

New age determinations of central Colorado Plateau laccoliths, Utah: Recognizing disturbed K-Ar systematics and re-evaluating tectonomagmatic relationships

Research Article| December 01, 1992 New age determinations of central Colorado Plateau laccoliths, Utah: Recognizing disturbed K-Ar systematics and re-evaluating tectonomagmatic relationships STEPHEN T. NELSON; STEPHEN T. NELSON 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 Search for other works by this author on: GSW Google Scholar JON P. DAVIDSON; JON P. DAVIDSON 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 Search for other works by this author on: GSW Google Scholar KIM R. SULLIVAN KIM R. SULLIVAN 2Department of Geology, Brigham Young University, Provo, Utah 84602 Search for other works by this author on: GSW Google Scholar Author and Article Information STEPHEN T. NELSON 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 JON P. DAVIDSON 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 KIM R. SULLIVAN 2Department of Geology, Brigham Young University, Provo, Utah 84602 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (12): 1547–1560. https://doi.org/10.1130/0016-7606(1992)104<1547:NADOCC>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation STEPHEN T. NELSON, JON P. DAVIDSON, KIM R. SULLIVAN; New age determinations of central Colorado Plateau laccoliths, Utah: Recognizing disturbed K-Ar systematics and re-evaluating tectonomagmatic relationships. GSA Bulletin 1992;; 104 (12): 1547–1560. doi: https://doi.org/10.1130/0016-7606(1992)104<1547:NADOCC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Published K-Ar ages of laccoliths of the Henry and La Sal Mountains range from 23 to 56 Ma, with a bimodal distribution in the Eocene and late Oligocene epochs. Eocene ages require a period of magmatism during the Laramide Orogeny, a time of little or no igneous activity in the western United States at these latitudes. On the basis of 40Ar/39Ar and fission-track ages, we conclude that intrusive activity occurred during middle to late Oligocene and early Miocene time (31-20 Ma) in the Henry, La Sal, and Abajo Mountains. These results imply a genetic link to magmatism in the Reno-Marysvale and San Juan volcanic fields, which lie to the west and east of the Colorado Plateau, respectively. We interpret the small volume of the laccoliths, relative to the surrounding volcanic fields, to reflect differences in crustal composition and tectonic history of the plateau and adjacent areas.Eocene K-Ar ages result from excess 40Ar or contamination by xenocrysts. Excess argon is manifested by trapped argon with 40Ar/36Ar ratios greater than atmosphere (295.5), resulting in total-gas or apparent K-Ar ages that are too old. Laser-fusion experiments and microprobe data show that some samples with Eocene or older total-gas ages and uncorrelated inverse correlation plots are affected by radiogenic argon residing in incompletely outgassed hornblende xenocrysts. All such samples contain middle- to late-Oligocene magmatic hornblende crystals as revealed by laser-fusion experiments. Xenocrysts may be derived from amphibolite xenoliths common in these rocks. Mixing calculations indicate that only 1%-4% contamination may be required to increase K-Ar ages by 10-15 Ma for xenocrysts outgassed as much as 75%. The 40Ar/39Ar method is capable of identifying small degrees of magmatic contamination by older K-bearing phases. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

CdTe in photoconductive applications fast detector for metrology and X-ray imaging

Operating as a photoconductor, the sensitivity and the impulse response of semi-insulating materials greatly depend on the excitation duration to electron compared and hole lifetimes. The requirement of ohmic contact is shortly discussed. Before developing picosecond measurements with an integrated autocorrelation system, this paper explains high energy industrial tomographic applications with large CdTe detectors (25 × 15 × 0.9 mm3). The excitation is typically in the μs range. X-ray flash radiography, with a 10 ns burst, is in an intermediate time domain where the excitation is similar to the electron lifetime. In laser fusion experiments excitation is in the 50 ps range and we develop photoconductive devices able to study very high speed X-ray emission time behaviour. Thin polycrystalline MOCVD CdTe films with picosecond response are suitable to perform optical correlation measurements of single shot pulses with a very large bandwidth (∼ 50 GHz).

Ultrafast photoconductive detectors for burn time and bang time measurements on Phebus

To study deuterium-tritium mixture implosion conditions on laser fusion experiments, we developed very high-speed photoconductive devices. The targets are irradiated by the 1.3-ns, 5-kJ blue light of the two beams of the Phebus laser. The neutron yield is a few 1010/4π. Some neutron sensitivity measurement results are presented on neutron-damaged GaAs and diamond. These devices are able to measure the time behavior of the thermonuclear neutron burst. So, we have measured the burn time and the bang time on direct drive experiments with 100 ps as accuracy.

A Fourier transform microscope for x-ray imaging

This is a progress report on development of a new x-ray imaging system, called a Fourier transform microscope, intended for use with x-ray emitting targets in laser fusion experiments. It is being built by Naval Research Laboratory and Lawrence Livermore National Laboratory. The system works at energies from 3 to 7 keV. We describe the development of the design, which utilizes fine etched grids to extract Fourier amplitudes for the source brightness distribution at selected spatial frequencies. The finest grids in the prototype system will have rib dimensions of 2 μm. The prototype system is expected to achieve position resolution of 4 μm in 3–7 keV. Simulations of the expected imaging performance are presented.

Numerical method for finding uniform irradiation conditions of a fusion capsule driven by X-ray radiation

A three-dimensional model to calculate X-ray intensity distribution on an indirectly driven fusion target is presented. The model includes conversion of laser light into X rays, radiation reemission from X-ray-heated wall of a cavity, and influence of an inner pellet (i.e., a fuel capsule) on radiation redistribution. Intensity distribution of an X ray inside a cylindrical cavity heated by intense blue laser light (wavelength 351 nm, energy 5.2 kJ, duration 0.7 ns) was determined by measuring a burn-through signal from a diagnostic foil integrated onto the cavity. The experimental result is well replicated by the model calculation. By using this model, optimum conditions for uniform irradiation of afusion capsule by X-ray radiation are evaluated for use in Gekko XII laser fusion experiments.

Fabrication of deuterated–tritiated polystyrene shells for laser fusion experiments by means of an isotope exchange reaction

Deuterated–tritiated polystyrene shells with a silicon dopant for use in laser fusion experiments were fabricated by means of an isotope exchange method. In order to increase the isotope exchange rate, deuterated polystyrene shells in a tritium atmosphere were exposed by ultraviolet (UV) radiation from a mercury lamp. The specific activity of tritium in the shells were 92 Ci/g for partially deuterated polystyrene and 150 Ci/g for fully deuterated polystyrene, respectively. The isotope exchange rate with UV irradiation was 230 times as fast as that without the UV irradiation. The tritium in the shell was found to be strongly localized at the outer surface.

New materials and their characterization for photonic device applications

Recent advances in materials technology has led to the development of new and improved optically nonlinear materials that can be used for a wide variety of applications ranging from deep UV conversion for laser fusion experiments to fast optical switching devices in future telecommunication systems. The processibility that many of these new materials have to offer will allow devices to be designed for specific applications. Some recent developments in the search for new materials are described and the future role that some of these may play in the design of nonlinear optical devices is discussed.

Intensity scaling of stimulated Raman forward scattering in laser-produced plasmas.

Measurement of the intensity scaling of stimulated Raman forward scattering (SRFS) at one observation angle is presented. A single, high-intensity 527-nm laser beam excited the instability in a preformed plasma whose properties are such that the predicted gain for SRFS spans the range anticipated for high-gain laser-fusion experiments. At low intensities, the gain of the instability was consistent with convective theory. At intensities greater than 1\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{cm}}^{2}$, the instability saturated at a level of less than ${10}^{11}$ W/sr nm ${\mathrm{cm}}^{2}$.

Preheating in the laser-driven compression of thermonuclear targets.

In laser-fusion experiments, the degree of preheating of the thermonuclear fuel and of the imploding shell that compresses the fuel is an important factor determining the efficiency of fuel compression. Results are presented here of fast-electron-, radiation-, and shock-wave-induced preheating in the laser-driven thermonuclear fusion experiments with high-aspect-ratio shells at the laser intensity ${10}^{14}$ W/${\mathrm{cm}}^{2}$. An alternative technique is proposed for the measurement of fast-electron-induced preheating. The technique is based on analysis of superthermal plasma x-ray radiation. The roles of radiation and shock-wave-induced preheating are also evaluated.

Gated neutron streak camera with a uranium cathode

A gated neutron streak camera with a uranium oxide cathode has been developed for laser fusion experiments. The energy spectrum of secondary electrons was improved by coating the uranium oxide cathode with thin cesium iodine. The tube design was carried out by computer calculation of electron trajectories. The newly developed streak tube showed a temporal resolution of about 70 ps. This streak camera can be applicable to a neutron yield greater than 1012 with a reasonable signal-to-noise ratio by gating a microchannel plate electron multiplier.

High-speed photoconductive device development for laser fusion experiments on Phebus facility (abstract)

The goal of laser fusion experiments conducted with the most powerful European laser Phebus is to study deuterium tritium mixture implosions close to thermonuclear conditions. For this purpose we develop very high speed (less than 100 ps) photoconductive devices able to measure the time behavior of the thermonuclear neutron burst or x-ray emission. Some test results on these specific detectors will be presented.

Gated neutron streak camera with a uranium cathode (abstract)

A gated neutron streak camera with a uranium oxide cathode has been developed for laser fusion experiments. The energy spectrum of secondary electrons was improved by coating the uranium oxide cathode with thin cesium iodine. The tube design was carried out by computer calculation of electron trajectories. The newly developed streak tube showed a temporal resolution of about 70 ps. This streak camera can be applicable to a neutron yield greater than 1012 with a reasonable signal-to-noise ratio by gating a microchannel plate electron multiplier.

Focusing x‐ray spectrograph for laser fusion studies

A modified Rowland‐circle x‐ray spectrograph for laser fusion experiments is described and analyzed. It can be used either as a high‐spectral‐resolution device or as a monochromatic imaging device (including a backlighting option). The relative merits in each configuration are discussed in detail, and an actual design is worked out numerically.

Arbitrary pulse shape synthesis via nonuniform transmission lines

A discrete inverse scattering technique is used to define the impedance profile for a nonuniform transmission line which reflects an arbitrary waveform. Initially charged nonuniform lines, switched out into a general load, can also be synthesized by this method, and are discussed. The direct or layer peeling algorithm is applied to generate profiles which are subsequently analyzed using the one-dimensional finite difference method and fabricated in stripline. Excitation for the nonuniform line is done by using a charged line connected to a photoconductive Si switch triggered by a mode-locked YLF laser. Several lines were fabricated relevant to amplitude modulation of the master oscillator laser pulse for fusion experiments. Using the layer peeling method, a complex high-voltage pulse shape for use in laser fusion experiments is synthesized to an extraordinary degree of precision. It is possible to generate any arbitrary pulse shape by reflecting a step pulse off a synthesized nonuniform transmission line provided the power spectrum of the reflected pulse does not exceed that of the input pulse at any frequency. >