K. A. Klare

Interpretation of neutron time-of-flight signals from current-mode detectors

Neutron time-of-flight (nToF) signals from current-mode neutron detectors are often used to determine burn-averaged ion temperature from ICF targets because the spread of the neutron energy distribution is a function of the temperature of the reacting ions. The measured signal will, however, be a convolution of the actual neutron signal, the detector response, and the response of the recording system. In addition, scattered neutrons will arrive at the detector later than unscattered neutrons, further broadening the signal. The ion temperature derived from nToF data depends strongly on the functions used to fit the data or the methods utilized to unfold the neutron energy spectrum. A functional form based on known and measured properties of the detectors is used to fit the integral of the time-of-flight signal, allowing ion temperature derivations from targets with lower neutron yield than previously possible.

Nuclear diagnostics for the National Ignition Facility (invited)

The National Ignition Facility (NIF), currently under construction at the Lawrence Livermore National Laboratory, will provide unprecedented opportunities for the use of nuclear diagnostics in inertial confinement fusion experiments. The completed facility will provide 2 MJ of laser energy for driving targets, compared to the approximately 40 kJ that was available on Nova and the approximately 30 kJ available on Omega. Ignited NIF targets are anticipated to produce up to 1019 DT neutrons. In addition to a basic set of nuclear diagnostics based on previous experience, these higher NIF yields are expected to allow innovative nuclear diagnostic techniques to be utilized, such as neutron imaging, recoil proton techniques, and gamma-ray-based reaction history measurements.

Experimental configuration of direct drive cylindrical implosions on the OMEGA laser

Details about the cylindrical implosions using direct-drive irradiation on the OMEGA Laser facility are provided. The experimental configuration, including orientation, construction, and mounting of the targets is described. An attempt to characterize the modulation transfer function of the primary x-ray framing camera diagnostic results in insufficient exposure contrast but relative agreement with other determinations. The x-ray intensity of the titanium backlighter driven by the 2.5-nsec linear ramp of the laser beams is described, and the relative intensity on film is compared to similar Nova experiments. The parallax effects of different length marker layers of high-opacity dichloropolystyrene is measured, resulting in the conclusion that the marker layer length should be matched to the laser drive illumination profile.

Moderate-convergence inertial confinement fusion implosions in tetrahedral hohlraums at Omega

A highly uniform thermal x-radiation field for indirect-drive inertial confinement fusion implosions may be obtained by irradiating a four-hole, tetrahedral geometry, spherical hohlraum with all 60 Omega laser beams. Implosion studies and calculations [J. M. Wallace et al., Phys. Rev. Lett. 82, 3807 (1999)] indicate a drive uniformity comparable to that expected for the National Ignition Facility [J. A. Painser et al., Laser Focus World 30, 75 (1994)]. With 60 beams distributed over the cavity wall, tetrahedral hohlraums have a natural insensitivity to power balance and pointing errors. Standard, smooth Nova capsules imploded with this drive indicate that moderate convergence-ratio implosions, Cr∼18, have measured-neutron yield to calculated-clean-one-dimensional-neutronyield ratios similar to those previously investigated using the comparatively poor drive uniformity of Nova cylindrical hohlraums. This may indicate that a nonsymmetry-related neutron yield degradation mechanism, e.g., hydrodynamic mixing ...

Detailed diagnosis of a double-shell collision under realistic implosion conditions

Double-shell implosions provide a noncryogenic path to inertial confinement fusion. In the double-shell target, the energy is absorbed in an outer shell that is accelerated inward and collides with an inner shell that implodes against the deuterium fuel. Symmetric collision of the shells requires that the shells be illuminated and built symmetrically. In reality, the targets are complicated and the construction is not symmetric, due to the seam that our current assembly method requires. Using the Omega laser [R. T. Boehly et al., Opt. Comm. 133, 495 (1997)], an illumination strategy was designed that uses 40 beams in an offset geometry, leaving 20 beams to perform radiography from two different directions. This places a significant nonsymmetric illumination challenge that may not exist in final targets shot on the National Ignition Facility. This paper presents a measurement of the time history of a collision of two shells in a double-shell capsule, briefly reviews the illumination geometry, gives the res...

Production of enhanced pressure regions due to inhomogeneities in inertial confinement fusion targets

It is shown that regions of enhanced pressure have been produced in targets with indirect radiation drive in planar and cylindrically convergent geometry through the interaction between the flows caused by target inhomogeneities and the main target drive. Design calculations for National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] targets with beryllium ablators formed by bonded hemi-shells [D. C. Wilson et al., Bull. Am. Phys. Soc. 43, 1667 (1998)] indicate that related behavior produces a seed perturbation in the ablator which can in some cases lead to the suppression of ignition. From simulation and analysis of the NIF problem in the planar geometry analog, a scaling for the perturbation, which should be useful for validation of the behavior with lower energy drive and smaller-scale geometries, is derived.

Neutron time-of-flight signals from expanding or contracting spherical sources

The width of the energy distribution of fusion-produced neutrons is often used as an indication of the temperature of the reacting ions. The Doppler broadening of the neutron energy is due to the center-of-mass velocity of reacting ion pairs and is characterized by the ion temperature for a Maxwellian distribution of ions with zero collective velocity. If there is bulk fluid motion or turbulence characterized by a velocity on the order of the ion thermal speed, a significant additional broadening may introduced. Suggestions of this phenomenon have been observed for two classes of laser targets. The first is a ``gas bag`` target, in which a deuterated hydrocarbon gas is contained in a thin spherical membrane and illuminated uniformly. The second target is an ICF capsule with a deuterated plastic inner layer. In both cases, measured neutron energy distributions were wider than expected from theoretical ion temperatures alone would predict, and if interpreted as indicative of the ion temperature, are inconsistent with the neutron yields observed.

Measurements of neutron spectra from Nova targets

We have measured neutron spectra from targets on the Nova laser. The three basic classes of targets are (1) directly driven capsules with ablative or exploding pushers, (2) indirectly driven imploding capsules with various hohlraum drives, capsule thicknesses, capsule fill pressures, and hohlraum gas fills, and (3) gas-filled hohlraums containing deuterated gas mixtures. In many capsule implosions, the neutron spectra are significantly broader than predictions of fusion-reaction-weighted ion temperatures from simulations. For implosions of deuterated capsules filled with H2, the observed spectra are consistent with the predicted combination of thermal and motional broadening. For normal capsules filled with D2, we conjecture that implosion asymmetries may induce fluid motion within the compressed fuel which contributes to the measured spectral width, but not the fusion reactivity, and which is not included in the simulations.

A sensitive neutron spectrometer for the National Ignition Facility

We are developing a sensitive neutron spectrometer for the National Ignition Facility laser at Livermore. The spectrometer will consist of a 1020 channel single-neutron-interaction time-of-flight detector array fielded 23 m from the neutron-producing target. It will use an existing detector array together with upgraded electronics for improved time resolution. Measurements of neutron yield, ion and electron temperatures, and density-radius product are all possible under certain conditions using one-, two-, or three-step reaction processes. The locations of the most important potential sources of scattered neutron backgrounds are determined as the first step in designing collimation to reduce these backgrounds.

Calibration and operation of a neutron time-of-flight scintillator array on Nova

Abstract The Tion diagnostic is a 1020-channel scintillator array on the Nova laser at Livermore. It measures the arrival time of single D-D or D-T neutrons at 27 m from the target with time resolution of 4 ns. The effect of neutron scattering has been obtained from Monte-Carlo neutron transport simulations and the scattered response is convolved with a Gaussian source to fit the data. The data analysis has been checked using targets which produce roughly equal numbers of D-T and D-D counts. The temperature obtained from Tion and from a current-mode neutron time-of-flight detector are in agreement for neutron yields above 1 × 10 9 .