R. L. Griffith

Development of nuclear diagnostics for the National Ignition Facility (invited)

The National Ignition Facility (NIF) will provide up to 1.8MJ of laser energy for imploding inertial confinement fusion (ICF) targets. Ignited NIF targets are expected to produce up to 1019 DT neutrons. This will provide unprecedented opportunities and challenges for the use of nuclear diagnostics in ICF experiments. In 2005, the suite of nuclear-ignition diagnostics for the NIF was defined and they are under development through collaborative efforts at several institutions. This suite includes PROTEX and copper activation for primary yield measurements, a magnetic recoil spectrometer and carbon activation for fuel areal density, neutron time-of-flight detectors for yield and ion temperature, a gamma bang time detector, and neutron imaging systems for primary and downscattered neutrons. An overview of the conceptual design, the developmental status, and recent results of prototype tests on the OMEGA laser will be presented.

CVD diamond as a high bandwidth neutron detector for inertial confinement fusion diagnostics

We characterize the response of chemical vapor deposition (CVD) diamond detectors to inertial confinement fusion (ICF) neutrons generated at the OMEGA laser fusion facility in Rochester, NY. Four detectors are tested: three utilizing “optical grade” CVD diamond, and one utilizing “electronic grade” CVD diamond. Using a 50 Ω measurement system, we find that the optical grade wafers, biased to 1000 V/mm, have an average sensitivity of 0.24 μV ns/n for 2.5 MeV (DD fusion) neutrons and 0.62 μV ns/n for 14.0 MeV (DT fusion) neutrons. At the same E field, the electronic grade wafer has a sensitivity of 0.56 and 1.43 μV ns/n for 2.5 and 14 MeV neutrons, respectively. Linear dynamic range for the optical grade material is shown to be at least 105. Average full width at half maximum response times, as measured with pulsed laser and 3 GHz scope, are 376 and 880 ps for optical and electronic grades, respectively. These characteristics make CVD diamond suitable for ICF applications such as neutron time-of-flight spec...

Observation of d-t fusion gamma rays (invited)

Deuterium–tritium (DT) reaction rates of imploding capsules have historically been measured using neutron detectors. Temporal resolution is limited by the size of the detector and distance from the source to detector. The reaction rates can also be measured using the 16.7 MeV gamma ray, which is produced by the same DT reaction, but statistically far less often than the 14.1 MeV neutron. Cherenkov detectors detect gamma rays by converting the gamma rays to electrons, which in turn produce Cherenkov light and record this visible light using a fast optical detector. These detectors can be scaled to large volumes in order to increase detection efficiency with little degradation in time resolution, and placed well away from the source since gamma rays do not suffer velocity dispersion between the source and detector. Gas-based Cherenkov detectors can also discriminate against lower-energy photons produced in and around the target. A prototype gas Cherenkov detector has been built and tested for detector respo...

Implementation of a near backscattering imaging system on the National Ignition Facility

A near backscattering imaging diagnostic system is being implemented on the first quad of beams on the National Ignition Facility. This diagnostic images diffusing scatter plates, placed around the final focus lenses on the National Ignition Facility target chamber, to quantitatively measure the fraction of light backscattered outside of the focusing cone angle of incident laser beam. A wide-angle imaging system relays an image of light scattered outside the lens onto a gated charge coupled device camera, providing 3 mm resolution over a 2 m field of view. To account for changes of the system throughput due to exposure to target debris the system will be routinely calibrated in situ at 532 and 355 nm using a dedicated pulsed laser source.

Ten-inch manipulator-based neutron temporal diagnostic for cryogenic experiments on OMEGA

Measurements of the neutron emission from inertial confinement fusion implosions provide important information about target performance that can be compared directly with numerical models. For room-temperature target experiments on OMEGA at the Laboratory for Laser Energetics (LLE) the neutron temporal diagnostic (NTD), originally developed at Lawrence Livermore National Laboratory, is used to measure the neutron burn history with high resolution and good timing accuracy. The NTD is mechanically incompatible with cryogenic target experiments because of the standoff required to remain clear of the cryogenic target handling system, A cryogenic-compatible neutron temporal diagnostic (cryoNTD) has been designed for LLE’s standard ten-inch manipulator diagnostic inserters. This instrument provides high-resolution neutron emission measurements for cryogenic implosions. Experimental results of the performance of cryoNTD compared to NTD on room-temperature direct-drive implosions and on cryogenic implosions are p...

Implementation of a high energy 4ω probe beam on the Omega laser

A high-energy, ultraviolet Thomson scattering probe beam has been implemented on the Omega laser facility at the University of Rochester. The new probe operates at a wavelength of 264 nm, with a maximum energy of 260 J in a pulse length of 1 ns. The probe is focused with an F/6.7 lens to a minimum focal spot of 40 μm within a pointing tolerance of <50 μm. Data obtained from this probe beam have provided new diagnostic information on plasmas relevant for inertial confinement fusion and atomic physics studies.

Multiplexed gas Cherenkov detector for reaction-history measurements

A diagnostic is being designed for the National Ignition Facility, using fusion gamma rays to measure highly time-resolved bang times and deuterium-tritium (d-t) interaction rates for imploding inertial fusion capsules. As a complement to neutron-based methods, gas Cherenkov detectors were chosen for this purpose because of proven ultrahigh bandwidth, thresholding versatility, and minimal time-of-flight dispersion. Gas Cherenkov detector prototypes, involving streak cameras and fast photomultiplier, microchannel plate detectors, are being tested using d-t implosions at the Omega Laser Facility. The possibility of simultaneous streak camera and photomultiplier, microchannel plate recordings of a source in one gas Cherenkov detector instrument is advantageous for reasons of independent measurement and extended reaction-history coverage. A multiplexed gas Cherenkov detector system was demonstrated successfully using electron pulses produced by the Idaho State University linear electron accelerator. A reactio...

Preliminary performance measurements for a streak camera with a large-format direct-coupled charge-coupled device readout

The University of Rochester’s Laboratory for Laser Energetics (Rochester, New York) is leading an effort to develop a modern, fully automated streak camera. Characterization of a prototype camera shows spatial resolution better than 20 lp/mm, temporal resolution of 12 ps, line-spread function of 40 μm (full width at half maximum) contrast transfer ratio of 60% at 10 lp/mm, system gain of 101 charge-coupled device electrons per photoelectron, and a dynamic range of 500 for a 2 ns window.

3ω transmitted beam diagnostic at the Omega Laser Facility

A 3{omega} transmitted beam diagnostic has been commissioned on the Omega Laser at the Laboratory for Laser Energetics, University of Rochester [Soures et.al., Laser Part. Beams 11 (1993)]. Transmitted light from one beam is collected by a large focusing mirror and directed onto a diagnostic platform. The near field of the transmitted light is imaged; the system collects information from twice the original f-cone of the beam. Two gated optical cameras capture the near field image of the transmitted light. Thirteen spatial positions around the measurement region are temporally resolved using fast photodiodes to allow a measure of the beam spray evolution. The Forward stimulated Raman scattering and forward simulated Brillion scattering are spectrally and temporally resolved at 5 independent locations within twice the original f-cone. The total transmitted energy is measured in two spectral bands ({delta}{lambda} 400 nm).

Compact optical technique for streak camera calibration

The National Ignition Facility is under construction at the Lawrence Livermore National Laboratory for the U.S. Department of Energy Stockpile Stewardship Program. Optical streak cameras are an integral part of the experimental diagnostics instrumentation. To accurately reduce data from the streak cameras a temporal calibration is required. This article describes a technique for generating trains of precisely timed short-duration optical pulses that are suitable for temporal calibrations.