Carlos Barrera

Downscattered neutron imaging

Images with 14 MeV neutrons of inertial confinement fusion (ICF) D,T fusion show the regions of most intense fusion burn, while images based on lower-energy “downscattered” neutrons can reveal regions of nonburning D,T fuel. The downscattered images can help to understand ICF implosion dynamics. Recording downscattered images is difficult because the images are relatively weak, and because they may be obscured by residual “afterglow” of more intense 14 MeV images. The effect of afterglow can be estimated by adding a sequence of images for neutron energies from 14 MeV down to the downscatteed energy of interest. The images will be subject to decay factors which depend on the time response of the neutron scintillator. Preliminary analyses suggest that afterglow will not prevent the recording of useful downscattered images.

Image reconstruction algorithms for inertial confinement fusion neutron imaging

A neutron imaging system is required to diagnose ignition implosions at the National Ignition Facility. Such a system is required to be able to resolve features in the imploded target core as small as 5μm. The system will use a pinhole-camera-type geometry with a nonideal coded aperture and will employ image restoration techniques. The choice of image reconstruction method will be important in recovering the best possible source images from the recorded data. Monte Carlo transport simulations with MCNP5 make it possible to estimate the performance of the neutron imaging system based on calculated energy-dependent image edits of a failed inertial confinement fusion implosion. Simulations of the recorded neutron images include specific aperture designs, a pixelated energy- and time-dependent scintillator array, and an intensified gated charge coupled device camera for recording the images. An initial series of simulations used a source that was binned into 1MeV increments from 6to18MeV, an imaging aperture ...

Performance Modeling of the NIF Neutron Imaging System

Summary form only given. The Neutron Imaging System (MS) for the National Ignition Facility (NIF) is currently under development. The system should be able to record hot-spot (13-15 MeV) and cold-fuel (6-10 MeV) images with a resolution of 10 microns and a signal-to-noise ratio (SNR) of 10 at the 20% contour. This diagnostic is valuable because lower energy neutrons reveal the distribution of the non-burning DT fuel surrounding the hot-spot, providing key information in the case of a failed implosion. An End-to-End model of the system has been developed to study the performance of different designs with respect to the resolution and SNR requirements. The model includes accurate source distributions of a failed implosion, five aperture geometries (ring, penumbral, square pinhole, small penumbral pinhole and triangular wedge), their associated point spread functions, and a pixelated plastic scintillator detector array. The simulated recorded images are deconvolved using a modified regularization algorithm, producing overall simulations of the expected source images.

High-Bandwidth Optical Detectors for Fusion Experiments

Summary form only given. Traditional nuclear diagnostics for inertial confinement fusion (ICF) experiments have relied on detectors which convert interactions with radiation to high-bandwidth electrical signals. As the power of the lasers and the nuclear yields from ICF experiments has increased, electrical background signals have become a serious problem that prevents the recording of high-quality data. A recent series of experiments at the University of Rochester Laboratory for Laser Energetics OMEGA facility studied the feasibility of using radiation-to-light converters and high bandwidth optical signal transmission as an alternate nuclear diagnostic method. The radiation to light converters (scintillators, pure glasses and plastics, and pressurized CO2) performed well and produced predictable signal amplitudes. Fiber optic transmission lines proved to have a number of fundamental problems, with radiation sensitivity being the most serious. Simple optical light pipes have performed very well and make it possible to design high-bandwidth diagnostic systems. The light pipes are hollow stainless steel tubes with polished interior surfaces which can provide efficient signal transmission with modal dispersion of the order of 10 picoseconds. Experimental results will be discussed, and the preliminary design of a diagnostic system will be described.