Coded imaging of fusion neutrons in inertial confinement fusion experiments

Abstract

In Inertial Confinement Fusion (ICF) experiments, radiation from the compressed core is increasingly reabsorbed. For the largest experiments, the only radiation to escape is the 14 MeV fusion neutrons in which we must turn to learn of the physical processes taking place. The most important parameters are the shape and the size of the compressed core and this involves imaging the neutrons produced by the fusion reactions. The penumbral technique is ideally suited to neutron imaging and the feasibility of this technique has been demonstrated at the Lawrence Livermore National Laboratory in the United States. At the Phebus laser facility in France, this method has been used in image compressed ICF cores with diameters of 150 micrometers yielding approximately 10<SUP>9</SUP> neutrons, and the overall spatial resolution obtained in the reconstructed source was approximately 100 micrometers . On the Laser Megajoule project which is the equivalent of the National Ignition Facility in the United States, the spatial resolution required to diagnose high-convergence targets is 10 micrometers . We wish first to obtain a spatial resolution of 30 micrometers to image source with a diameter &lt;EQ 100 micrometers at a neutron yield in the range of 10<SUP>11</SUP> - 10<SUP>14</SUP> neutrons/4(pi) . A collaborative experimental program with the Laboratory for Laser Energetics at the University of Rochester in this perspective is planned. At the same time, there is a research program in collaboration with Laval University (Quebec) concerning coded aperture designs and the associated reconstruction techniques. In this article we first review the basic requirements of such imagery and the concept of the penumbral imaging technique. Then we concentrate on the parameters that condition the spatial resolution and the description of our imaging system. Finally, we survey the reconstruction techniques used followed by results and comparative evaluation of those methods.