Goals for and design of a neutron pinhole imaging system for ignition capsules

Abstract

Neutron yield at the National Ignition Facility (NIF) or the Laser MegaJoule (LMJ) will range from 10 19 for a capsule that ignites and burns well to below 10 15 for one that fails to ignite. Expected image sizes in deuterium–tritium (DT) neutrons decrease with the neutron yield. At 10 18 –10 19 yields the capsules have ignited and are burning main fuel, producing images with full width at half maximum (FWHM) of ∼100 μm which require a 200 μm field of view and would need 10 μm resolution. Marginally igniting capsules, with yields of 10 17 to 10 18 , burn the hot spot and some main fuel. Their neutronimages are smaller, ∼60 μm FWHM, and require ∼120 μm field of view, with 7 μm resolution needed. Below ∼10 17 , the capsule fails to ignite and a field of view of ∼100 μm suffices to image the hot spot which might be ∼30 μm FWHM with a resolution of ∼5 μm. Images in downscattered neutrons are as large or larger than the time integrated images, have ∼5% of the brightness, and require correspondingly larger fields of view and are useful at lower resolution. A neutron imaging system can be designed to meet these requirements. One design for a neutron aperture is symmetric, biconic, with no clear opening. The opening angle of the cone defines the aperture’s effective area, point spread function, and hence the resolution. The field of view is determined by the cone angle and the distance from the capsule, and hence is coupled to the aperture resolution. Detector resolution determines the required magnification and thereby the minimum capsule to detector distance. At NIF a 20 cm long tungsten aperture with its front face 15 cm from the capsule could produce images with 5 μm resolution at yields down to 10 16 in directions normal to the indirect drive Hohlraum axis. Image noise levels could be comparable to those already achieved on the Omega laser. Imaging in downscattered DT neutrons, typically 9.4–13 MeV and several percent in number compared to the DT neutrons, requires a detector with relatively fast decay time to recover after the arrival of the 14 MeV neutrons. The larger, fainter images reflecting the cold, main DT fuel, which demand less resolution, might be imaged through a second aperture with larger effective area located in the same pinhole assembly. In addition to a line of sight normal to the Hohlraum axis, a second line of sight along the axis is needed to diagnose nonaxisymmetric asymmetries.