Gamma bang time/reaction history diagnostics for the National Ignition Facility using 90° off-axis parabolic mirrors^a) Available to Purchase

Gas Cherenkov detectors (GCDs) have been used to convert fusion gamma into photons to achieve gamma bang time and reaction history measurements. The GCDs designed for OMEGA used Cassegrain reflector optics in order to fit inside a $10in.$ manipulator. A novel design for the National Ignition Facility using 90° off-axis parabolic mirrors will increase light collection efficiency from fusion gammas and achieve minimum time dispersion. The broadband Cherenkov light (from $200to800nm$⁠) is relayed into a high-speed detector using three parabolic mirrors. Because light is collected from many source planes throughout the $CO2$ gas volume, the detector is positioned at the stop position rather than at an image position. The stop diameter and its position are independent of the light-generation location along the gas cell. The current design collects light from a $100mm$ diameter by $500mm$ long gas volume. Optical ray tracings demonstrate how light can be collected from different angled trajectories of the Compton electrons as they fly through the $CO2$ gas volume. A cluster of four channels will allow for increased dynamic range as well as for different gamma energy threshold sensitivities.