High dynamic range Gamma Reaction History (GRH-15m) for diagnosing ignition plasmas*

The Gamma Reaction History (GRH) diagnostic has provided a fusion reaction history at the National Ignition Facility (NIF) by detecting 16.75 MeV gamma rays generated from deuterium-tritium (DT) fusion reactions. While the 4-channel GRH positioned at 6 meters from the target chamber center (GRH-6m) achieves a high temporal resolution (i.e., 10 ps), it suffers from a limited signal-to-background ratio of approximately 17 due to the intense radiation environment in the NIF target bay. Consequently, the current GRH-6m configuration fails to capture the onset of alpha heating. To address this background issue and enable the detection of alpha heating initiation, we propose the implementation of GRH-15m, a new configuration positioned at a distance of 15 meters from the target chamber center. The GRH-15m setup offers improved signal-to-background ratio compared to GRH-6m. The approach involves transporting Cherenkov light from gas cells within the target bay at a 15-meter distance to optical detectors such as photo multiplier tube (PMT) or pulse-dilation (PD) PMTs located on the other side of the six-foot-thick cement shield wall. By adopting this approach, the radiation background experienced by the PMT or PD-PMT is significantly reduced, thereby enhancing the signal-to-background ratio. With these advancements, the proposed GRH-15m will provide a reaction history measurement with a high dynamic range.