Advanced time-resolved neutron time-of-flight diagnostics via refractive index modulation*

Understanding the onset of thermonuclear burn and ignition in inertial confinement fusion (ICF) implosions is critical, and time-resolved measurements of ion temperature and fuel density are necessary to do so. There are currently no diagnostics at the National Ignition Facility (NIF) that can constrain the ion temperature as a function of time, T_ion(t). Modulation of the refractive index of an optically transparent medium by ionizing particles can provide a novel and ultra-fast (sub-ps) measurement technique that could be adopted for neutron time-of-flight (nToF) spectroscopy. Advantages of the novel technique discussed here are its relative affordability and shortened development times. Limited data exists on the sensitivity of the index of refraction to 14.1 MeV neutrons. Tests are being conducted to determine the sensitivity of the index of refraction to neutrons at the NIF. This data will inform the design of an intermediate nToF, placed ~ 0.3-1.0m from target chamber center (TCC). Demonstration of an intermediate nToF, based on index of refraction modulation, could enable constraining measurements of T_ion(t). [1] This work will discuss current efforts on the development of a nToF based on refractive index modulation.  

[1] A. S. Moore at al., Constraining time-dependent ion temperature measurements in inertial confinement fusion (ICF) implosions with an intermediate distance neutron time-of-flight (nToF) detector, Rev. Sci. Instrum. 93, 113536 (2022).