When the electronic components of crucial U.S. weapon systems are exposed to radiation, such as electromagnetic pulses and neutrons, the electronics can be degraded and this may cause the weapons to malfunction. As part of NIF’s National Security Applications (NSA) mission, LLNL researchers are working with Sandia National Laboratories colleagues to develop a neutron effects platform on NIF aimed at helping determine the survivability of non-nuclear weapon system components in hostile radiation environments.
“NIF is the best prompt source of high-intensity 14-MeV (million-electron-volt) neutrons in the world,” said NSA Program Manager Brent Blue. The neutron effects platform “will give the researchers access to new regimes in intensity and fluence (neutrons per unit area) they were unable to get to before on any other facility.”
During NIF inertial confinement fusion (ICF) experiments, quadrillions of high-energy neutrons are released into the Target Chamber as the hydrogen (deuterium and tritium) atoms in the target capsule fuse, producing helium. Electronics mounted on NIF’s new neutron effects diagnostic (NED) are irradiated by some of those neutrons, and the results are studied to determine the test sample’s “hardness” against adverse effects.
“Transistors, in particular those that are used for gain and switches, are susceptible to degradation of their performance when subjected to high neutron fluence,” said Sandia nuclear engineer Billy Martin, who is leading the neutron effects program along with LLNL Staff Scientist Charles Yeamans. “We’re performing a wide range of experiments to inform us of these effects.” The experiments are part of a multiyear Sandia Laboratory Directed Research and Development (LDRD) Grand Challenge initiative titled, “New Capabilities for Hostile Environments.”
While many experiments in the program have been performed at Sandia’s Z Pulsed Power Facility, the Z Machine currently is operating without tritium, which limits the available neutron flux, Martin said. “Together with our LDRD External Advisory Board, we thought, why not look at other sources? So we developed hostile environment test platforms at NIF and at OMEGA” (the OMEGA Laser Facility at the University of Rochester).
The NIF neutron effects diagnostic got its first test on Dec. 28 of last year, when it participated in a diagnostic calibration shot using an indirectly driven deuterium-tritium-filled exploding pusher target. The shot produced an average yield of about 500 trillion neutrons, with 220 billion neutrons per square centimeter impinging on the test sample. The test had “extremely promising results,” Martin said. “It measured the damage we expected based on the yields produced.”
The NED sample, a power transistor, was positioned half a meter from the fusion target; this summer a further test is planned in which the sample will be placed just 10 centimeters from the target, increasing the neutron fluence reaching the sample by 25 times. Tests with even higher fluence levels are contemplated for the future.
The NED assembly is about the size of a small, 7.5-oz. soda can made of aluminum and stainless steel to protect the circuit board and transistor from the extreme temperature and x-ray environment. It is mounted on a diagnostic instrument manipulator, which simultaneously carries x-ray pinhole assemblies, solid radiochemistry sample collector diagnostics, and nuclear activation diagnostics.
The neutron effects platform is a parallel effort with the continuing National Security Applications system-generated electromagnetic pulse (SGEMP) campaign that began on NIF in 2014 (see “NIF Experiments Help Validate Electromagnetic Pulse Codes”). Along with Martin and Yeamans, LLNL researchers Charlie Brown, Ben Hatch, and Fred Allen and SNL researchers Ed Parma, Rodney Keith, and Gerald Naranjo participated in the NED’s development.
