Measurement of Non-Maxwellian Ion Velocity Distributions in a Planar-Geometry Shock-Driven Implosion Mockup

Inertial confinement fusion (ICF) experiments at both the National Ignition Facility (NIF) and the OMEGA Laser Facility have demonstrated evidence of kinetic, non-Maxwellian ion velocity distributions, but the actual distributions involved remain unknown. A planar-geometry mockup of a shock-driven ICF implosion was fielded at the OMEGA Laser Facility to directly measure such non-Maxwellian ion velocity distributions in ICF-relevant conditions for the first time. The optical Thomson scattering (OTS) technique was used in the collective regime to extract the ion velocity distributions at the center of the experimental configuration. The OTS spectrum indicates ion beams streaming into the center of the implosion, a kinetic effect which is reproduced in Vlasov-Fokker-Planck simulations of the system. The inferred density and relative flow velocity of the beams suggest that the beaming ions contribute significantly to the neutron yield, which is also corroborated by nuclear yield measurements. These results suggest that similar kinetic effects could impact both plasma dynamics and neutron yields in spherical shock-driven implosions as well.
The experiment was conducted at the Omega Laser Facility with the beam time through the National Laser Users’ Facility (NLUF) under the auspices of the U.S. DOE/NNSA by the University of Rochester’s Laboratory for Laser Energetics under Contract DE-NA0003856 and was funded by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, under Award Number DE-SC0024306. B. Foo is supported by NNSA SSGF DE-NA0003960.