Neutron Time-of-Flight Measurements of Charged-Particle Energy Loss in Inertial Confinement Fusion Plasmas.

Neutron spectra from secondary ^{3}H(d,n)α reactions produced by an implosion of a deuterium-gas capsule at the National Ignition Facility have been measured with order-of-magnitude improvements in statistics and resolution over past experiments. These new data and their sensitivity to the energy loss of fast tritons emitted from thermal ^{2}H(d,p)^{3}H reactions enable the first statistically significant investigation of charged-particle stopping via the emitted neutron spectrum. Radiation-hydrodynamic simulations, constrained to match a number of observables from the implosion, were used to predict the neutron spectra while employing two different energy loss models.

Fractionation of copper activation products in debris samples from the National Ignition Facility.

Nuclear fusion experiments performed at the National Ignition Facility produce radioactive debris, arising in reactions of fast neutrons with the target assembly. We have found that postshot debris collections are fractionated, such that isotope ratios in an individual debris sample may not be representative of the radionuclide inventory produced by the experiment. We discuss the potential sources of this fractionation and apply isotope-correlation techniques to calculate unfractionated isotope ratios that are used in measurements of nuclear reaction cross sections.

Effects of asymmetry and hot-spot shape on ignition capsules.

Asymmetric implosion of inertial confinement fusion capsules is known, both experimentally and computationally, to reduce thermonuclear performance. This work shows that low-mode asymmetries degrade performance as a result of a decrease in the hydrodynamic disassembly time of the hot-spot core, which scales with the minimum dimension of the hot spot. The asymmetric shape of a hot spot results in decreased temperatures and areal densities and allows more alpha particles to escape, relative to an ideal spherical implosion, thus reducing alpha-energy deposition in the hot spot.