Thermal decoupling of deuterium and tritium during the inertial confinement fusion shock-convergence phase.

A series of thin glass-shell shock-driven DT gas-filled capsule implosions was conducted at the OMEGA laser facility. These experiments generate conditions relevant to the central plasma during the shock-convergence phase of ablatively driven inertial confinement fusion (ICF) implosions. The spectral temperatures inferred from the DTn and DDn spectra are most consistent with a two-ion-temperature plasma, where the initial apparent temperature ratio, T_{T}/T_{D}, is 1.5. This is an experimental confirmation of the long-standing conjecture that plasma shocks couple energy directly proportional to the species mass in multi-ion plasmas. The apparent temperature ratio trend with equilibration time matches expected thermal equilibration described by hydrodynamic theory. This indicates that deuterium and tritium ions have different energy distributions for the time period surrounding shock convergence in ignition-relevant ICF implosions.

Diagnostic signature of the compressibility of the inertial-confinement-fusion pusher.

Carbon shell areal density measurements from many types of inertial confinement fusion implosions at the National Ignition Facility (NIF) demonstrate that the final state of the outside portion of the shell is set primarily by capsule coast time, the coasting period between main laser shut off and peak fusion output. However, the fuel areal density does not correlate with the increasing carbon compression. While two-dimensional (2D) radiation-hydrodynamic simulations successfully capture the carbon compression, energy must be added to the simulated fuel-ice layer to reproduce fuel areal density measurements. The data presented demonstrates that the degradation mechanisms that reduce the compressibility of the fuel do not reduce the compressibility of the ablator.

Implementation of the foil-on-hohlraum technique for the magnetic recoil spectrometer for time-resolved neutron measurements at the National Ignition Facility.

The next-generation Magnetic Recoil Spectrometer, called MRSt, will provide time-resolved measurements of the deuterium-tritium-neutron spectrum from inertial confinement fusion implosions at the National Ignition Facility. These measurements will provide critical information about the time evolution of the fuel assembly, hot-spot formation, and nuclear burn. The absolute neutron spectrum in the energy range of 12-16 MeV will be measured with high accuracy (∼5%), unprecedented energy resolution (∼100 keV) and, for the first time ever, time resolution (∼20 ps). Crucial to the design of the system is a CD conversion foil for the production of recoil deuterons positioned as close to the implosion as possible. The foil-on-hohlraum technique has been demonstrated by placing a 1-mm-diameter, 40-µm-thick CD foil on the hohlraum diagnostic band along the line-of-sight of the current time-integrated MRS system, which measured the recoil deuterons. In addition to providing validation of the foil-on-hohlraum technique for the MRSt design, substantial improvement of the MRS energy resolution has been demonstrated.

Multibeam Stimulated Raman Scattering in Inertial Confinement Fusion Conditions.

Stimulated Raman scattering from multiple laser beams arranged in a cone sharing a common daughter wave is investigated for inertial confinement fusion (ICF) conditions in a inhomogeneous plasma. It is found that the shared electron plasma wave (EPW) process, where the lasers collectively drive the same EPW, can lead to an absolute instability when the electron density reaches a matching condition dependent on the cone angle of the laser beams. This mechanism could explain recent experimental observations of hot electrons at early times in ICF experiments, at densities well below quarter critical when two plasmon decay is not expected to occur.