Using secondary-proton spectra to study the compression and symmetry of deuterium-filled capsules at OMEGA

With new measurement techniques, high-resolution spectrometry of secondary fusion protons has been used to study compression and symmetry of imploded ${\mathrm{D}}_2$ -filled capsules in direct-drive inertial-confinement-fusion experiments at the 60-beam OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Data from target capsules with ∼15 atmospheres of ${\mathrm{D}}_2$ fuel, in CH shells 19–27 μm thick, were acquired with a magnet-based, charged-particle spectrometer and with several new “wedge-range-filter”-based spectrometers incorporating special filters and CR39 nuclear track detectors. Capsules with 19-μm shells, imploded with similar laser energies (∼23 kJ) but different methods of single-beam laser smoothing, were studied and found to show different compression characteristics as indicated by the fuel areal density (determined by the ratio of secondary-proton yield to primary-neutron yield) and the total areal density (determined by the energy loss of protons due to slowing in the fuel and shell). In going from 0.3-THz SSD (smoothing by spectral dispersion) to 1-THz SSD and PS (polarization smoothing), the fuel areal density increased by at least 30%, while the total areal density increased by 40% (from ∼52 to ∼72 mg/cm²). In addition, significant low-mode-number spatial asymmetries in implosions were indicated by spectra measured at different angles with respect to the target. The mean energies of protons, measured at different angles during the same shot, varied by as much as 1 MeV, implying angular variations in areal density of order 30 mg/cm². To the best of our knowledge, this is the first experimental demonstration that capsule symmetry can be sensitively studied by measuring the energy loss of charged particles.