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

Abstract

Articles you may be interested inA compact neutron spectrometer for characterizing inertial confinement fusion implosions at OMEGA and the NIF Rev. Sci. Instrum. 85, 063502 (2014); 10.1063/1.4880203 Improving the hot-spot pressure and demonstrating ignition hydrodynamic equivalence in cryogenic deuterium-tritium implosions on OMEGAa) Phys. Plasmas 21, 056315 (2014); 10.1063/1.4876618 Dynamic symmetry of indirectly driven inertial confinement fusion capsules on the National Ignition Facilitya) Phys. Plasmas 21, 056313 (2014); 10.1063/1.4876609 High-resolution spectroscopy used to measure inertial confinement fusion neutron spectra on Omega (invited)a) Rev. Sci. Instrum. 83, 10D919 (2012); 10.1063/1.4742926Study of direct-drive, deuterium-tritium gas-filled plastic capsule implosions using nuclear diagnostics at OMEGA With new measurement techniques, high-resolution spectrometry of secondary fusion protons has been used to study compression and symmetry of imploded 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 ͑1997͔͒. Data from target capsules with ϳ15 atmospheres of D 2 fuel, in CH shells 19-27 m 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-m shells, imploded with similar laser energies ͑ϳ23 kJ͒ 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 yield͒ and the total areal density ͑determined by the energy loss of protons due to slowing in the fuel and shell͒. In going from 0.3-THz SSD ͑smoothing by spectral dispersion͒ to 1-THz SSD and PS ͑polarization smoothing͒, the fuel areal density increased by at least 30%, while the total areal density increased by 40% ͑from ϳ52 to ϳ72 mg/cm 2 ͒. 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/cm 2 . 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.

Using nuclear data and Monte Carlo techniques to study areal density and mix in D2 implosions

“…5d and 6d (Fig. A.2) because the primary T are, on average, less energetic when they fuse with thermal D. 12 Measurements of secondary-neutron spectra from more recent cryogenic implosions also show the same characteristics.…”

“…Wedge-range-filter proton spectrometers 12,27 provided secondary-proton spectra from up to six different directions simultaneously.…”

“…In previous work, 12 high-resolution secondary-proton spectra were obtained during experiments at the OMEGA laser facility. 17 The yields were used with measured neutron yields to estimate ρR hot with the hot-spot and uniform models and it was shown that the Y 2p /Y 1n -inferred ρR hot was often lower than the Y 2n /Y 1n , -inferred ρR hot .…”

“…[1][2][3] One method used to estimate ρR hot of D 2 -filled capsule implosions is to measure the yields of secondary protons (Y 2p ) and/or secondary neutrons (Y 2n ) relative to the primary neutron yield (Y 1n ). [4][5][6][7][8][9][10][11][12] These secondary particles result from sequential reactions in which the energetic primary products of reactions, D + D → n(2.45 MeV) + 3 He(0.82 MeV),…”

“…The secondary particle yields are typically two to three orders of magnitude lower than the primary yield, and the ratios Y 2n /Y 1n , and Y 2p /Y 1n (which are linearly dependent on ρR hot in certain plasma regimes) can each be used to infer a value of ρR hot for implosions of D 2 -filled capsules in both direct-and indirect-drive experiments. [12][13][14][15] In those studies, the simple "hotspot" and/or the "uniform" models were used to relate these ratios to ρR hot . Although these simple models have been widely used to infer a value of ρR hot , they have some serious limitations which can result in misinterpretation and errors (as described in Section II); one manifestation of these problems is often disagreement between the proton-and neutron-inferred values of ρR hot calculated from experimental data (see Fig.…”