High-resolution spectrometry of charged particles from inertial-confinement-fusion ͑ICF͒ experiments has become an important method of studying plasma conditions in laser-compressed capsules. In experiments at the 60-beam OMEGA laser facility ͓T. R. Boehly et al., Opt. Commun. 133, 495 ͑1997͔͒, utilizing capsules with D 2 , D 3 He, DT, or DTH fuel in a shell of plastic, glass, or D 2 ice, we now routinely make spectral measurements of primary fusion products ͑p, D, T, 3 He, ␣͒, secondary fusion products ͑p͒, ''knock-on'' particles ͑p, D, T͒ elastically scattered by primary neutrons, and ions from the shell. Use is made of several types of spectrometers that rely on detection and identification of particles with CR-39 nuclear track detectors in conjunction with magnets and/or special ranging filters. CR-39 is especially useful because of its insensitivity to electromagnetic noise and its ability to distinguish the types and energies of individual particles, as illustrated here by detailed calibrations of its response to 0.1-13.8 MeV protons from a Van de Graaff accelerator and to p, D, T, and ␣ from ICF experiments at OMEGA. A description of the spectrometers is accompanied by illustrations of their operating principles using data from OMEGA. Sample results and discussions illustrate the relationship of secondary-proton and knock-on spectra to capsule fuel and shell areal densities and radial compression ratios; the relationship of different primary fusion products to each other and to ion temperatures; the relationship of deviations from spherical symmetry in particle yields and energies to capsule structure; the acceleration of fusion products and the spectra of ions from the shell due to external fields; and other important physical characteristics of the laser-compressed capsules.
The performance of gas-filled, plastic-shell implosions has significantly improved with advances in on-target uniformity on the 60-beam OMEGA laser system ͓T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 ͑1997͔͒. Polarization smoothing ͑PS͒ with birefringent wedges and 1-THz-bandwidth smoothing by spectral dispersion ͑SSD͒ have been installed on OMEGA. The beam-to-beam power imbalance is р5% rms. Implosions of 20-m-thick CH shells ͑15 atm fill͒ using full beam smoothing ͑1-THz SSD and PS͒ have primary neutron yields and fuel areal densities that are ϳ70% larger than those driven with 0.35-THz SSD without PS. They also produce ϳ35% of the predicted one-dimensional neutron yield. The results described here suggest that individual-beam nonuniformity is no longer the primary cause of nonideal target performance. A highly constrained model of the core conditions and fuel-shell mix has been developed. It suggests that there is a ''clean'' fuel region, surrounded by a mixed region, that accounts for half of the fuel areal density.
A systematic review of the literature has been conducted and studies reporting investigations of genotoxicity biomarkers in pesticide workers have been assessed with view to establishing whether there was evidence for any risk to those using pesticides approved in the United Kingdom. Each of the studies was evaluated using a set of criteria drawn up by members of the UK Committee of Mutagenicity based upon the guidelines proposed by the International Programme on Chemical Safety (IPCS) working group [R. J. Albertini, D. Anderson, G. R. Douglas, L. Hagmar, K. Hemminki, F. Merlo, A. T. Natarajan, H. Norppa, D. E. Shuker, R. Tice, M. D. Waters and A. Aitio (2000) Mutat. Res., 463, 111-172]; 24 out of 70 studies met the criteria for inclusion in the substantive evaluation. Positive findings were compared with occupational practices and evidence of exposure to specific pesticides with view to developing hypotheses for further consideration. Seventeen of the 24 studies reported positive findings, although in the majority of these the magnitude of increase was small. There was some limited evidence that the use of benzimidazoles was more consistently associated with positive findings. However, limitations in the data, particularly evidence of exposure, did not allow definitive conclusions to be drawn. Also, it was noted that the use (or not) of personal protective equipment (PPE) was not well documented and in the few studies in which its use was reported, the findings were more likely to be positive in the absence of PPE usage. An independent epidemiological review concluded that all studies were of limited design, particularly with regards to study size, the assessment of subject selection and potential recruitment bias. Variance in genotoxicity indices in the control population and a lack of understanding of the factors influencing this variability complicate attempts to characterize positive responses. More substantive data are needed in this respect so that the significance of relatively small increases in biomonitoring indices can be accurately assessed. Once these data are available, a study in workers using benzimidazoles would be appropriate.
For the first time high areal-density ͑R͒ cryogenic deuterium-tritium ͑DT͒ implosions have been probed using downscattered neutron spectra measured with the magnetic recoil spectrometer ͑MRS͒ ͓J. A. Frenje et al., Rev. Sci. Instrum. 79, 10E502 ͑2008͔͒, recently installed and commissioned on OMEGA ͓T. R. Boehly et al., Opt. Commun. 133, 495 ͑1997͔͒. The R data obtained with the MRS have been essential for understanding how the fuel is assembled and for guiding the cryogenic program at the Laboratory for Laser Energetics ͑LLE͒ to R values up to ϳ300 mg/ cm 2 . The R data obtained from well-established charged particle spectrometry techniques ͓C. K. Li et al., Phys. Plasmas 8, 4902 ͑2001͔͒ were used to authenticate the MRS data for low-R plastic capsule implosions, and the R values inferred from these techniques are in excellent agreement, indicating that the MRS technique provides high-fidelity data. Recent OMEGA-MRS data and Monte Carlo simulations have shown that the MRS on the NIF ͓G. H. Miller et al., Nucl. Fusion 44, S228͑2004͔͒ will meet most of the absolute and relative requirements for determining R, ion temperature ͑T i ͒ and neutron yield ͑Y n ͒ in both low-yield, tritium-rich, deuterium-lean, hydrogen-doped implosions and high-yield DT implosions.
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.
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