With new measurement techniques, high-resolution spectrometry of secondary fusion protons has been used to study compression and symmetry of imploded D2-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 D2 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.

In vivo imaging of cells tagged with light-emitting probes, such as firefly luciferase or fluorescent proteins, is a powerful technology that enables a wide range of biological studies in small research animals. Reporters with emission in the red to infrared (>600 nm) are preferred due to the low absorption in tissue at these wavelengths. Modeling of photon diffusion through tissue indicates that bioluminescent cell counts as low as a few hundred can be detected subcutaneously, while approximately 10(6) cells are required to detect signals at approximately 2 cm depth in tissue. Signal-to-noise estimates show that cooled back-thinned integrating charge coupled devices (CCDs) are preferred to image-intensified CCDs for this application, mainly due to their high quantum efficiency (approximately 85%) at wavelengths >600 nm where tissue absorption is low. Instrumentation for in vivo imaging developed at Xenogen is described and several examples of images of mice with bioluminescent cells are presented.

The use of measured spectra of secondary fusion protons for studying physical characteristics of D2-filled inertial confinement fusion capsules is described theoretically and demonstrated with data from implosions in the OMEGA 60-beam laser facility. Spectra were acquired with a magnet-based charged-particle spectrometer and with a range-filter-based spectrometer utilizing filters and CR39 nuclear track detectors. Measurement of mean proton energy makes possible the study of a capsule’s total areal density (ρR), since that is what affects the energy loss suffered by protons as they pass through fuel and shell while leaving the capsule. Details of specific shots will be presented. It is also shown that similar techniques should prove useful for diagnosis of future experiments with cryogenic D2-filled capsules.

With charged-particle spectroscopy implemented on OMEGA, we have been able to routinely measure the particle spectra (both nuclear lines and continua) from a variety of capsule implosions. Important parameters such as fusion yields, fuel and shell areal densities, and ion temperatures can be readily deduced. We will report on details of this work with emphasis on the implosion physics.

With the operation of successively more intense and powerful lasers, such as the NOVA petawatt laser with I∼3×1020 W/cm2, several novel (to laser physics) nuclear diagnostics were used to determine the nature of the laser/matter interaction at the target surface. A broad beam of hot electrons, whose centroid varied from shot to shot, width was remarkably constant, and intensity was about 40% of the incident laser energy was observed. New nuclear phenomenon included photonuclear reactions [e.g., (γ,xn)], photofission of U238 and intense beams of ions. Photonuclear reactions were observed and quantified in Cu, Ni, and Au samples, and produced activation products as neutron deficient as Au191 [a (γ,6n) reaction!], requiring gamma rays exceeding 50 MeV in energy. The spectral features of the gamma-ray source have been investigated by comparing activation ratios in Ni and Au samples, and angular distributions of higher energy photons have been measured with activation of spatially distributed Au samples. Extraordinarily intense beams of charged particles (primarily protons) were observed normal to the rear surface of the target and quantified using the charged particle reaction Ti48(p,n)48V, radiochromic film and CR39 plastic track detectors. Approximately 3×1013 protons, with energies up to 55 MeV were observed in some experiments. Collimation of this beam increases with increasing proton energy. Correlations of activation with laser performance will be discussed.

Recent work has resulted in the first high-resolution, spectroscopic measurements of energetic charged particles on OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 496 (1997)]. Energy spectra of charged fusion products have been obtained from two spectrometers, and have been used to deduce various physical quantities in imploded capsules. In this paper the first use of 14.7 MeV deuterium–helium3 (D–3He) proton spectra for diagnosing shell areal density (ρR) and fuel ion temperature (Ti) is discussed. For thick-plastic shell capsules, shell areal densities between 20 and 70 mg/cm2 and ion temperatures between 3 and 5 keV have been determined. The spectral linewidths associated with such capsules are found to be wider than the doppler widths. This effect, the focus of future study, is the result of ρR evolution during the burn; or is the result of an extended burn region; or results from nonuniformities in the shell. For thin-glass shell capsules, the spectral linewidths are dominated by the doppler width, and ion temperatures between 10 and 15 keV were determined. These measurements have been also compared and contrasted with the results from neutron measurements and from one-dimension hydrodynamic simulations.

We have recently conducted a series of experiments on Omega in which we utilized a charged-particle spectrometer. It consists of a 7.6 kG magnet, CR-39 track for particle detection, and an assortment of range filters to either discriminate against or emphasize certain particles. Because of excellent particle selectivity and energy resolution, the spectrometer can accurately measure the ratios of various fusion products from a single shot. Among other uses, these ratios will be used to determine ion temperatures.

Solid targets irradiated with 1019 W/cm2 or greater of 1 μm light in picosecond pulses are found to be radioactive. The strongest activities observed are the result of photonuclear reactions in which an energetic photon excites the nucleus sufficiently to produce particle emission leaving a radioactive daughter. The photoreaction cross sections are known for a wide range of nuclei and provide a quantitative measure of the photon flux produced in the target. Both the delayed daughter activities and measurements of the prompt particles emitted in the reaction can be used as diagnostics. Examples of these techniques applied in diagnosing experiments at the Nova laser facility adapted to generate petawatt pulses using chirped pulse amplification will be presented. These results will be compared with bremsstrahlung photon spectra calculated using electron spectra measured in a magnetic spectrograph.

In an experimental study of the physics of fast ignition, the characteristics of the hot electron source at laser intensities up to 1020 W cm−2 have been measured and a diagnosis of the heating at depth by hot electrons has been initiated. Generation of hot electrons with more than 30% efficiency has been observed. Preliminary heating data suggest temperatures kT in the range 300–800 eV.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation M. H. Key, M. D. Cable, T. E. Cowan, K. G. Estabrook, B. A. Hammel, S. P. Hatchett, E. A. Henry, D. E. Hinkel, J. D. Kilkenny, J. A. Koch, W. L. Kruer, A. B. Langdon, B. F. Lasinski, R. W. Lee, B. J. MacGowan, A. MacKinnon, J. D. Moody, M. J. Moran, A. A. Offenberger, D. M. Pennington, M. D. Perry, T. J. Phillips, T. C. Sangster, M. S. Singh, M. A. Stoyer, M. Tabak, G. L. Tietbohl, M. Tsukamoto, K. Wharton, S. C. Wilks; Atomic and nuclear processes produced in ultra-high intensity laser irradiation of solid targets. AIP Conference Proceedings 28 September 1998; 443 (1): 283–286. https://doi.org/10.1063/1.56575 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search