We present measurements of absolute rates for multiphoton ionization of the ground state of atomic hydrogen by a linearly polarized, subpicosecond KrF laser at a wavelength of 248 nm. The irradiance ...was varied from 3{times}10{sup 12} to 2{times}10{sup 14} W/cm{sup 2}, and three above-threshold-ionization peaks were observed. The measured rate for total electron production was less than that predicted by Floquet theory (S.-I. Chu and J. Cooper, Phys. Rev. A 32, 2769 (1985)) and perturbation calculations (S. V. Khristenko and S. I. Vetchinkin, Opt. Specktrosk. 40, 417 (1976)), but significantly higher than calculated by the Reiss (Phys. Rev. A 22, 1786 (1980)) and Keldysh (Sov. Phys.---JETP 20, 1307 (1965)) methods using Volkov final states.
Non-burning thermonuclear fuel implosion experiments have been fielded on the National Ignition Facility to assess progress toward ignition by indirect drive inertial confinement fusion. These ...experiments use cryogenic fuel ice layers, consisting of mixtures of tritium and deuterium with large amounts of hydrogen to control the neutron yield and to allow fielding of an extensive suite of optical, x-ray and nuclear diagnostics. The thermonuclear fuel layer is contained in a spherical plastic capsule that is fielded in the center of a cylindrical gold hohlraum. Heating the hohlraum with 1.3 MJ of energy delivered by 192 laser beams produces a soft x-ray drive spectrum with a radiation temperature of 300 eV. The radiation field produces an ablation pressure of 100 Mbar which compresses the capsule to a spherical dense fuel shell that contains a hot plasma core 80 µm in diameter. The implosion core is observed with x-ray imaging diagnostics that provide size, shape, the absolute x-ray emission along with bangtime and hot plasma lifetime. Nuclear measurements provide the 14.1 MeV neutron yield from fusion of deuterium and tritium nuclei along with down-scattered neutrons at energies of 10-12 MeV due to energy loss by scattering in the dense fuel that surrounds the central hot-spot plasma. Neutron time-of-flight spectra allow the inference of the ion temperature while gamma-ray measurements provide the duration of nuclear activity. The fusion yield from deuterium-tritium reactions scales with ion temperature, which is in agreement with modeling over more than one order of magnitude to a neutron yield in excess of 1014 neutrons, indicating large confinement parameters on these first experiments.
Radiation transport in the trans-sonic regime, when the speed of a Marshak wave is approximately equal to the shock speed, is relevant to various problems involving radiating plasmas. For example, ...some inertial confinement fusion reactor concepts invoke the use of a gas to stop target produce x-rays and ions which creates a fireball in the gas that exhibits trans-sonic radiation transport. To calculate the trans-sonic radiation transport, hydrodynamics of the foam needs to be correctly modeled. The Z facility at Sandia National Laboratories is an excellent environment to do experiments that study radiation transport in SiO2 foams. In these experiments, samples of 25 - 38 mg/cm3 SiO2 foam are mounted on the sides of hohlraums that are filled with radiation from W wire-arrays. This radiation drives Marshak waves in the foams. Experimental results will be presented that show radiographic data of the shock motion and Si-diode measurements of the radiation that transits the foam. These results are compared with computer code simulations to demonstrate the codes' ability to model trans-sonic radiation transport.
We report on the first experiment dedicated to the study of nuclear reactions on dopants in a cryogenic capsule at the National Ignition Facility (NIF). This was accomplished using bromine doping in ...the inner layers of the CH ablator of a capsule identical to that used in the NIF shot N140520. The capsule was doped with 3\(\times\)10\(^{16}\) bromine atoms. The doped capsule shot, N170730, resulted in a DT yield that was 2.6 times lower than the undoped equivalent. The Radiochemical Analysis of Gaseous Samples (RAGS) system was used to collect and detect \(^{79}\)Kr atoms resulting from energetic deuteron and proton ion reactions on \(^{79}\)Br. RAGS was also used to detect \(^{13}\)N produced dominantly by knock-on deuteron reactions on the \(^{12}\)C in the ablator. High-energy reaction-in-flight neutrons were detected via the \(^{209}\)Bi(n,4n)\(^{206}\)Bi reaction, using bismuth activation foils located 50 cm outside of the target capsule. The robustness of the RAGS signals suggest that the use of nuclear reactions on dopants as diagnostics is quite feasible.
One-photon detachment and two-photon nonresonant excess photon detachment of electrons from the H{sup -} ion (outer-electron binding energy = 0.7542 eV) are observed with 1.165 eV laser pulses from a ...Nd:YAG laser (where YAG denotes yttrium aluminum garnet). A Penning ion source produces a pulsed 8 {mu}A, 35 keV H{sup -} beam that intersects a laser beam cylindrically focused down to a 17 {mu}m full width at half maximum waist in the ion beam direction, creating a high-intensity interaction region with peak intensities of up to 10{sup 11} W/cm{sup 2}. The interaction time is 7 ps. The detached electrons are detected by a time-of-flight apparatus enabling us to detect a very small two-photon signal in the presence of a very large signal from single photon detachments. By rotating the linear polarization angle, we study the angular distribution of the electrons for both one- and two-photon detachments. The spectra are modeled to determine the asymmetry parameters and one- and two-photon cross sections. We find {beta}{sub 2} to be 2.54+0.44/-0.60 and {beta}{sub 4} to be 2.29+0.07/-0.31, corresponding to a D state of 89+3/-12% of the S wave and D wave detachments for the two-photon results. The relative phase angle between the S and D amplitudes is measured to be less than 59 degree sign . The measured cross sections are found to be consistent with theoretical predictions. The one-photon photodetachment cross section is measured to be (3.6{+-}1.7)x10{sup -17} cm{sup 2}. The two-photon photodetachment generalized cross section is (1.3{+-}0.5)x10{sup -48} cm{sup 4} sec, consistent with theoretical calculations of the cross section. The three-photon generalized cross section is less than 4.4x10{sup -79} cm{sup 6} sec{sup 2}. (c) 1999 The American Physical Society.
In the field of inertial confinement fusion (ICF), work has been consistently progressing in the past decade toward a more fundamental understanding of the plasma conditions in ICF implosion cores. ...The research presented here represents a substantial evolution in the ability to diagnose plasma temperatures and densities, along with characteristics of mixing between fuel and shell materials. Mixing is a vital property to study and quantify, since it can significantly affect implosion quality. We employ a number of new spectroscopic techniques that allow us to probe these important quantities. The first technique developed is an emissivity analysis, which uses the emissivity ratio of the optically thin Lybeta and Hebeta lines to spectroscopically extract temperature profiles, followed by the solution of emissivity equations to infer density profiles. The second technique, an intensity analysis, models the radiation transport through the implosion core. The nature of the intensity analysis allows us to use an optically thick line, the Lyalpha, to extract information on mixing near the core edge. With this work, it is now possible to extract directly from experimental data not only detailed temperature and density maps of the core, but also spatial mixing profiles.
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