D. W. Phillion

Conversion efficiencies from laser‐produced plasmas in the extreme ultraviolet regime

The conversion efficiency of spectral emission from laser‐irradiated solid targets was investigated for short wavelength source development. The plasma brightness was quantified using absolutely calibrated detectors for 20 materials and spectra were obtained between 50 and 200 A. Laser parameters such as wavelength, pulse length, intensity, and spot size were systematically varied to establish a comprehensive database for source optimization. Qualitative differences in the underlying dominant emission features as a function of atomic number and laser wavelength were observed that accounted for the relatively high spectral conversion efficiencies produced. In the specific case of Sn, a conversion efficiency greater than 0.8%/eV has been observed in the technologically important region of λ=134.0 A using a laser intensity of 1–2×1011 W/cm2.

General methods for generating phase-shifting interferometry algorithms

Two completely independent systematic approaches for designing algorithms are presented. One approach uses recursion rules to generate a new algorithm from an old one, only with an insensitivity to more error sources. The other approach uses a least-squares method to optimize the noise performance of an algorithm while constraining it to a desired set of properties. These properties might include insensitivity to detector nonlinearities as high as a certain power, insensitivity to linearly varying laser power, and insensitivity to some order to the piezoelectric transducer voltage ramp with the wrong slope. A noise figure of merit that is valid for any algorithm is also derived. This is crucial for evaluating algorithms and is what is maximized in the least-squares method. This noise figure of merit is a certain average over the phase because in general the noise sensitivity depends on it. It is valid for both quantization noise and photon noise. The equations that must be satisfied for an algorithm to be insensitive to various error sources are derived. A multivariate Taylor-series expansion in the distortions is used, and the time-varying background and signal amplitudes are expanded in Taylor series in time. Many new algorithms and families of algorithms are derived.

The interaction of 1.06 μm laser radiation with high Z disk targets

Gold disks have been irradiated with 1.06 μm laser light at intensities between 7 × 1013 and 3 × 1015 W/cm2, and pulse lengths between 200 and 1000 psec. Due to the high Z and long pulse, inverse bremsstrahlung becomes an important absorption mechanism and competes strongly with resonance absorption and stimulated scattering. In addition to measured absorptions, data on the temporal, spatial, angular, and spectral characteristics of the x‐ray emission are presented. Temporally and spectrally resolved back‐reflected light, and polarization‐dependent sidescattered light are detected, providing estimates for the amount of stimulated scattering and of the coronal electron temperature. Inhibited electron thermal conduction and nonlocal thermodynamic equilibrium ionization physics play key roles in bringing numerical simulations of these experiments into agreement with all of the above‐mentioned data.

The Gemini Planet Imager

The next major frontier in the study of extrasolar planets is direct imaging detection of the planets themselves. With high-order adaptive optics, careful system design, and advanced coronagraphy, it is possible for an AO system on a 8-m class telescope to achieve contrast levels of 10-7 to 10-8, sufficient to detect warm self-luminous Jovian planets in the solar neighborhood. Such direct detection is sensitive to planets inaccessible to current radial-velocity surveys and allows spectral characterization of the planets, shedding light on planet formation and the structure of other solar systems. We have begun the construction of such a system for the Gemini Observatory. Dubbed the Gemini Planet Imager (GPI), this instrument should be deployed in 2010 on the Gemini South telescope. It combines a 2000-actuator MEMS-based AO system, an apodized-pupil Lyot coronagraph, a precision infrared interferometer for real-time wavefront calibration at the nanometer level, and a infrared integral field spectrograph for detection and characterization of the target planets. GPI will be able to achieve Strehl ratios > 0.9 at 1.65 microns and to observe a broad sample of science targets with I band magnitudes less than 8. In addition to planet detection, GPI will also be capable of polarimetric imaging of circumstellar dust disks, studies of evolved stars, and high-Strehl imaging spectroscopy of bright targets. We present here an overview of the GPI instrument design, an error budget highlighting key technological challenges, and models of the system performance.

25 ps neutron detector for measuring ICF‐target burn history

We have developed a fast, sensitive neutron detector for recording the fusion reaction‐rate history of inertial‐confinement fusion (ICF) experiments. The detector is based on the fast rise time of a commercial plastic scintillator (BC‐422) and has a response <25 ps FWHM. A thin piece of scintillator material acts as a neutron‐to‐light converter. A zoom lens images scintillator light to a high‐speed (15 ps) optical streak camera for recording. A retractable nose cone positions the scintillator between 1 and 50 cm from a target. A simultaneously recorded optical fiducial pulse allows the streak camera time base to be calibrated relative to the incident laser power. Burn histories have been measured for deuterium‐tritium filled targets with yields ranging between 108 and 2×1013 neutrons.

Stimulated Raman scattering in large plasmas

In long pulse, high‐energy experiments (4000 J, 2 nsec, 5×1014 W/cm2, 1.064 μm) at the Shiva laser facility, several percent of the laser light has been observed to be Raman scattered. The spectrum of the Raman‐scattered light was measured from 1.48 to 2.17 μm. The Raman scattering occurred principally at electron densities much lower than the quarter‐critical electron density. The high‐energy electrons expected in Raman scattering were observed indirectly in these experiments via their bremsstrahlung radiation. Additional experiments show that the Raman instability has a much lower intensity threshold for longer laser pusle length and larger laser spot size. Raman light measurements for 5320 A irradiated disk target experiments are also reported. The light near 2λ0 or 1.064 μm had both a red‐ and blue‐shifted component. At high intensities, Raman scattering also occurred in the very underdense plasma.

Radiation drive in laser‐heated hohlraums

Nearly 10 years of Nova [E. M. Campbell, Laser Part. Beams 9, 209 (1991)] experiments and analysis have lead to a relatively detailed quantitative and qualitative understanding of radiation drive in laser‐heated hohlraums. Our most successful quantitative modeling tool is two‐dimensional (2‐D) LASNEX numerical simulations [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Analysis of the simulations provides us with insight into the physics of hohlraum drive. In particular we find hohlraum radiation conversion efficiency becomes quite high with longer pulses as the accumulated, high‐Z blow‐off plasma begins to radiate. Extensive Nova experiments corroborate our quantitative and qualitative understanding.

Laser ionization and heating of gas targets for long‐scale‐length instability experiments

This paper examines the use of gas targets to create low‐ and mid‐Z plasmas ≊3 mm in size at 5% to 10% critical density for green and blue light (ne≊2×1020 to 1021 cm−3) with an electron temperature of several keV. At sufficiently high intensities (≊1014 W/cm2) the gas is ionized and heated by a laser absorption wave propagating faster than the sound speed. For pulses under 2 ns, the bulk of the plasma remains stationary, resulting in efficient heating minimizing density and velocity gradients, which are particularly important for instability thresholds in nonuniform plasmas. The propagation of a laser absorption wave in a preionized plasma is derived analytically. Ionization resulting from multiphoton and electron avalanche processes is studied by numerical methods and dimensional analysis. This establishes the length and time scales over which an absorption wave can be observed. Computer simulations, using the lasnex code, are presented for several implementations of this concept, applicable to the rele...

Drive characterization of indirect drive targets on the Nova laser (invited)

The indirect drive method of inertial confinement fusion uses a high‐Z radiation case to convert energy from high‐powered laser beams to x rays which implode fusion capsules. Experiments have been performed on the Nova laser to characterize the x‐ray production in high‐Z cavities for studying the efficiency for x‐ray production using two methods for characterization. One method measures the shock velocity produced in low‐Z materials by the radiation. The shock velocity is measured by observing the optical signal from the rear of a stepped or continuously varying thickness of Al placed over a hole in the cavity wall. The other method measures the reradiated x‐ray flux from the cavity wall viewing through a hole in the cavity. Both methods have been shown to provide a consistent characterization of the x‐ray drive in the cavity target.

Time‐resolved observations of stimulated Raman scattering from laser‐produced plasmas

Temporally and spectrally resolved measurements of stimulated Raman scattering from high‐intensity, 0.532 μm laser–plasma experiments on disk targets are reported. The observed scattering is principally from densities below quarter‐critical, and is observed to occur nearly simultaneously over a wide range of wavelengths (0.72 μm<λ<0.9 μm), in short (<50 psec) bursts. Several possible explanations of these data are briefly discussed.