R. E. Stewart

Laboratory measurement of opacity for stellar envelopes

Abstract We have measured the frequency dependent opacity of a low density iron plasma in Local Thermodynamic Equilibrium (LTE). The measured iron plasma conditions of 20 eV temperature and 10 −4 g/cc density, match those of stellar envelopes where iron dominates the radiative transport. Properties of the M-shell Δn = 0 transition arrays in iron are measured in this experiment, providing the first direct test of opacity models used in stellar pulsation and evolution calculations. We describe new methods to obtain LTE opacity data for plasmas at 100 times lower density than previous measurements. Experimental requirements include: high spectral resolution, large homogenous plasma sources, and Planckian radiation fields lasting tens of nanoseconds. These conditions were achieved using the 500 kJ SATURN facility at Sandia National Laboratory.

High-resolution x-ray spectrometer based on spherically bent crystals for investigations of femtosecond laser plasmas

Ultrashort-pulse, laser-produced plasmas have become very interesting laboratory sources to study spectroscopically due to their very high densities and temperatures, and the high laser-induced electromagnetic fields present. Typically, these plasmas are of very small volume and very low emissivity. Thus, studying these near point source plasmas requires advanced experimental techniques. We present a new spectrometer design called the focusing spectrometer with spatial resolution (FSSR-2D) based on a spherically bent crystal which provides simultaneous high spectral (λ/Δλ≈104) and spatial resolution (≈10 μm) as well as high luminosity (high collection efficiency). We described in detail the FSSR-2D case in which a small, near point source plasma is investigated. An estimate for the spectral and spatial resolution for the spectrometer is outlined based on geometric considerations. Using the FSSR-2D instrument, experimental data measured from both a 100 fs and a nanosecond pulse laser-produced plasma are pr...

Equation of state measurements of hydrogen isotopes on Nova

High intensity lasers can be used to perform measurements of materials at extremely high pressures if certain experimental issues can be overcome. We have addressed those issues and used the Nova laser to shock-compress liquid deuterium and obtain measurements of density and pressure on the principal Hugoniot at pressures from 300 kbar to more than 2 Mbar. The data are compared with a number of equation of state models. The data indicate that the effect of molecular dissociation of the deuterium into a monatomic phase may have a significant impact on the equation of state near 1 Mbar.

Ultrafast x‐ray streak camera for use in ultrashort laser‐produced plasma research

In recent years there has been growing interest in energetic (≳100 eV), temporally short (<10 ps) x rays produced by ultrashort laser‐produced plasmas. The detection and temporal dispersion of the x rays using x‐ray streak cameras has been limited to a resolution of 2 ps, primarily due to the transit time dispersion of the electrons between the photocathode and the acceleration grid. The transit time spread of the electrons traveling from the photocathode to the acceleration grid is inversely proportional to the accelerating field. By increasing the field by a factor of 7, we have minimized the effects of transit time dispersion in the photocathode/accelerating grid region and produce an x‐ray streak camera with subpicosecond temporal resolution (≊900 fs). The streak camera has been calibrated using a Michelson interferometer and 100 fs, 400 nm laser light. The characteristics of the streak camera, along with the most recent x‐ray streak data will be presented.

Experiment demonstration of a 100‐ps microchannel plate framing camera

We report on the construction and testing of a 100‐ps framing camera based on a gated microchannel plate. The gating is obtained by applying a fast voltage pulse directly across the microchannel plate detector. An infinite ground‐plane microstrip transmission‐line configuration is used. We have measured the transit time for electrons through the microchannel plate to be 150±35 ps and the rise and fall times of the detector to be less than 100 ps. The time resolution computed using the experimentally measured edge response times is less than 100 ps.

Interferometric investigation of femtosecond laser-heated expanded states

Simultaneous temporally and spatially resolved measurements of the phase change and reflectivity of S- and P-polarized femtosecond laser probes are obtained from hot expanded states produced by femtosecond laser heating of a solid aluminum target. The combined set of data provides an integral test of equation-of-state models in a regime up to 10 Mbar and densities of 0.01–1 times solid. The results suggest that target stoichiometry at the few A level should be considered in the analysis of phase and reflectivity measurements in such experiments.

Time and space resolved vacuum‐ultraviolet spectroscopy of an argon gas‐puff Z pinch

Time and radially resolved vacuum‐ultraviolet spectra from an argon gas‐puff Z pinch have been obtained using a grazing incidence spectrometer with gated microchannel plates curved to the Rowland circle. Most of the 50–300‐A radiation is emitted from a dense core (r≊0.4 mm, ne≳1019 cm−3, Te≊150 eV) which forms when the plasma assembles on the axis.

X-ray detection by direct modulation of an optical probe beam—Radsensor: Progress on development for imaging applications

We present a progress report on our new x-ray detection technique based on optical measurement of the effects of x-ray absorption and electron hole pair creation in a direct band-gap semiconductor. The electron–hole pairs create a frequency dependent shift in optical refractive index and absorption. This is sensed by simultaneously directing an optical probe beam through the same volume of semiconducting medium that has experienced an x-ray induced modulation in the electron–hole population. If the wavelength of the optical probe beam is close to the semiconductor band-edge, the optical probe will be modulated significantly in phase and amplitude. We have analyzed the physics of the imaging radsensor, developed modeling tools for device design, and are cautiously optimistic that we will achieve single x-ray photon sensitivity, and picosecond response. These predictions will be tested with Cu Kα xrays at the LLNL USP facility this spring and summer, with a cavity-based radsensor detector suitable for use i...

Spectroscopic studies of an argon plasma produced in a relativistic electron beam gas puff Z pinch

The spectra of argon ions emitted from a plasma created by discharging a 3‐TW relativistic electron beam generator through an argon gas column are reported. New lines have been classified for 2s 2pn–2pn+1 transition arrays in Ar XI–XV and energy levels have been derived for lithiumlike argon XVI. The effects of mass motion Doppler shifts on the spectra have been measured and systematic shifts of transition energies with charge state are observed in the data.

RadSensor: Xray Detection by Direct Modulation of an Optical Probe Beam

We present a new x-ray detection technique based on optical measurement of the effects of x-ray absorption and electron hole pair creation in a direct band-gap semiconductor. The electron-hole pairs create a frequency dependent shift in optical refractive index and absorption. This is sensed by simultaneously directing an optical carrier beam through the same volume of semiconducting medium that has experienced an xray induced modulation in the electron-hole population. If the operating wavelength of the optical carrier beam is chosen to be close to the semiconductor band-edge, the optical carrier will be modulated significantly in phase and amplitude. This approach should be simultaneously capable of very high sensitivity and excellent temporal response, even in the difficult high-energy xray regime. At xray photon energies near 10 keV and higher, we believe that sub-picosecond temporal responses are possible with near single xray photon sensitivity. The approach also allows for the convenient and EMI robust transport of high-bandwidth information via fiber optics. Furthermore, the technology can be scaled to imaging applications. The basic physics of the detector, implementation considerations, and preliminary experimental data are presented and discussed.