David J. Erskine

Shock‐induced martensitic transformation of highly oriented graphite to diamond

Shock‐wave profiles of highly ordered pyrolytic graphite shocked normal to the basal plane of the graphite crystal structure have been measured. For graphite with sufficient orientational order a martensitic transformation to a diamond‐like phase is observed with a transition onset pressure 19.6±0.7 GPa, the stability limit of the graphite structure under shock compression. The minimum overpressure required for the transformation is not more than 6 GPa and the two‐wave structure of the transition is overdriven to a single wave above 40 GPa.

An Externally Dispersed Interferometer for Sensitive Doppler Extrasolar Planet Searches

A new kind of instrument for sensitive Doppler extrasolar planet searches, called an externally dispersed interferometer, is described in this paper. It is a combination of an optical Michelson-type interferometer and an intermediate-resolution grating spectrometer. The interferometer measures Doppler radial velocity (RV) variations of starlight through the phase shifts of moire fringes, created by multiplication of the interferometer fringes with stellar absorption lines. The intermediate-resolution spectrograph disperses the moire fringes into thousands of parallel-wavelength channels. This increases the instrument bandwidth and fringe visibility by preventing fringe cross-talk between neighboring spectral lines. This results in a net increase in the signal-to-noise ratio over an interferometer used alone with broadband light.Compared to current echelle spectrometers for extrasolar planet searches, this instrument offers two unique instrument properties: a simple, stable, well-defined sinusoidal instrument response function (point-spread function) and magnification of Doppler motion through moire fringe techniques. Since instrument noise is chiefly limited by the ability to characterize the instrument response, this new technique provides unprecedented low instrumental noise in an economical compact apparatus, enabling higher precision for Doppler RV measurements. In practice, the moire magnification can be 5-10 times depending on the interferometer comb angle. This instrument has better sensitivity for smaller Doppler shifts than echelle spectrometers. The instrument can be designed with much lower spectral resolving power without losing Doppler sensitivity and optimized for higher throughput than echelle spectrometers to allow a potential survey for planets around fainter stars than current magnitude limits.Lab-based experiments with a prototype instrument with a spectral resolution of R ≈ 20,000 demonstrated ~0.7 m s–1 precision for short-term RV measurements. A fiber-fed version of the prototype with R ≈ 5600 was tested with starlight at the Lick 1 m telescope and demonstrated ~7 m s–1 RV precision at 340 A bandwidth. The increased velocity noise is attributed to the lower spectral resolution, lower fringe visibility, and uncontrolled instrument environment.

An Externally Dispersed Interferometer Prototype for Sensitive Radial Velocimetry: Theory and Demonstration on Sunlight

A theory of operation of a wideband interferometric Doppler spectroscopy technique, called externally dispersed interferometry (EDI), is presented. The first EDI prototype was tested on sunlight and detected the 12 m s 1 amplitude lunar signature in Earths motion. The hybrid instrument is an undispersed Michelson interferometer having a fixed delay of about 1 cm, in series with an external spectrograph of about 20,000 resolution. The Michelson provides the Doppler shift discrimination, while the external spectrograph boosts net white-light fringe visibility by reducing cross talk from adjacent continuum channels. A moireeffect between the sinusoidal interferometer transmission and the input spectrum heterodynes high spectral details to broad moire ´ patterns, which carry the Doppler information in its phase. These broad patterns survive the blurring of the spectrograph, which can have several times lower resolution than grating-only spectrographs typically used now for the Doppler planet search. This enables the net instrument to be dramatically smaller in size (∼1 m) and cost. The EDI behavior is compared and contrasted to the conventional grating-only technique.

High-Resolution Broadband Spectroscopy Using an Externally Dispersed Interferometer

An externally dispersed interferometer (EDI) is a series combination of a fixed delay interferometer and an external grating spectrograph. We describe how the EDI can boost the effective resolving power of an echelle or linear grating spectrograph by a factor of 2-3 or more over the spectrographs full bandwidth. The interferometer produces spectral fringes over the entire spectrographs bandwidth. The fringes heterodyne with spectral features to provide a low spatial frequency moire pattern. The heterodyning is numerically reversed to recover highly detailed spectral information unattainable by the spectrograph alone. We demonstrate resolution boosting for stellar and solar measurements of two-dimensional echelle and linear grating spectra. An effective spectral resolution of ~100,000 has been obtained from the ~50,000 resolution Lick Observatory two-dimensional echelle spectrograph, and that of ~50,000 from an ~20,000 resolution linear grating spectrograph.

Equation of state of shock-compressed liquids: Carbon dioxide and air

Equation‐of‐state data were measured for liquid carbon dioxide and air shock‐compressed to pressures in the range 28–71 GPa (280–710 kbar) using a two‐stage light‐gas gun. The experimental methods are described. The data indicate that shock‐compressed liquid CO2 decomposes at pressures above 34 GPa. Liquid air dissociates above a comparable shock pressure, as does liquid nitrogen. Theoretical intermolecular potentials are derived for CO2 from the data. The calculated shock temperature for the onset of CO2 decomposition is 4500 K at a volume of 17 cm3/mol.

Technique for high‐pressure electrical conductivity measurement in diamond anvil cells at cryogenic temperatures

A technique is described for making four‐probe electrical conductivity measurements on bulk samples in a diamond anvil cell. The technique has been successfully applied up to 48 GPa and at temperatures below 4.2 K to measure the superconducting transition temperatures (Tc, of Pb, GaP, and Si. A method for analyzing the resistance versus temperature curve in the vicinity of the superconducting transition is also described. This method is applied to determine the pressure dependence of Tc in Si in the region where Tc varies rapidly with pressure.

A high-resolution two-dimensional imaging velocimeter

Velocity interferometers are typically used to measure velocities of surfaces at a single point or along an imaged line as a function of time. We describe an optical arrangement that enables high-resolution measurements of the two-dimensional velocity field across a shock front or shocked interface. The technique is employed to measure microscopic fluctuations in shock fronts that have passed through materials being considered as ablators for indirect-drive inertial confinement fusion. With picosecond time resolution the instrument captures velocity modes with wavelengths as short as 2.5 microm at a resolution of approximately 10 m/s rms on velocity fields averaging many km/s over an 800 microm field of view.

Interferometric resolution boosting for spectrographs

Externally dispersed interferometry (EDI) is a technique for enhancing the performance of spectrographs for wide bandwidth high resolution spectroscopy and Doppler radial velocimetry. By placing a small angle-independent interferometer near the slit of a spectrograph, periodic fiducials are embedded on the recorded spectrum. The multiplication of the stellar spectrum times the sinusoidal fiducial net creates a moire pattern, which manifests high detailed spectral information heterodyned down to detectably low spatial frequencies. The latter can more accurately survive the blurring, distortions and CCD Nyquist limitations of the spectrograph. Hence lower resolution spectrographs can be used to perform high resolution spectroscopy and radial velocimetry. Previous demonstrations of ~2.5x resolution boost used an interferometer having a single fixed delay. We report new data indicating ~6x Gaussian resolution boost (140,000 from a spectrograph with 25,000 native resolving power), taken by using multiple exposures at widely different interferometer delays.

METALLIC PROPERTIES OF ORTHORHOMBIC HIGH PRESSURE PHASE OF GaAs: THEORY & EXPERIMENT

Contrary to current beliefs that GaAs II is a semiconductor or semimetal, we find that the two high pressure orthorhombic phases of GaAs (GaAs II and GaAs III) are metallic and superconducting with Tc close to 4.5 K. Results from the experimental measurements and first principles pseudopotential calculations are found to be consistent.

Heterogeneous flow and brittle failure in shock-compressed silicon

We combine a recently developed high-resolution two-dimensional (2D) imaging velocimetry technique (velocity interferometer system for any reflector (VISAR)) with 1D VISAR measurements to construct a moving picture of heterogeneous deformation in shock-compressed single crystal silicon. The 2D VISAR takes an intensity snapshot of target velocity and reflectivity over a mm field-of-view while the compression history is simultaneously recorded by the 1D VISAR. Our data show particle velocity surface roughening due to the anisotropic onset of plasticity and, above ∼13 GPa, a structural phase transformation. Shock arrival at the Si free-surface is characterized by the formation of fracture networks and incipient velocity jetting.