Radiation, optical, power flow, and electrical diagnostics at the Z facility: Layout and techniques utilized to operate in the harsh environment.

The Z machine is a current driver producing up to 30 MA in 100 ns that utilizes a wide range of diagnostics to assess accelerator performance and target behavior conduct experiments that use the Z target as a source of radiation or high pressures. We review the existing suite of diagnostic systems, including their locations and primary configurations. The diagnostics are grouped in the following categories: pulsed power diagnostics, x-ray power and energy, x-ray spectroscopy, x-ray imaging (including backlighting, power flow, and velocimetry), and nuclear detectors (including neutron activation). We will also briefly summarize the primary imaging detectors we use at Z: image plates, x-ray and visible film, microchannel plates, and the ultrafast x-ray imager. The Z shot produces a harsh environment that interferes with diagnostic operation and data retrieval. We term these detrimental processes "threats" of which only partial quantifications and precise sources are known. We summarize the threats and describe techniques utilized in many of the systems to reduce noise and backgrounds.

Converting existing optical detectors into fast x-ray detectors.

The very short burn time and small size of burning plasmas created at advanced laser-fusion facilities will require high-spatial-resolution imaging diagnostics with fast time resolution. These instruments will need to function in an environment of extremely large neutron fluxes that will cause conventional diagnostics to fail because of radiation damage and induced background levels. One solution to this challenge is to perform an ultrafast conversion of the x-ray signals into the optical regime before the neutrons are able to reach the detector and then to relay image the signal out of the chamber and into a shielded bunker, protected from the effects of these neutrons. With this goal in mind, the OMEGA laser was used to demonstrate high-temporal-resolution x-ray imaging by using an x-ray snout to image an imploding backlighter capsule onto a semiconductor. The semiconductor was simultaneously probed with the existing velocity interferometry system for any surface reflector (VISAR) diagnostic, which uses an optical streak camera and provided a one-dimensional image of the phase in the semiconductor as a function of time. The phase induced in the semiconductor was linearly proportional to the x-ray emission from the backlighter capsule. This approach would then allow a sacrificial semiconductor to be attached at the end of an optical train with the VISAR and optical streak camera placed in a shielded bunker to operate in a high neutron environment and obtain time-dependent one-dimensional x-ray images or time-dependent x-ray spectra from a burning plasma.

Extreme measurements with Photonic Doppler Velocimetry (PDV).

Photonic Doppler Velocimetry (PDV) is a fiber-based diagnostic for the extreme conditions created by high-speed impact, explosive detonation, electrical pulsed power, and intense laser ablation. PDV is a conceptually simple application of the optical Doppler effect, but measurements above 1 km/s only became practical at the beginning of the twenty-first century. This review discusses the evolution of PDV, its operational details, practical analysis, and outstanding challenges.

Time-stretched photonic Doppler velocimetry.

Inertial confinement fusion facilities generate implosions at speeds greater than 100 km/s, and measuring the material velocities is important and challenging. We have developed a new velocimetry technique that uses time-stretched spectral interferometry to increase the measurable velocity range normally limited by the detector bandwidth. In this approach, the signal is encoded on a chirped laser pulse that is stretched in time to reduce the beat frequency before detection. We demonstrate the technique on an imploding liner experiment at the Sandia National Laboratories' Z machine, where beat frequencies in excess of 50 GHz were measured with 20 GHz bandwidth detection.

Compression Freezing Kinetics of Water to Ice VII.

Time-resolved x-ray diffraction (XRD) of compressed liquid water shows transformation to ice VII in 6 nsec, revealing crystallization rather than amorphous solidification during compression freezing. Application of classical nucleation theory indicates heterogeneous nucleation and one-dimensional (e.g., needlelike) growth. These first XRD data demonstrate rapid growth kinetics of ice VII with implications for fundamental physics of diffusion-mediated crystallization and thermodynamic modeling of collision or impact events on ice-rich planetary bodies.

Note: Heated flyer-plate impact system.

A technique for launching heated flyer plates was developed on a single-stage gas gun. This type of impact creates a well-posed mechanical state and a tunable thermal state, which is useful for calibrating dynamic temperature measurements. Proof-of-principle thermoreflectance measurements were performed using this technique. Since the target remains at room temperature until the moment of impact, heated flyers avoid differential expansion and annealing issues, allowing novel impact experiments to be performed.

Note: visible reflectivity system for high-pressure studies.

A near-normal visible reflectivity system was constructed for investigating the optical properties of materials at high pressure and temperatures. The system consists of focusing optics, spectrometers, and imaging cameras for use with a resistively-heated membrane-controlled diamond anvil cell (DAC). The DAC allows optical access on two sides: one dedicated to pressure measurement via fluorescence, the other side dedicated to absolute reflectance measurements. With this configuration, pressure and temperature may be controlled and measured without disturbing the sample position, permitting quantitative reflectivity measurements as a function of thermodynamic state variables. The system was tested on gold at high pressure conditions, and compared to data obtained on a commercial ellipsometer system at 1 bar.

Tracking an imploding cylinder with photonic Doppler velocimetry.

Cylindrical implosion offers a path to extreme material states, reaching considerably higher pressures than planar geometry. However, diagnosing compressed material in cylindrical geometry is challenging. Time-resolved velocimetry, a standard technique in planar compression, is difficult to incorporate into cylindrical experiments. This paper describes the use of photonic Doppler velocimetry (PDV) in magnetically driven cylindrical compression experiments at the Sandia Z machine. With this diagnostic, it is possible to track the interior of an imploding cylinder beyond 20 km/s. A "leapfrog" implementation is described to support velocities well above the bandwidth limits of standard PDV measurements.

Note: Frequency-conversion photonic Doppler velocimetry with an inverted circulator.

Photonic Doppler velocimetry (PDV) is a fiber-based interferometer used in dynamic compression research. Conventional PDV systems are simple to construct but do not perform well in all measurement conditions, while universal PDV systems that support many different configurations are complex and expensive. A simpler approach is the use of external, inverted circulators which can be added and removed in a modular fashion. This technique permits frequency-conversion measurements with a conventional PDV system. Using a correction to remove baseline effects, frequency conversion systems can resolve low velocity transients that conventional PDV cannot.

Effect of window reflections on photonic Doppler velocimetry measurements.

Photonic Doppler velocimetry (PDV) has rapidly become a standard diagnostic for measuring velocities in dynamic compression research. While free surface velocity measurements are fairly straightforward, complications occur when PDV is used to measure a dynamically loaded sample through a window. Fresnel reflections can severely affect the velocity and time resolution of PDV measurements, especially for low-velocity transients. Shock experiments of quartz compressed between two sapphire plates demonstrate how optical window reflections cause ringing in the extracted PDV velocity profile. Velocity ringing is significantly reduced by using either a wedge window or an antireflective coating.

Accuracy and precision in photonic Doppler velocimetry.

While photonic Doppler velocimetry (PDV) is becoming a common diagnostic in dynamic compression research, its limiting accuracy and precision are not well understood. Velocity resolution is known to be inversely proportional to the time resolution, but resolution estimates differ by one to two orders of magnitude. Furthermore, resolution varies with the number of recorded signals and how these signals are analyzed. Numerical simulations reveal factors that affect accuracy and precision in PDV, and the results may be extended to a broad class of measurements. After systematic effects are eliminated, the limiting velocity uncertainty in a PDV measurement is governed by the sampling rate, the signal noise fraction, and the analysis time duration.

Shock-wave exploration of the high-pressure phases of carbon.

The high-energy density behavior of carbon, particularly in the vicinity of the melt boundary, is of broad scientific interest and of particular interest to those studying planetary astrophysics and inertial confinement fusion. Previous experimental data in the several hundred gigapascal pressure range, particularly near the melt boundary, have only been able to provide data with accuracy capable of qualitative comparison with theory. Here we present shock-wave experiments on carbon (using a magnetically driven flyer-plate technique with an order of magnitude improvement in accuracy) that enable quantitative comparison with theory. This work provides evidence for the existence of a diamond-bc8-liquid triple point on the melt boundary.

Push-pull analysis of photonic Doppler velocimetry measurements.

A robust analysis method is presented for multiple-phase heterodyne velocimetry measurements. By combining information from three phase-shifted signals, it is possible to eliminate coherent intensity variations and incoherent light from the measurement. The three data signals are reduced to a pair of quadrature signals, allowing unambiguous calculation of target displacement. The analysis relies on a minimum number of adjustable parameters, and these parameters can be precisely determined from simple interferometer characterization.

Nanosecond freezing of water under multiple shock wave compression: continuum modeling and wave profile measurements.

Using real time optical transmission and imaging measurements in multiple shock wave compression experiments, water was shown to solidify on nanosecond time scales [D. H. Dolan and Y. M. Gupta, J. Chem. Phys. 121, 9050 (2004)]. Continuum modeling and wave profile measurements, presented here, provide a complementary approach to examine the freezing of shocked water. The water model consisted of thermodynamically consistent descriptions of liquid and solid (ice VII) water, relationships for phase coexistence, and a time-dependent transition description to simulate freezing dynamics. Continuum calculations using the water model demonstrate that, unlike single shock compression, multiple shock compression results in pressure-temperature conditions where the ice VIII phase is thermodynamically favored over the liquid phase. Wave profile measurements, using laser interferometry, were obtained with quartz and sapphire windows at a peak pressure of 5 GPa. For water confined between sapphire windows, numerical simulations corresponding to a purely liquid response are in excellent agreement with the measured wave profile. For water confined between quartz windows (to provide a nucleating surface), wave profile measurements demonstrate a pure liquid response for an incubation time of approximately 100 ns followed by a time-dependent transformation. Analysis of the wave profiles after the onset of transformation suggests that water changes from a metastable liquid to a denser phase, consistent with the formation of a high-pressure ice phase. Continuum analyses and simulations underscore the need for multiple time scales to model the freezing transition. Findings from the present continuum work are extremely consistent with optical results reported previously. These studies constitute the first comprehensive investigation reported for freezing of a liquid at very short time scales.

Nanosecond freezing of water under multiple shock wave compression: optical transmission and imaging measurements.

Water samples were subjected to multiple shock wave compressions, generating peak pressures of 1-5 GPa on nanosecond time scales. This loading process approximates isentropic compression and leads to temperatures where the ice VII phase is more stable than the liquid phase above 2 GPa. Time resolved optical transmission and imaging measurements were performed to determine the solidification rate under such conditions. Freezing occurred faster at higher pressures as water was compressed further into the ice VII phase, in agreement with classical micleation theory. Water consistently froze when in contact with a silica window, whereas no solidification occurred in the presence of sapphire windows. The transition was determined to be a surface initiated process--freezing began via heterogeneous nucleation at the water/window interface and propagated over thicknesses greater than 0.01 mm. The first optical images of freezing on nanosecond time scales were obtained. These images demonstrate heterogeneous nucleation and irregular solid growth over 0.01-0.10 mm lateral length scales and are consistent with latent heat emission during the transformation. The combination of optical transmission and imaging measurements presented here provide the first consistent evidence for freezing on short time scales.

Cold- and ethanol-induced hypothermia reduces cellular levels of mRNA-encoding Thyrotropin-Releasing Hormone (TRH) in neurons of the preoptic area.

Thyrotropin-releasing hormone (TRH)-containing neurons have been implicated in the central control of body temperature. TRH-containing neurons are located in brain areas known to influence body temperature, and TRH injected into these areas can produce changes in body temperature. While these lines of evidence support the view that central TRH is involved in thermoregulation, it has been difficult to confirm that TRH-containing neurons of the preoptic area are involved in this process. We used a different approach to test this hypothesis, based on recent evidence that changes in cellular levels of neuropeptide mRNA are linked to changes in neurosecretory processes. Hence, we predicted that if TRH neurons of the preoptic area are involved in body temperature regulation, cellular levels of TRH mRNA would be altered in animals in which body temperature had been experimentally altered. TRH mRNA levels were measured by in situ hybridization histochemistry in neurons of the preoptic area (POA) of animals that had been exposed to cold (5 degrees C) or that had been given a hypothermic dose of ethanol. Cellular levels of TRH mRNA were reduced by both treatments. However, cellular levels of the mRNA-encoding gastrin-releasing peptide were not affected by these treatments in neurons of the POA, indicating that hypothermia exerted selective effects on TRH neurons in this brain region. Considering that both cold exposure and ethanol administration increase blood pressure, that the POA contains neurons which are both thermosensitive and barosensitive, and that TRH has been implicated in the control of blood pressure, we manipulated arterial blood pressure pharmacologically without changing body temperature to determine whether TRH neurons were also responsive to cardiovascular changes. Infusions with either nitroprusside, a vasodilator, or phenylephrine, a vasoconstrictor, produced significant changes in arterial blood pressure and heart rate, but did not affect TRH mRNA in the POA. These findings demonstrate that TRH neurons of the POA are thermoresponsive, supporting the view that they play a role in the central control of body temperature.