Quinn McCulloch

Fiber Bragg grating sensing of detonation and shock experiments at Los Alamos National Laboratory

An all optical-fiber-based approach to measuring high explosive detonation front position and velocity is described. By measuring total light return using an incoherent light source reflected from a fiber Bragg grating sensor in contact with the explosive, dynamic mapping of the detonation front position and velocity versus time is obtained. We demonstrate two calibration procedures and provide several examples of detonation front measurements: PBX 9502 cylindrical rate stick, radial detonation front in PBX 9501, and PBX 9501 detonation along a curved meridian line. In the cylindrical rate stick measurement, excellent agreement with complementary diagnostics (electrical pins and streak camera imaging) is achieved, demonstrating accuracy in the detonation front velocity to below the 0.3% level when compared to the results from the pin data. In a similar approach, we use embedded fiber grating sensors for dynamic pressure measurements to test the feasibility of these sensors for high pressure shock wave research in gas gun driven flyer plate impact experiments. By applying well-controlled steady shock wave pressure profiles to soft materials such as PMMA, we study the dynamic pressure response of embedded fiber Bragg gratings to extract pressure amplitude of the shock wave. Comparison of the fiber sensor results is then made with traditional methods (velocimetry and electro-magnetic particle velocity gauges) to gauge the accuracy of the approach.

Atlas Line-Imaging ORVIS Diagnostic

Many pulsed-power facilities used for high energy density experiments require diagnostics that can measure the velocity histories of shocked materials. The Atlas pulsed-power Z-pinch machine (located at the Nevada Test Site) is a 23-megajoule capacitor bank capable of delivering 28 mega-amperes in an approximately 5 microsecond rise time pulse into a cylindrical imploding liner. Experimental data are needed for the hydro-modeling of dynamic friction. For this set of experiments, shocks in two adjacent materials will produce differential shear velocities. An optically recording velocity interferometer system (ORVIS) has been designed to measure the differential velocity from the inner surface of a load after shock breakout. The moving target surface located inside an imploding load is illuminated with an f/10 laser pulse at 532 nm, focused down to a 12-mm long line. An optical relay collects light from the middle 8-mm of this line at f/15. Relay lenses pass collimated light through a two-arm interferometer, in the same fashion as VISAR (velocity interferometer system for any reflector). Different thicknesses of e acutetalons in one arm allow recording of different velocity ranges. After the interferometer, a dove prism rotates the line image into the slit of the recording streak camera. Alignment techniques are discussed and test calibration data from laser-driven mini-flyer plates are presented.

Studying Shocked Material Dynamics with Ultrafast X-rays

The response of micron-scale inhomogeneities dictates the overall dynamic, structural and chemical response of many materials. Of particular interest is the response of micron scale voids. It is believed that such micron scale voids are responsible for the nucleation of damage leading to structural failure in metals and to initiation of detonation in explosive material under high strain-rates. A critical step towards developing safer, stronger, and longer lasting materials in a range of applications from energy to defense requires understanding the dynamic response of these inhomogeneties on the micron-scale.

Nondestructive calibration of Chirped Fiber Bragg Grating sensors using a fiber-based ultrafast laser

Chirped Fiber Bragg Gratings (CFBG) sensors are powerful high-speed strain detectors for various applications like traffic monitoring or detonation measurement. Here we present a novel, non-destructive calibration for CFBGs using a femtosecond fiber laser.

Ultra-thin and strong formvar-based membranes with controlled porosity for micro- and nano-scale systems

We present a methodology for developing ultra-thin and strong formvar-based membranes with controlled morphologies. Formvar is a thin hydrophilic and oleophilic polymer inert to most chemicals and resistant to radiation. The formvar-based membranes are viable materials as support structures in micro- and macro-scale systems depending on thinness and porosity control. Tunable sub-micron thick porous membranes with 20%-65% porosity were synthesized by controlling the ratios of formvar, glycerol, and chloroform. This synthesis process does not require complex separation or handling methods and allows for the production of strong, thin, and porous formvar-based membranes. An expansive array of these membrane characterizations including chemical compatibility, mechanical responses, wettability, as well as the mathematical simulations as a function of porosity has been presented. The wide range of chemical compatibility allows for membrane applications in various environments, where other polymers would not be suitable. Our formvar-based membranes were found to have an elastic modulus of 7.8 GPa, a surface free energy of 50 mN m-1 and an average thickness of 125 nm. Stochastic model simulations indicate that formvar with the porosity of ∼50% is the optimal membrane formulation, allowing the most material transfer across the membrane while also withstanding the highest simulated pressure loadings before tearing. Development of novel, resilient and versatile membranes with controlled porosity offers a wide range of exciting applications in the fields of nanoscience, microfluidics, and MEMS.

Optically induced lattice dynamics probed with ultrafast X-ray diffraction

We have studied the picosecond-dynamics of optically pumped hexagonal LuMnO/sub 3/ using ultrafast X-ray diffraction. Experimental data are compared with a simulation based on dynamical diffraction theory modified to account for the hexagonal structure of LuMnO/sub 3/.

A Series of Joint VNIIEF/LANL Explosive Magnetic Experiments RHSR-0,1,2 on Radiographic Study of Perturbations Growth at a Copper Liner Boundary with Polyethylene or Water

In the three joint VNIIEFILANL experiments RHSR-0, 1,2 with a disk EMG and a three-layer liner system (Al-dielectric-eu) a method of radial radiography was used to study growth of perturbations amplitude at the boundary between copper layer and polyethylene (in experiments RHSR-0, 1) and water (in experiment RHSR-2); the growth occurs during the evolution of the Raleigh-Taylor instability of this boundary.

An Ultrafast X‐Ray Diffraction Apparatus for the Study of Shock Waves

The use of X‐ray diffraction for the study of shock physics has been pursued for decades. Conceptually, changes in the diffraction line, including broadening and shifts, provide details about the nature of compression, plasticity, phase, and kinetics of the phase transition for the material being shock‐loaded. In practice, X‐ray source brightness, sample preparation, and turn‐around times have limited the applicability to a few crystalline systems. We report our development of an ultrafast X‐ray diffraction instrument suitable for studying rapid phase changes, including both solid‐solid and solid‐melt, in shock‐loaded materials. Due to the relatively small sample sizes needed and to the ability to conduct thousands of shock physics experiments with these small samples, we can build up the statistics required to study elastic‐plastic transitions, the kinetics of phase changes, as well as the mechanistic details of phase changes in nearly all materials, including high‐Z samples. An overview of the technique...

Singlet and triplet states of charged excitons in ZnSe-based QWs probed by high magnetic fields

Singlet and triplet states of negatively (X-) and positively (X+) charged excitons (trions) in ZnSe-based quantum wells have been studied by means of photoluminescence in pulsed magnetic fields to 50 Tesla. Singlet state binding energies of X- show a monotonic increase with growing magnetic fields with a tendency to saturation. Contrary to that a decrease of X+ binding energy is found. A crossover of the triplet and singlet states is observed in magnetic fields 35 - 50 T.