Session J1: Experimental Developments V: Novel Techniques

Quantifying the Hydrodynamic Performance of an Explosively-Driven Two-Shock Source

An explosively-driven experimental package capable of generating a tunable two-shock drive would enable a host of experiments in shock physics. To make the best use of such a platform, though, its symmetry, reproducibility, and performance must be characterized thoroughly. We report on a series of experiments on a particular two-shock design that used shock reverberation between the sample and a heavy anvil to produce a second shock. Drive package diameters were varied between 50 and 76 mm in order to investigate release wave propagation. We used proton radiography to characterize the detonation and reverberation fronts within the high explosive elements of the packages, as well as surface velocimetry to measure the resulting shock structure in the sample under study. By fielding more than twenty channels of velocimetry per shot, we were able to quantify the symmetry and reproducibility of the drive.   

Development of converging-shock experiments for higher pressures, higher-Z samples, and off-Hugoniot states

We are extending our converging-shock platforms at the National Ignition Facility and the Omega laser to reach pressures in the PPa range, to study samples that are opaque to x-ray backlighters available for streak radiography, and to determine states along isentropes produced by compression and release behind the initial shock. Given a radiographic marker layer outside the region of interest in a 1D converging shock, the displacement field inside can be deduced. The continuum dynamics equations then give the sound speed, pressure, and energy. A planar shock of high pressure can be generated by Mach reflection of a converging shock in conical geometry; this is one avenue for measuring shock states when radiography is not practicable. Mach reflection from a rigid boundary was calculated for arbitrary equations of state, enabling efficient studies above the peak compression for a single shock. Our existing converging-shock platform can be extended to ~PPa pressures in several ways. The hohlraum temperature can be increased and the drive history optimized for each sample.   

Evidence of multi-petapascal pressures in converging shock compression of deutero-polyethene at the National Ignition Facility

A converging shock was induced in a sphere of deuterated polyethene using a hohlraum x-ray drive at the National Ignition Facility. A CH ablator was deposited over the sample, including a Ge-doped radiographic marker layer near its inner edge. Density and opacity profiles were deduced from streaked x-ray radiography, giving a measurement of the shock Hugoniot from ~10-80 TPa. As the shock reached the center of the sample, intense x-rays and neutrons were produced, detected with x-ray cameras and neutron scintillators respectively. Penumbral imaging of the x-ray flash showed that shock convergence was spherical to ~20 percent or better. The neutron time-of-flight record showed a well-resolved D-D peak, and also a lower D-T peak from tritons produced in the D-D reactions. The x-ray and neutron signals were in very encouraging agreement with radiation hydrodynamics simulations. Analysis of the peak shapes and comparison with the simulations indicates that reaction-averaged temperatures in the hotspot were in the kilovolt range with pressures of several petapascals (tens of gigabars). The hotspot can provide valuable insight on the limits of shock compression before transport perturbs the state ahead.   

A novel method to create molecular mixtures at high pressures

We have successfully created a segregated mixture of hydrogen and oxygen at high pressure in a diamond anvil cell (DAC) using \textit{useful hard x-ray photochemistry}. A keyhole (two holes connected by an opening) sample chamber was created in a metallic gasket to support two segregated powders of ammonia borane and potassium perchlorate in each hole, respectively at $\sim$ 5.0 GPa. Both holes were separately irradiated with synchrotron hard x-rays to release molecular oxygen (via KClO$_{4} + $ hv$\to $ KCl $+$2O$_{2})$ and molecular hydrogen respectively. Upon irradiation of the first KClO$_{4}$ -- containing hole, solid reddish-orange O$_{2}$ appeared in the irradiated region and molecular oxygen diffused throughout the entire sample region. The second ammonia borane-containing hole was then irradiated and H$_{2}$ was observed to form via Raman spectroscopy. Water was observed in the ammonia borane-containing hole and possibly (in the form of ice VII) in the second hole. This unique experiment demonstrates the ability to easily create solid mixtures of simple molecular systems via x-ray irradiation and then react them via further irradiation which will aid chemistry at extreme conditions. In particular, the ability to easily determine intermolecular potentials of detonation products and better understand diffusion and molecular mixing or segregation under extreme conditions.   

Influence of thick surface coatings on jet formation in flyer plate impacts

The hypervelocity impact of two flyer plates will produce a fast jet, provided that the collision angle is sufficiently large. At small collision angles, the flow in the reference frame of the impact point is supersonic. The flyers are then deflected by an oblique shock, and no jet is generated. As the impact angle is increased, the oblique shock solution no longer exists. The shock is thus detached, a condition that is associated with the production of a forward-directed jet. The role that surface coatings with significantly different impedance might have on the jetting process has not been previously investigated. In this work, we have studied the effect a thick coating has on the existence of a jet. Aluminum flyers thermally sprayed with a thick layer of aluminum oxide were launched towards each other at 2 km/s at various impact angles, above and below the critical angle for jetting of the respective materials (aluminum and aluminum oxide). Jets for various impact angles where detected and their velocity measured using photon Doppler velocimetry (PDV).   

Hyper-velocity impact experiments with electrostatic dust accelerators

Hypervelocity impacts (HVI) of micrometer-sized particles play an important role in a variety of fields such as the investigation of matter at extreme pressures and temperatures, shock waves in solid bodies, planetology and cosmic dust. The physical phenomena occurring upon impact are fragmentation and cratering, shock waves, the production of neutral and ionized gas, and light flashes. Advanced analysis techniques promise new insights into short time-scale high-pressure states of matter, requiring the production of high speed projectiles. Electrostatic accelerators act as a source of micrometer and sub-micrometer particles as projectiles for HVI experiments. This paper describes an HVI facility, capable of accelerating particles to over 100 km/s, currently located at the Max Planck Institute for Nuclear Physics in Heidelberg, together with planned improvements. The facility is about to be relocated to the University of Stuttgart. This is an opportunity to enhance the facility to meet the requirements of future experimental campaigns, necessary to better understand the micrometeoroid hypervelocity impact process and develop new in situ dust experiments. We will present the design of the new facility and the planned enhancements, including new diagnostic apparatus.