Novel fabrication tools for dynamic compression targets with engineered voids using photolithography methods.

Mesoscale imperfections, such as pores and voids, can strongly modify the properties and the mechanical response of materials under extreme conditions. Tracking the material response and microstructure evolution during void collapse is crucial for understanding its performance. In particular, imperfections in the ablator materials, such as voids, can limit the efficiency of the fusion reaction and ultimately hinder ignition. To characterize how voids influence the response of materials during dynamic loading and seed hydrodynamic instabilities, we have developed a tailored fabrication procedure for designer targets with voids at specific locations. Our procedure uses SU-8 as a proxy for the ablator materials and hollow silica microspheres as a proxy for voids and pores. By using photolithography to design the targets' geometry, we demonstrate precise and highly reproducible placement of a single void within the sample, which is key for a detailed understanding of its behavior under shock compression. This fabrication technique will benefit high-repetition rate experiments at x-ray and laser facilities. Insight from shock compression experiments will provide benchmarks for the next generation of microphysics modeling.

Multi-frame, ultrafast, x-ray microscope for imaging shockwave dynamics.

Inertial confinement fusion (ICF) holds increasing promise as a potential source of abundant, clean energy, but has been impeded by defects such as micro-voids in the ablator layer of the fuel capsules. It is critical to understand how these micro-voids interact with the laser-driven shock waves that compress the fuel pellet. At the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS), we utilized an x-ray pulse train with ns separation, an x-ray microscope, and an ultrafast x-ray imaging (UXI) detector to image shock wave interactions with micro-voids. To minimize the high- and low-frequency variations of the captured images, we incorporated principal component analysis (PCA) and image alignment for flat-field correction. After applying these techniques we generated phase and attenuation maps from a 2D hydrodynamic radiation code (xRAGE), which were used to simulate XPCI images that we qualitatively compare with experimental images, providing a one-to-one comparison for benchmarking material performance. Moreover, we implement a transport-of-intensity (TIE) based method to obtain the average projected mass density (areal density) of our experimental images, yielding insight into how defect-bearing ablator materials alter microstructural feature evolution, material compression, and shock wave propagation on ICF-relevant time scales.

Evidence of shock-compressed stishovite above 300 GPa.

SiO2 is one of the most fundamental constituents in planetary bodies, being an essential building block of major mineral phases in the crust and mantle of terrestrial planets (1-10 ME). Silica at depths greater than 300 km may be present in the form of the rutile-type, high pressure polymorph stishovite (P42/mnm) and its thermodynamic stability is of great interest for understanding the seismic and dynamic structure of planetary interiors. Previous studies on stishovite via static and dynamic (shock) compression techniques are contradictory and the observed differences in the lattice-level response is still not clearly understood. Here, laser-induced shock compression experiments at the LCLS- and SACLA XFEL light-sources elucidate the high-pressure behavior of stishovite on the lattice-level under in situ conditions on the Hugoniot to pressures above 300 GPa. We find stishovite is still (meta-)stable at these conditions, and does not undergo any phase transitions. This contradicts static experiments showing structural transformations to the CaCl2, α-PbO2 and pyrite-type structures. However, rate-limited kinetic hindrance may explain our observations. These results are important to our understanding into the validity of EOS data from nanosecond experiments for geophysical applications.

Ultrafast Photodissociation Dynamics of Nitromethane.

Nitromethane (NM), a high explosive (HE) with low sensitivity, is known to undergo photolysis upon ultraviolet (UV) irradiation. The optical transparency, homogeneity, and extensive study of NM make it an ideal system for studying photodissociation mechanisms in conventional HE materials. The photochemical processes involved in the decomposition of NM could be applied to the future design of controllable photoactive HE materials. In this study, the photodecomposition of NM from the nπ* state excited at 266 nm is being investigated on the femtosecond time scale. UV femtosecond transient absorption (TA) spectroscopy and excited state femtosecond stimulated Raman spectroscopy (FSRS) are combined with nonadiabatic excited state molecular dynamics (NA-ESMD) simulations to provide a unified picture of NM photodecomposition. The FSRS spectrum of the photoproduct exhibits peaks in the NO2 region and slightly shifted C-N vibrational peaks pointing to methyl nitrite formation as the dominant photoproduct. A total photolysis quantum yield of 0.27 and an nπ* state lifetime of ∼20 fs were predicted from NA-ESMD simulations. Predicted time scales revealed that NO2 dissociation occurs in 81 ± 4 fs and methyl nitrite formation is much slower having a time scale of 452 ± 9 fs corresponding to the excited state absorption feature with a decay of 480 ± 17 fs observed in the TA spectrum. Although simulations predict C-N bond cleavage as the primary photochemical process, the relative time scales are consistent with isomerization occurring via NO2 dissociation and subsequent rebinding of the methyl radical and nitrogen dioxide.

Photoactive high explosives: linear and nonlinear photochemistry of petrin tetrazine chloride.

Pentaerythritol tetranitrate (PETN), a high explosive, initiates with traditional shock and thermal mechanisms. In this study, the tetrazine-substituted derivative of PETN, pentaerythritol trinitrate chlorotetrazine (PetrinTzCl), is being investigated for a photochemical initiation mechanism that could allow control over the chemistry contributing to decomposition leading to initiation. PetrinTzCl exhibits a photochemical quantum yield (QYPC) at 532 nm not evident with PETN. Using static spectroscopic methods, we observe energy absorption on the tetrazine (Tz) ring that results in photodissociation yielding N2, Cl-CN, and Petrin-CN as the major photoproducts. The QYPC was enhanced with increasing irradiation intensity. Experiment and theoretical calculations imply this excitation mechanism follows sequential photon absorption. Dynamic simulations demonstrate that the relaxation mechanism leading to the observed photochemistry in PetrinTzCl is due to vibrational excitation during internal conversion. PetrinTzCl's single photon stability and intensity dependence suggest this material could be stable in ambient lighting, yet possible to initiate with short-pulsed lasers.