H. Jarmakani

Experiments for the validation of debris and shrapnel calculations

The debris and shrapnel generated by laser targets are important factors in the operation of a large laser facility such as NIF, LMJ, and Orion. Past experience has shown that it is possible for such target debris to render diagnostics inoperable and also to penetrate or damage optical protection (debris) shields. We are developing the tools to allow evaluation of target configurations in order to better mitigate the generation and impact of debris, including development of dedicated modeling codes. In order to validate these predictive simulations, we briefly describe a series of experiments aimed at determining the amount of debris and/or shrapnel produced in controlled situations. We use glass and aerogel to capture generated debris/shrapnel. The experimental targets include hohlraums (halfraums) and thin foils in a variety of geometries. Post-shot analysis includes scanning electron microscopy and x-ray tomography. We show the results of some of these experiments and discuss modeling efforts.

MODELING OF THE SLIP‐TWINNING TRANSITION IN NANOCRYSTALLINE NICKEL AND NICKEL‐TUNGSTEN UNDER SHOCK COMPRESSION

A constitutive description of the slip‐to‐twinning transition, based on the critical shear stress, is applied to determine the twinning transition pressures in nanocrystalline nickel and nickel‐tungsten (13 at. %) under shock compression. The model predicts a critical transition pressure of 78 GPa in 30 nm Ni and 16 GPa in 10 nm Ni‐W. These predicted results are in good agreement with laser shock experiments carried out on the same materials.

Dynamic Response of Copper Subjected to Quasi‐Isentropic, Gas‐Gun Driven Loading

A transmission electron microscopy study of quasi‐isentropic high‐pressure loading (peak pressures between 18 GPa and 52 GPa) of polycrystalline and monocrystalline copper was carried out. Deformation mechanisms and defect substructures at different pressures were analyzed. Current evidence suggests a deformation substructure consisting of twinning at the higher pressures and heavily dislocated laths and dislocation cells at the intermediate and lower pressures, respectively. Evidence of stacking faults at the intermediate pressures was also found. Dislocation cell sizes decreased with increasing pressure and increased with distance away from the surface of impact.

Dynamic response of single crystalline copper subjected to quasi-isentropic laser and gas-gun driven loading

Single crystalline copper was subjected to quasi-isentropic compression via gas-gun and laser loading at pressures between 18GPa and 59GPa. The deformation substructure was analyzed via transmission electron microscopy (TEM). Twins and laths were evident at the highest pressures, and stacking faults and dislocation cells in the intermediate and lowest pressures, respectively. The Preston-Tonks-Wallace (PTW) constitutive description was used to model the slip-twinning process in both cases.