Tantalum Strength Experiments on National Ignition Facility

We are conducting Ta strength experiments using the NIF laser to test Ta strength models at high pressures ($\sim$ 5 Mbar), high strain rates ($\sim$ 10$^{7}$ s$^{-1})$ and high strains (\textgreater 30{\%}). We use 800 kJ of laser energy to create a ramped drive via a 4-layer reservoir - gap configuration. The target package includes sinusoidal Ta surface ripples that are used to infer the plastic flow stress of the sample from a measurement of the Rayleigh-Taylor instability ripple growth. The inferred flow stress is approximately twice greater than predictions by the multiscale strength model. It is conjectured that homogeneous nucleation behind the leading shock at $\sim$ 1 Mbar promptly generates a very high dislocation density, thus increasing the strength through the work hardening term. This paper will present the experimental results comparing them with various strength models.