Effect of artificially implanted helium bubbles on material strength in the high energy density regime*

Understanding plastic deformation dynamics of materials under extreme conditions is of high interest to a number of fields, including meteor impact dynamics and advanced inertial confinement fusion. We infer the strength of samples at pressures up to 8 Mbar, strain rates of ~10⁷ s^-1, and high strains (> 30%) by measuring the growth of Rayleigh-Taylor instabilities (RTI) under ramped compression. We are now studying the dynamic response of materials that are aged by the radioactive alpha decay process. One consequence of alpha radiation is the formation of helium bubbles.  We fabricate lead samples that are artificially implanted with helium bubbles to study the effect of those bubbles on strength. We conducted side-by-side comparisons of pure versus helium-implanted lead samples using the NIF laser facility. Relative strength effects in the high energy density regime are rather different from those exhibited by irradiated materials in conventional stress-strain testing. Initial simulation results help to rationalize the experimental observations, with a porosity-mechanics-based model being used for the material containing helium bubbles.