Vorticity induced mixing in the Rayleigh-Taylor Instability *

 In high-energy-density physics (HEDP), mixing due to the Rayleigh-Taylor instability (RTI) is prevalent in a wide range of flows relevant to inertial-confinement fusion (ICF) and astrophysics. As the mixing increases, nonlinearities and secondary instabilities develop. Eventually, these instabilities dominate in the mixing process and contribute to cooling the hot fuel in ICF.  HED experiments conducted on the National Ignition Facility (NIF) have primarily focused on measuring the layer width of mixed materials caused by RTI.  With diagnostic improvements, recent experimental radiographs have imaged  secondary instability growth along the RT spike tip. In contrast, simulations that assume an ideal interface as an initial condition fail to reproduce this growth of secondary structures. Here we discuss what causes these secondary instabilities to appear, as well as their contribution to the mixing process. We then use simulations and synthetic radiographs to highlight the contribution of a non-ideal initial interface to vorticity-induced mixing on the RT rollup.  Continued improvements in resolution can shed further light on the involvement of the RTI and secondary instabilities in mixing.