Experimental study of energy transfer in double shell implosions

Advances in target fabrication have made double shell capsule implosions a viable platform to study burning fusion plasmas. Central to the double shell capsule is a high-Z (e.g., Au) metal pusher that accesses the volume-burn regime by reducing radiative losses through radiation trapping and compressing a uniform fuel volume at reduced velocities. A double shell implosion relies on a series of energy transfer processes starting from x-ray absorption by the outer shell, followed by transfer of kinetic energy to an inner shell, and finally conversion of kinetic energy to fuel internal energy. We present simulation and experimental results on momentum transfer to different layers in a double shell. We also present the details of the development of the NIF cylindrical hohlraum double shell platform including an imaging shell design with a mid-Z inner shell necessary for imaging the inner shell shape and the trajectory with the current 2DConA platform capability. We examine 1D energy transfer between shell layers using trajectory measurements from a series of surrogate targets; the series builds to a complete double shell layer by layer, isolating the physics of each step of the energy transfer process. The measured energy transfer to the foam cushion and the inner shell suggests that our radiation-hydrodynamics simulations capture most of the relevant collision physics. With a 1 MJ laser drive, the experimental data indicate that 22% ± 3% of the ablator kinetic energy couples into inner shell KE, compared to a 27% ± 2% coupling in our xRAGE simulations. Thus, our xRAGE simulations match experimental energy transfer to ∼5%, without inclusion of higher order 2D and 3D effects.