Experimental study of energy and shape transfer in double shell implosions*

As high-velocity, high-convergence ratio ICF experiments continue to push for higher DT fuel burn-up fraction and -particle heating at the NIF, researchers continue to explore additional methods to study burning fusion plasmas. Recently, advances in target fabrication have made double shell (DS) implosions a viable burn platform. Core to the DS capsule is a high-Z (e.g., Au) metal pusher that accesses the “volume”-burn regime by trapping radiation losses and compressing a uniform fuel volume at reduced velocities. The DS system relies on a series of energy transfer processes starting from x-ray absorption by the outer shell, followed by collisional transfer of kinetic energy to an inner shell, and final conversion to fuel internal energy. Beyond efficient energy transfer, we must also design double shells for robust performance against engineering features and implosion asymmetry.

We present simulation and experiment results on momentum and shape transfer between the outer and inner shell. 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. Closely matching theory and RAGE simulations, experiments show a reduction in ablator velocity with the addition of a foam cushion, and a reduction in ablator/foam kinetic energy due to collision with the inner shell. We present ablator-only experiments demonstrating control of outer-shell shape, since simulations suggest that ablator shape during the collision primarily determines the fuel shape at stagnation. We also present results of shape transfer studies using our surrogate “imaging”-shell platform. Included in these studies is an examination of the role of the ablator joint engineering feature on implosion shape.