Numerical analysis of dynamic heating modulation during rapid cooling of fuel layer in an indirect-drive cryogenic target

Abstract

The thermal shimming technology is initially used to adjust the temperature field around the capsule in order to obtain an isothermal distribution and meet the requirement of a high-quality uniform deuterium-tritium (DT) ice layer. In this paper, we focus on the asymmetric heating scheme on upper and lower heaters, which are used both in the quasi-steady layering process and the rapid-cooling process. A dynamic modulation scheme with asymmetric heating is first proposed to attenuate the cooling-induced thermal perturbation around the capsule. Thermal and hydrodynamic simulations of the cryogenic target are numerically conducted. An axisymmetric model based on the NIF-scale indirect-drive target is built and used in simulations. A variety of dynamic modulation schemes are studied and compared with each other. In the layering process, the asymmetric heating scheme is validated to be effective in reducing the low-mode roughness of the DT ice layer. In the rapid cooling, the thermal uniformity around the capsule can be greatly improved by using the dynamic heating modulation scheme when the DT layer is frozen to 1.5 K below its triple point temperature. At a constant cooling rate of 0.4 K/s, the temperature difference on the capsule outer surface reaches ∼24.5 mK without dynamic heating. By employing a nonlinear modulation with symmetric heating, this value can be decreased by 25%. Based on these results, further improvement can be achieved by optimizing the modulation with asymmetric heating and the temperature difference can be significantly reduced to ∼6.0 mK, which is a factor of four lower than the original value.