Yuanzineng kexue jishu (Oct 2022)
Transient Thermal Characteristics of Cryogenic Target with Multiple Laser Entrance Holes during Quick-freezing Process
Abstract
Cryogenic target is one of the most widely used technologies in the inertial confinement fusion (ICF) research. The formation of a uniform and smooth ice layer inside the fuel capsule is a prerequisite for the successful ignition of an ICF cryogenic target. The ice layer homogenization process requires precise temperature control of the capsule, so the temperature control of the cryogenic target system is an important part of the capsule preparation and ignition experiments. The smoothness of the ice layer is mainly affected by the crystal growth during the solidification process, and the ice layer experiments show that single crystal growth is conducive to reduce the layer defects and improve the final ice roughness. Since the fast cooling of fuel capsule often results in a polycrystalline ice layer with many defects, further warming and melting follow the freezing process to form the single crystal seed, which requires a precise temperature control of the cryogenic target. In the present study, a full-scale three-dimensional model was established for the cryogenic target with multiple laser entrance holes and cold energy transfer components such as silicon arms and cooling rods. The simulation was based on the DO radiation model using Fluent simulation software, where the precision structure in the model was processed using shell conduction, considering both radial and axial heat transfer. The steadystate distribution and the transient quickfreezing characteristics of the cryogenic target temperature field were numerically studied by controlling the power of the heating blocks on the silicon arms. Moreover, the effects of contact thermal resistance and helium pressure on the temperature stability of the cryogenic target during the fast cooling process were analyzed. The results show that the temperature on the capsule surface reaches its minimum value in the vicinity of the fuel filling tube and peaks at the regions facing the laser entrance holes under constant cooling conditions with a maximum temperature difference of 0.03 mK. When the heaters on the silicon cooling arms reduce their power, the maximum temperature difference on the capsule surface sharply increases to 87.88 mK in 0.25 s, and the uniformity of the temperature field on the capsule surface deteriorates significantly. The temperature difference gradually falls back and stabilizes with time. Compared with the zero thermal contact resistance condition, the presence of ultralow temperature glue between the silicon cooling arm and the thermal mechanical packages (TMP) can weaken the propagation of temperature fluctuation on the heat conduction path and thus reduce the maximum temperature difference peak. This can prevent the uniform temperature environment from deteriorating, but is unfavourable for the real time control of capsule temperature. From the viewpoints of the rapid capsule temperature response and the small maximum temperature difference, the helium pressure ranging from 1 kPa to 10 kPa is conducive to maintaining the temperature uniformity of the capsule surface.
