Numerical Study of Effect of Helium Filling on Temperature Characteristic of Cryogenic Target

In the design of the inertial confinement fusion cryogenic target, the helium gas with low pressure in the cryogenic target is filled to the high pressure in order to achieve ignition. Due to convective heat transfers, the fillings of helium cause thermal environment around the capsule, which may generate the melting of the DT ice layer. The CFD numerical methods coupled with multiple heat transfer mechanisms under three-dimensional finite volume frame were employed to investigate the thermophysical phenomena in the helium filling process in this paper. The effects of the filling velocity, temperature and steady pressure on the flow and temperature characteristics in the cryogenic target were studied. The distributions of temperature in the capsule surface were analyzed. The numerical results show that the filling process can be divided into two stages: rapid filling and steady state, according to the variation of maximum temperature in the capsule surface. In the steady state, two modes of cyclic flow are found in the cryogenic target, and the temperature in the southern capsule is significantly higher than that in the north because of the helium with high temperature baking. The highest temperature is found at 62.4° south of the equator on the windward side, and the lowest temperature is on the opposite side of the highest temperature. The locations of temperature extremum point are not affected by filling velocities and temperatures. What’s more, the temperatures in the capsule surface increase (relative to 19.7 K) by 69.2, 99.8 and 134.6 mK, when the filling temperatures are 200, 250 and 300 K, respectively. However, the filling velocity of helium has little effect on the flow and temperature fields in the cryogenic target. After helium fillings were completed, the influences of the steady pressure in the cryogenic target on the temperature characteristics of the capsule surface were also evaluated. When the steady pressure is 10 kPa, the temperatures of the capsule surface are symmetrically distributed about the equator. With the increase of steady pressure, the natural convection effects are enhanced, resulting in the widening temperature differences between the north pole and the south pole. The critical pressure of 100 kPa is obtained by parameter analyses, and the corresponding Rayleigh number is about 1.12. The conclusions in this paper have certain guiding significance for the further design and experiment of cryogenic target.