Crystallization Behavior Of Deuterium Deuterium / Deuterium And Tritium

Deuterium-tritium cryogenic target is the main target type to achieve ignition laser inertial confinement fusion(ICF) within the higher initial fuel density and lower shock wave preheating sensitive characteristics. The target is composed of amorphous CHx that contains a 60μm～100μm thick DT ice. Both the microshell and DT solid layer must be highly uniform in thickness and roughness to overcome hydrodynamic Rayleigh-Taylor instabilities during the implosion process. In order to obtain a uniform DT ice,β and IR layering methods were adopted mainly. For a uniform DT layer(the routhness requirement for a NIF scale target is rms<1μm), we need to start with a single crystal though it is difficult to obtain one. The crystallization process is mainly affected by the target temperature fluctuation, cooling rate and the inner surface roughness of target. Thus the understanding of DT fuel crystal growth behavior and the precise control of of single crystal growth temperature become the present technical challenges.Literature data indicate that DT fuel and D2 fuel both form hcp crystal structure at temperature higher than 4K, We choose to study the D2 crystal crystal growth process to simulate DT. Experimental apparatus include cryostat, temperature control and optical characterizing units. The glass microsphere (248-μm diameter,2.6-μm thickness and less than 10-nm inner surface roughness)containing high pressure D2(13.8MPa) was fixed in Hohlraum center using (15-μm diameter) carbon fiber. Backlit and reflected light(λ=620nm) imaging were used to characterize the target. When the D2 vapor is liquefied, it shows "meniscus"shape due to gravity and surface tension.In order to obtain a uniform temperature enviroment on the hohlraum, the thermal arms have been designed to transfer heat isotropically. the target surface temperature uniformity is mainly affected by the radiation temperature. The lower the cold shield temperature, the better the target surface temperature uniformity we can have, The silmulation results show that the temperature fluctuation of contact area between hohlraum and thermal arms is better than 0.5 mK.High pressure He lead to convection flow in the hohlruam and thus lower the target surface temperature uniformity, and too low helium pressure can not achieve a good thermal conduction. Knudsen number (Kn) for different pressure were calculated to describe the thermal state of tamping gas around 18K. When the pressure is higher than 15Pa (Kn<0.01), He was in continuous thermal region in which the thermal conductivity almost stay the same. He thermal conductivity changes positively with pressure between 1.5 Pa to 15 Pa(0.01<Kn<0.1) corresponding to the transition and slip flow region. Because the thermal resistance between the hohlraum and the target become larger with He pressure lowering, the temperature uniformity of the target surface was improved gradually at the same time. As the pressure continues decreasing(<1.5Pa), helium reaches molecul arregion gradually and the heat transfer approaches the adiabatic condition. In experiments we found that when He pressure was lower than 10 Pa, it requires even lower temperature for D2 to crystallize. Actual measured results were coincided with the theoretical predictions. For this reason, the experimental results introduced in the following parts are at an operating pressure of 10 Pa without specific statement.The grain boundary defects and crystal cracks appeared apparently inside the D2 ice at higher cooling rates. The number of D2 crystal defects gradually decrease as the decrease of cooling rate, When the cooling rate is decreased to 2mK/min, we obtained a relatively uniform D2 ice layer. Two crystal belt were formed on both side of the target, because of exhaustion of D2 in the crystallization process. The He pressure was controlled around 10 Pa for above experiments, In experiments we found two types of D2 crystal growth behavior, one is ripple growth, the other is belt growth. Ripple growth was observed repeatedly and have the same growth rate of 48μm/s under the same experimental conditions. Two crystallization rates were obtained during the growth of the belt. The belt closed itself with a rate of 3600 μm/s and the following expansion rate was 18μm/s. A detail explanation of this crystal growth behavior will be presented in the next part.Through analysis of D2 crystals surface, we found that ripple growth mode always have more grain boundary defects compared to the belt growth mode. For ripple growth mode, D2 fuel firstly formed a thin crystal film on the target inner surface due to the layer growth. After the crystal film closes itself inside the target, the inner fuel then grow randomly. For this reason, more grain boundary defects appeared during the random crystallization processes, When belt growth occurres, the belt structure firstly closes itself, then grow along both sides of the belt by two-dimensional nucleation. Crystal orientation does not change in the crystal growth process, so D2 ice surface has no obvious defect. However, experimental results show that belt growth condition can not be meet initially inside the target. If we control seed crystal to form on the micro-tube wall, with its c-plane parallel to micro-tube wall,the fuel in the target will grow according to the belt mode. High-quality single crystal DT ice will be obtained ultimately.