Researches on forced convection heat transfer to supercritical helium have been surveyed. Introduction of the recent experiments, discussion on the impairment of heat transfer and the influence of buoyant force on the impairment, and critical review of the available experimental data have been done.
The JT-60 neutral beam injector consists of 14 beamlines. Each beamline has a cryocondensation pump of capacity 1, 400m3/sec. The purpose is to evacuate hydrogen gas of 2-4 Pa·m3/sec from the ion sources, thereby keeping the pressure in the beamline chamber in the order of 10-3 Pa during beam extraction. The cryopump consists of four large modules, two small modules and two phase separators. These modules are divided into three pairs and constitute differential pumping. In each module, liquid helium and liquid nitrogen are supplied from the phase separators by natural circulation based on thermal siphon effect. The cryogenic system for the cryopumps are helium and nitrogen systems. The helium system is a complete closed loop of a refrigerator (cold box, main and pumping compressors), distribution transfer lines, a liquid helium storage dewar and helium gas storage tanks. Design capacity of the refrigerator is 2, 400W at 3.7K. The construction was started in autumn of 1983. Each performance of the 14 cryopumps was tested one by one in the test bed facility with good results. Capacity of helium refrigerator exceeded 2, 500W at 3.6K. Completion of the system is scheduled for July 1986.
The acoustic emission (AE) from FRP cryostats has been studied in order to establish the monitoring systems of FRP Cryostat. Kaiser effect was found in FRP cryostat and hence using AE method we can estimate the stress history of any FRP cryostat. Condition of coolant was also detected by AE techniques. Sudden decrease of internal pressure which leads to the rapid evaporation of coolant can be detected as sudden increase of AE counts. It is also found that the vacuum leak area caused by gaseous pressure could be located by the location of AE sources.
Creep tests and constant load rate tensile tests at cryogenic temperatures were made on austenitic stainless steels, SUS 304L, 310S and 316L. Creep rate of about 10-9s-1 was observed in 200 hour creep test under the proof stress of the materials at 77K, and creep rate at 4K was decreased to a tenth at 77K. On the other hand, an abrupt and large deformation occurs in a constant load rate tensile test at 4K. The magnitude of the abrupt deformation depends on the workhardening rate and the stability of austenitic phase. Higher proof stress materials, especially SUS 310S, do not always show better characteristics for the creep or the abrupt deformation at 4K.
Measurements were made on the temperature rise of tensile test specimens of austenitic stainless steels undergoing plastic deformation and discontinuous flow in normal liquid helium and superfluid helium. As the strain rate increased, the ultimate tensile strength and elongation decreased and the temperature rise increased. Compared with the results in normal liquid helium, the frequency of stress drop increased and the temperature rise was suppressed in superfluid helium.
Measurements have been made of the heat transfer in liquid helium II in a capillary glass tube with an inside diameter of 93.3μm and a length of 8.5cm. Heat supplied to a lower part of a cell produced a thermal counterflow in the tube. The temperature difference (ΔT) across the tube was measured using a thermocouple (Au [0.03 at% Fe]-Nb⋅Ti) with a SQUID detector. The obtained value of the critical vortex line density (L0)c associated with the transition from a turbulent flow to a laminar flow was larger than that given by Vinen. The spontaneous periodic fluctuation (ΔTf) of the temperature difference was observed at the transition. The period of the fluctuation was about 20 seconds and the magnitude of ΔTf/ΔT was about 0.15 at 1.5K.