The Development And Experimental Study On GM Cryocooler Cryostat For Research In The Pulsed High Magnetic Field

With the rapid development of magnet technology and the high speedmeasurement electronics, pulsed high magnetic field is playing a more and moreimportant role in scientific research. For research in condense matter physics,, inparticular the observation of the quantum phenomenon, it is essential to cool thesample to an extremely low temperature and measure its properties in high magneticfield. This is achieved by the cryostat inserted in the bore of the magnet. The cryostatand its cooling system are critical components for the scientific experiment in pulsedhigh magnetic fields at extreme condition.At present, the helium-4cryostat, the dilution refrigeration cryostat and helium-3cryostat are the three main types of cryostat for the scientific research in pulsed highmagnetic fields. The helium-4cryostat, helium gas-flow cryostat, helium-3cryostat,dilution refrigeration cryostat and the GM cryocooler cryostat with conductionfilaments have been developed in the Wuhan National High Magnetic Field Center.Except for the GM cryocooler cryostat, all the other systems require liquid helium asthe cooling source. However, the shortage of helium in China results in a very highrunning cost for these system and restricts the development of the scientific experimentin pulsed high magnetic fields. While the lowest temperature provided by the GMcryocooler cryostat with conduction filaments was only10K, which cannot meet therequirements of lower temperature. Therefore, the design of a new type GM cryocoolercryostat which can reach the temperature range of helium-4cryostat is of greatimportance.In this dissertation, based on the concept of heat pipe using GM cryocooler toliquefy room temperature helium gas from a bottle and transferring it to the samplechamber, we have developed a prototype of such a cooling system and carried outexperiment in pulsed high magnetic field with it in the temperature range from1.4K upto300K. Temperature as low as1.4K has been achieved Compared to theconventional Helium flow system that is normally used for experiment in pulsed highfield, this system saves80%of helium. A bottle of Helium gas can be used for a weekwhen conducting experiments12hours per day, seven days per week.In this paper, the heat transfer and the temperature distribution are analyzed. The results show that the selected1.5W@1.4KGM cryocooler and the heat exchangers canprovide adequate cooling power for the sample chamber, and the sample chambertemperature can go down to4.2K by the liquefied Helium and even lower temperaturecan be achieved by pumping the liquid helium.Temperature stability of the samplechamber at1.5K during a pulse of60T magnetic field is studied taking into account ofthe eddy current effects in all the conductive components. The study shows the cryostatcan keep the temperature of the sampler chamber stable at1.5K during the pulse of60T magnetic field. According to the above analysis results and ordinary cryostat designpoints, and in view of the special condition of the scientific experiments in pulsed highmagnetic fields, the design of the heat transfer structure, material selection, sealing andwelding, temperature measurement and control and vacuum system of the cryostat havebeen performed. Therefore, the GM cryocooler cryostat uses the structure described inthis paper.Based on the above theoretic analysis, the author developed a set of GMcryocooler cryostat prototype. The helium gas cold trap, multi-stage cooling solenoidand porous condensing heat exchanger are used to condense the helium. The sample iscooled by introducing the liquid helium into the sample chamber by a1.5mm vacuuminterlayer. The performance of the prototype was tested by the the cooling andtemperature control experiments. The cooling experimental results show that the lowesttemperature of the cryostat can reach as low as1.4K, superior to the design value of1.5K. The1.4K temperature can maintain up to17minutes, which meets therequirement of the scientific experiment in pulsed high magnetic fields. Thetemperature control experimental results show that the temperature stability of thesample chamber is±0.1K at20K-300K,±0.2K at5K-20K and±0.02K at1.5K-5K. Finally, the electrical transport measurement in (Ga,Mn)As films using thiscryostat in pulsed high magnetic fields was carried out. The hole density in (Ga,Mn)Asfilms at different temperatures was obtained from this experiment. The experimentverified that the GM cryocooler cryostat can be applied to the scientific experiment inpulsed high magnetic fields.