Fig.5 Schematic view of GFRP Dewar

Extracts from the Article

There are three kinds of heat exchange: solid conduction, convection and thermal radiation. The amount of heat exchange is proportional to the conduction cross-section. Reducing the cross-section can reduce heat by solid conduction. Convection comes from the collision of gas molecules, by which heat automatically transfers from a high temperature zone to one at low temperature. High vacuum can reduce heat transfer by convection. As long as the temperature is above 0 K, any material can radiate heat outwards. Glazed metal foil can reflect the radiation effectively and reduce heat-leakage by thermal radiation. Conventional cryostat is made of high-strength stainless steel, and the wall of the cryostat may be very thin and can thus effectively reduce heat-leakage by solid conduction. On the other hand, stainless steel is high in tightness and does not deflate. The cryostat made of stainless steel can therefore maintain a vacuum, which greatly reduces heat-leakage from the convection. There are multi-layers of wrapping glazed metal foil between the inner and outer walls of the cryostat, and they are useful for preventing heat radiation from the environment. At the same time, a certain amount of active carbon is inserted into the space between the inner and outer walls to absorb remanent gas molecules. Therefore, the conventional cryostat is generally made of stainless steel material and thus has advantages of high strength, the ability to maintain a vacuum over a long period of time, and good thermal insulation. The vacuum of this cryostat can be pumped down to 10–7 Pa and can maintain this level for more than 2 years. However, because the cryostat of a superconducting transformer surrounds the magnetic circuit, this kind of cryostat cannot be made of metal materials wherein a large amount of eddy current will be induced. GFRP, with excellent insulation, is a kind of available candidate material used to make the cryostat of superconducting transformers. Different from the conventional cryostat, the radiation-protection shield between the inner and outer walls is made of glazed thin metal foil with cuts to prevent the foil from forming a short circuit current. The schematic structure is shown in Fig. 5. Figure 6 is a photo of the real cryostat made of GFRP. Unfortunately, since GFRP has intrinsic characteristics of air-bleed, the vacuum of the cryostat cannot last a long time, and thus needs pumping periodically. Active-carbon is usually used as gas absorption material between the inner and outer walls of the conventional cryostat. Semiconductors cannot be used as gas absorption material to avoid short circuit between the walls of the cryostat. Therefore, in the cryostat of superconducting transformers, organic materials should be chosen as gas absorption material that can avoid all of the above disadvantages. Additionally, since the cryostat of a superconducting transformer operates in an AC magnetic field environment, any magnetic material should not be used in the design and fabrication of the cryostat.