John M. Rowell
The cooling requirement is the primary barrier to the widespread commercial acceptance of superconducting devices. The superconducting materials or superconducting electronic (SCE) devices discussed in previous chapters constitute only a fraction of the size and the cost of a superconducting system, and the customer reacts, often negatively, to the appearance, size, noise, and (perceived) unreliability of the refrigerator or cryocooler. With the discovery of HTS superconductors, much was (wrongly) claimed for the advantages of cooling with liquid nitrogen instead of the liquid helium generally used to cool low-temperature SCE systems. In fact, the benefit of using HTS devices is that smaller, cheaper, and more reliable refrigerators can be used in HTS systems than in LTS ones. Initially cryocoolers available for HTS systems were adapted from ones developed for other markets such as those for vacuum cryopumps and for cooling infrared sensors in missiles, and they weren't ideal for superconducting applications. More recently, some SCE companies (e.g., SCT and STI) have begun development of their own coolers, or refrigerator manufacturers (e.g., CTI) have begun to modify their products to suit the needs of the SCE industry. As a result, the cryocoolers used in the wireless products of U.S. SCE companies are now almost "invisible" to the user (although the cost is not!).
Besides the refrigerator it is also essential to provide a cryopackage (sometimes called the Dewar), which is the means to mechanically and thermally attach the SCE component to the cold head of the refrigerator within an evacuated enclosure while providing electrical or optical connections through the enclosure and vacuum space. One challenge is to ensure low loss in the electrical connections without allowing too much heat to travel from room temperature to the SCE component; a second is to guarantee that the enclosure maintains its vacuum for the lifetime of the refrigerator, hopefully many years. Much of the development and engineering effort and budgets of the small U.S. SCE companies over the past two years have been spent on cryopackaging.
It was not possible during the week-long WTEC visit to Japanese sites working on SCE for the panel to gather an accurate and complete view of the Japanese cryocooler industry. Panelists were told that there are at least 10 manufacturers of cryocoolers in Japan, of which the panel visited or met with three. In hindsight and given the importance of refrigeration to SCE (as well as other industries), it would have been valuable for the WTEC panel to have increased its numbers by one or two members so that two people could have spent the whole week visiting Japanese refrigerator companies. As the panel was only able to visit two of the companies, Daikin and Denso (which has a joint effort with AMTEL), and only met with representatives of Aisin Seiki during our visit to Nagoya University, we do not feel that we obtained a balanced view of the industry. However, as these three companies were chosen because of their interest in SCE applications, it is likely that the panel did accomplish an important sampling of the activity that is most relevant to this study.