Liquid nitrogen freezing has made its debut in the U.S.A. along with the growth of space industry and, today, it is spreading rapidly as a characteristic method of freezing. Although it has not been long yet since the liquid nitrogen freezing was introduced into this country, a considerable number of units have already been put into commercial operations and the range of application for foods is being expanded gradually. In this report, the writer intends to explain about the liquid nitrogen freezing, i.e. growth history, theory of freezing, freezing systems and features.
The rectifier system where a small alternating current is fed into the dewar vessel, transformed and rectified into a large direct current may present a method to reduce the liquid helium vaporization during the operation of a superconducting magnet Two kinds of superconductive rectifiers were tested. One of them has Nb Zr wires as its rectifier elements, whose S-N transition for rectifier operation is caused by pulse heating. The other is a contactor type rectifier with Indium-Tin contacts which are also superconductive. The S-N transition type rectifier showed more vaporization of liquid helium than the counter flow leads in 100mV operation, but this vaporization may be reduced comparable, if the design is optimized. The contactor type rectifier was tested in the output of 500mV and 150A, and showed much less vaporization, which will mean the possibility of the superconductive rectifier of several volts and several thousand amperes.
The research work on manufacturing and testing a superconducting saddle shaped magnet for MHD power generator has been in progress since 1966 at the Electrotechnical Laboratory. So far we have succeeded in making 29cm bore, 2.3 Tesla fully stabilized magnet. It has the following characteristics: Bore diameter 29cm, Outer diameter 76cm, Length 91cm, Number of winding layers 11, Max. central flux density B0=2.3T (at 460Amp) Fully stabilized to at least B0=2.5T (to 500Amp) Max. flux density in magnet windings 3.5T (at 460Amp) Superconductor AVCO SG-500 (4×0.25 in dia. Nb-33% Zr) Weight 1, 120kg The magnet was precooled to about 48°K in about 16hours by helium gas refrigerator, which supplies the cooled helium gas of 42°K to the dewar in a closed loop. Then liquid helium was poured into the dewar to cool down the magnet to 4.2°K. The magnet has the energy of about 370kJ at the critical current of 460A. The transfer of the magnet energy to the parallel resistor outside the dewar was also successfully tested, that is, about 90% of the magnet energy could be transfered.
A superconducting lens system for ultra high voltage electron microscope has been completed. This is designed to have such construction that it can be replace the standard image forming system of electron microscope. The magnetic circuit of the lens system is made of Permendur. The maximum axial field strength at the excitation of 40, 000 Amper turns is about 46kG, and the computed focal length for this field distribution is 2.7mm. The liquid Helium is stored in a reservoir tank which is located above the lens part. The reservoir has the capacity of about 1.5 litres and it is enough to keep the lens in the superconducting condition for about 1hour. Electron micrographs taken with the superconducting lens operated under the persistent current mode are demonstrating.