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.