Patent GB-914541-A (GB)
Title: Improvements in or relating to cryotron arrangements
IPC: G11C11/44
Filed: 1959-03-26
Abstract: 914,541. Cryotrons. PHILIPS ELECTRICAL INDUSTRIES Ltd. March 26, 1959 [March 31, 1958], No. 10648/59. Class 37. In a cryotron arrangement comprising a current source connected to a thermally-insulated superconductor element a stable normallyconductive condition of the element is achieved at an ambient temperature below or equal to the effective transition temperature of the superconductor by selecting the current through the element so that the heat generated thereby is sufficient in relation to the amount of thermal insulation to ensure that the actual temperature of the element is above the effective transition temperature. At an ambient temperature below the effective transition temperature a stable superconductive state is possible under the same conditions since when in this state the element dissipates no heat. The conductor may be switched from the superconductive to the normal state by increasing the magnetic field therein momentarily to lower the effective transition temperature to less than the ambient temperature. The conductor then heats up rapidly so that when the original field condition returns it is above the effective transition temperature. The required increase in field may be produced by a current pulse through the conductor itself to raise the self-induced field or by a current pulse in a separate control conductor. Switching back to the superconductive state is effected by applying a negative current pulse to reduce the heat dissipation momentarily and allow the temperature to fall below the effective transition temperature. Alternatively, or in addition, if the temperature of the conductor in its normal state is lower than the transition temperature for zero magnetic field switching may be effected by reducing the externally produced magnetic field to raise the effective transition temperature above the conductor temperature. A suitable cryotron for use in the above manner consists of a tantalum wire in an appropriately dimensioned heat-insulating jacket. Another cryotron with built-in control conductors is shown in Fig. 3, in which 4 is a tantalum cylinder in which a control conductor 6, having a higher transition temperature, e.g. niobium, and insulated from 4 is concentrically disposed. An additional control conductor may be wound around the heatinsulating jacket 5. It is desirable that both the normal current in the superconductor and the switching current pulses be as small as possible. This is achieved by reducing the difference between the ambient and effective transition temperatures to a low level, either by selecting an ambient temperature very close to the zero field transition temperature or by providing an external magnetic field to lower the effective transition temperature to a suitable level. Using the device with two control conductors a constant current is passed through one, preferably the inner, conductor 6 and switching current pulses through the other. The switching time, which is determined by the rate at which the superconductor heats up and cools down, may be reduced by using a conductor of high resistance per unit length. High resistivity material may be used but use of a thinwalled hollow conductor is desirable in that it also leads to a low heat capacity. The highspeed cryotron shown in Fig. 4 comprises a vapour-deposited tantalum strip 4 on a support 8, overlain by a heat insulating silica layer 5 and a zig-zag control conductor 9. Suitable dimensions for such a device are calculated in the Specification. The devices may be used to memorize pulses of switching magnitude, and in logical circuits.