Patent GB-878377-A (GB)
Title: Improvements in and relating to superconductive switching devices
IPC: H01L39/20
Filed: 1959-09-15
Abstract: 878,377. Superconductive circuits. INTERNATIONAL BUSINESS MACHINES CORPORATION. Sept. 15, 1959 [Sept. 15, 1958], No. 31499/59. Class 40 (9). [Also in Group XXXVI] A superconductive switching device comprises inner, outer and intermediate conductors all capable of exhibiting superconductivity and so arranged that the outer conductor is effected by field produced in both the inner conductors but the inner conductors are unaffected by the outer. Fig. 1 shows an arrangement comprising three concentric cylindrical superconductors 10, 12, 14 respectively control, bias and gating windings to which batteries are connected by switches 18, 22 so as to produce magnetic fields in the same sense. The supply to the gating conductor, however, is in opposite sense. Conductor 14 is " soft," that is it has a low critical field, and may be of tantalum or tin depending on the operating temperature; the control and bias conductors are relatively " hard " and may be of niobium or lead although the control conductor may sometimes be of the same material as the gate conductor. When the device of Fig. 1 is operated as a biased cryotron switch 18 remains closed. The current in conductor 12 is less than the critical field for the gate conductor but it reduces the field to be applied to control conductor 10 to drive the gate conductor resistive while increasing the current in the gate circuit necessary to drive the gate conductor resistive. Thus the device has a gain which is greater than unity. Fig. 2 shows a practical construction of the device. An inner conductor 10 is first evaporated on to a core 34. An insulating film 32, of, for example, silicon monoxide is evaporated on followed by a hard biased conductor 112, insulating layer 30, and " soft " superconductor gate conductor 14. The radius of the core is large compared with the film thickness which is approximately 10,000 . The almost equal radii of the films means that current in any conductor produces the same effect at the outer surface of the gate conductor. The Specification explains why the bias conductor does not act as a magnetic screen to prevent the field of the control conductor from affecting the gate. The construction of the conductors means that each conductor produces only an external field so that each affects the conductors outside it but is not in turn affected by those outside. As a result the current in the gate conductor can flow only in the outer layers so that the bias and control fields are able to drive the inner part of the gate cylinder resistive so that the gate current is forced outwards and with it its field until the whole gate cylinder is resistive. When this occurs the gate current tends to re-distribute forming a superconductive path at the outer surface of the gate cylinder. The effect of this is that the total bias and control field is independent of the current in the gate conductor. The device of Fig. 1 may be used as an "and" circuit in which case switches 18, 22 are individually operable to apply pulses to cylinders 10 and 12 so that the cylinder 14 becomes resistive if both are closed at once. The circuit may also be used for gating. Fig. 5 shows a planar arrangement with a central control layer 10A spaced biasing layer 12A and gating layers 14A. In the arrangement of Fig. 3 the control field produced by 56 is unable to affect the gating cylinder 65 because intermediate cylinder 58 is provided with a superconducting path 65 which enables it to act as a magnetic screen. The screen is destroyed by a magnetic field produced in coil 64 to make part of the superconducting path resistive. In the device of Fig. 4 (not shown) the external superconductive path is provided by a further superconductive cylinder connected by superconductive strips to the intermediate cylinder. Each strip is enclosed by a coil which may drive it resistive. In the arrangement of Fig. 6 a sending coil 56B is coupled to a receiving coil 54B only when the superconductivity of a loop attached to coil 58B is destroyed by a field produced by gating coil 64B. Specifications 862,178 and 877,626 are referred to.