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Removal of Magnetic Flux Trapped in Josephson Junctions Comprising a Computing System

IP.com Disclosure Number: IPCOM000089621D
Original Publication Date: 1977-Nov-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Basavaiah, S: AUTHOR [+2]

Abstract

A magnetic flux trapped in a Josephson junction device reduces its zero voltage threshold current. Logic and memory circuits having such trapped flux devices will not be operational. Thus, all or part of a computing system could be rendered inoperative because of trapped magnetic fluxes.

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Removal of Magnetic Flux Trapped in Josephson Junctions Comprising a Computing System

A magnetic flux trapped in a Josephson junction device reduces its zero voltage threshold current. Logic and memory circuits having such trapped flux devices will not be operational. Thus, all or part of a computing system could be rendered inoperative because of trapped magnetic fluxes.

The method described below eliminates the trapped flux without removing any part of the system from its liquid helium bath. This involves heating the chips or modules "locally" using the following techniques.

It is recognized that local heating need only raise the temperature of the chip/module by about a degree or so above 4.2 Degrees K, enough for the creation of helium film boiling around the chip/module. Due to much lower thermal conductivity of helium gas created by the film boiling, the heat input is not conducted away to the liquid helium bath, and thus the temperature of the chip/module is very quickly raised to a few degrees above the T(c) of the electrodes. The device electrodes thus become normal, dissipating the circulating currents responsible for the trapped fluxes. As the heat input is removed, the electrodes return to the superconducting state.

During this period, it is important to insure that there are no external magnetic fields in the vicinity of the devices, so that they are not trapped again. High Mu metal shields surrounding the liquid He dewars [an essential requirement in the operation of the Josephson devices] insure negligible earth's magnetic fields. A sufficiently low heater current I of about a few tens of milliamps [the heater resistance R being such that the heater power I/2/R is enough to cause the helium film t...