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Generalized Tightly Coupled Low Noise Squid

IP.com Disclosure Number: IPCOM000051476D
Original Publication Date: 1981-Jan-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Jaycox, JM: AUTHOR [+2]

Abstract

This article relates to a generalized design for a low noise, tightly coupled Superconducting Quantum Interference Device (SQUID), with input inductance that may be made many orders of magnitude larger than the SQUID self-inductance. These qualities are jointly required for a SQUID to be of practical value as some type of measuring instrument. Low noise SQUIDs have been previously designed, fabricated, and tested 1,2 , but they have not exhibited strong coupling or input inductance large enough to be of use in a practical detector.

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Generalized Tightly Coupled Low Noise Squid

This article relates to a generalized design for a low noise, tightly coupled Superconducting Quantum Interference Device (SQUID), with input inductance that may be made many orders of magnitude larger than the SQUID self- inductance. These qualities are jointly required for a SQUID to be of practical value as some type of measuring instrument. Low noise SQUIDs have been previously designed, fabricated, and tested 1,2 , but they have not exhibited strong coupling or input inductance large enough to be of use in a practical detector. The design described below incorporates many of the features of the low noise SQUIDs previously constructed, but is general enough to allow easy modification of the SQUID loop dimensions to separately select both a desired SQUID self inductance and an input coil inductance, with the assurance of tight coupling.

The SQUID loop 1 appears, as shown in Fig. 1, as a large square conductor with a square hole in the center and a slit 2 down one side to interrupt the rectangular current path. The inductance of this shape has been calculated using a three-dimensional superconductor inductance program. With the exception of the inductance of slot 2, the computed value very nearly equals the SQUID loop inductance. The calculations show that when the SQUID loop width, w, is comparable or greater than the hole width, d, (Fig. 2) the inductance is only a very weak function of d. Thus, after choosing a hole size for proper loop inductance, the SQUID loop width may be chosen for any value w/d without necessitating a recalculation of the SQUID loop inductance. Using the result supplied in Fig. 2, the inductance of any hole size may be estimated using a linear scaling relation. One should make careful note that this inductance calculation is not based on the presence of a nearby superconducting screening plane [3].

Coupling to the SQUID is accomplished by winding a square spiral coil 3 over loop 1. If the conductor width of coil 3 is much less than the width of SQUID loop 1, then coil 3 inductance would be very high in the absence of SQUID loop 1. However, by bringing SQUID loop 1 close to coil 3, a stripline-like current configuration will result where the coil current is imaged in loop 1 below, and the return loop current is distributed across the conductor, lowering the energy of the system. To estimate coupling efficiency with a two-dimensional superconductor inductance program, a cross-section of the loop 1 and coil 3 system has been treated as a two-dimensional problem. The results indicate that the return current in SQUID loop 1 generates a field linking the turns of the coil almost c...