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SQUID sensors are also employed to detect very small changes in various physical quantities (current, voltage, magnetic susceptibility,magnetization, etc.), which can be transformed into changes in the magnetic flux treading the SQUID ring. Among them, particular interest is devoted to the SQUID based current sensors because they can be effectively employed as readout of gravitational wave detectors,transition-edge sensors and for current sensing noise thermometry. A suitable way to obtain a practical and reliable SQUID current noise is the design based on the flux transformer which converts easily the electrical current into a magnetic flux treading the SQUIDloop and allows to obtain fully integrated sensors. An efficient way to couple the signal coil with the SQUID is also obtained by using a double transformer coupling consisting of a matching (intermediary) transformer inserted between the signal coil and the input terminals of the conventional configuration.
Double transformer configuration
We have developed an ultra-low noise current sensors based on a suitable superconducting intermediary (matching) magnetic flux transformer magnetically coupled to a niobium based dc-SQUID (Superconducting Quantum Interference Device) have been developed. The signal current to magnetic flux transfer factor (current sensitivity) is equal to 62 nA/Φ0 measured by using a current sensing noise thermometer technique. Despite the circuit complexity, the sensor has exhibited a smooth and free resonance voltage-flux characteristic ensuring a stable working operation. Considering a SQUID magnetic flux noise SΦ1/2=1.8 μΦ0/Hz1/2 at T=4.2 K, a current noise as low as 110 fA/Hz1/2 is obtained. Due to his high performance such sensor can be employed in all application requiring an extremely current sensitivity, like the readout of the gravitational wave detectors and the current sensing noise thermometry.
Performance
Design
Main references:
An ultralow noise current amplifier based on superconducting quantum interference device for high sensitivity applications
C. Granata, A. Vettoliere, M. Russo
Review of Scientific Instruments 82 (1), 013901
An ultra high sensitive current sensor based on Superconducting Quantum Interference Device
A. Vettoliere, C. Granata, B. Ruggiero, M. Russo
Sensors and Microsystems, Lecture Notes in Electrical Engineering,Volume 109, 2012, pp 175-180
Double washer configuration
In the double-washer parallel configuration two thin films niobium washers are connected in such a way to form a first order planar gradiometer with respect to background fields. The external signal is coupled to the SQUID loop by a multiturn input coil via the mutual inductance M=k (Li L)1/2, where Li is the inductance of the input coil, L is the total SQUID inductance, and k is the coupling constant. Excellent coupling to the SQUID is obtained using a Ketchen type design. So the input coil inductance can be adjusted to match a particular load by varying the outer dimension of the washer to accommodate the required number of input coil turns. The geometry results in a value of SQUID inductance L=130 pH. The outer dimension of the washer (800 µm) accommodates an input coil consisting of 40 turns of a 4 mm wide niobium strip, giving an inductance value of the input coil of about 650 nH. The high input coil inductance allows a good matching with external load inductance. Being the flux noise of the SQUIDs configuration 2mΦ0/Hz1/2 measured and an input current sensitivity of 0.2 mA/Φ0, the resulting spectral density of input current noise is 400 fA/Hz1/2.
Performance
Design
Main references:
LTC Josephson device technology for low noise applications
C. Granata, A. Monaco, M. Russo
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