Super safety switch

1 August 2013

Superconducting switch with a twisted wire and connected to an optic fibre

Superconducting switch with a twisted wire and connected to an optic fibre

Complete setup includes a cryostat surrounded by aluminum foil and coils

Complete setup includes a cryostat surrounded by aluminum foil and coils

Researchers from Sorbonne University in France have developed a superconducting switch for low field nuclear magnetic resonance (NMR) applications that drastically improves controllability and integration, and reduces contact and noise problems.  Furthermore, their switch is controlled by laser beam and can flip in milliseconds, resulting in more durable and accurate test equipment.

In a spin

NMR is a powerful technique for investigating materials by using the intrinsic rotation – more specifically the spin – of their nuclei in an applied magnetic field. Céline Filloy-Corbrion, one of the authors of the research, explained its basic operation: “The rotation frequency depends on the magnitude external field, on the nucleus considered and, to a lesser extent, on the surrounding atoms since they modify the local magnetic field. Hydrogen, for example, has a rotation frequency of approximately 42.6 MHz/T.”

Given these properties, a number of tests can be performed, she explained: “A pre-polarisation procedure is often used at low magnetic fields. The material to be tested is placed in a medium magnetic field, say 100 mT, so nuclei align in this field within seconds. Then the medium magnetic field is removed and the nuclei begin to rotate around the low magnetic field, the earth field for instance. If the low and medium magnetic fields are arranged perpendicularly together, the rotation generates an alternating variation of the magnetic field which can be measured by induction.”

The switch that the team have developed is designed to control the sensor equipment that measures the induction, which must be protected from the medium magnetic field.

Weathering a magnetic storm

During the sensing phase, the NMR signal is very weak as the magnetic field is very low.  Consequently, the – highly sensitive – sensors are particularly delicate and when the medium magnetic field is introduced, very large signals can be induced resulting in saturation – and therefore in a long blanking delay – or even in damage. The switch is then used to protect the sensor as well as to control the measurement procedure.

In the case of very sensitive magnetic sensors, noise has to be reduced as much as possible, so the number of leads and contacts must be reduced to a minimum. Filloy-Corbrion explained that they previously “used a commercial superconducting switch, but that required two command leads carrying electromagnetic noise, and it often broke due to the thermal cycles during the design phase between 4.2K to room temperature.”

To fix these shortcomings, Filloy-Corbrion and her team have “designed a switch without command leads and a special arrangement of a wire to completely avoid contacts.” This arrangement first adds a small inductance and then drastically simplifies the switch mounting. "Without that switch", said Filloy-Corbrion, “our NMR measurement system would not work.”

Colder and more sensitive

Ultimately, the group’s goal is to develop a complete efficient Low Field NMR and MRI system. With the laser controlled superconducting switch they have demonstrated good signal-to-noise ratio, even in a non-shielded noisy environment. The current switch has operated at liquid helium temperatures, but the next step is towards liquid nitrogen temperatures and finally to combine NMR with magnetic tomography.

In the short time since they sent their Letter, Filloy-Corbrion said that they have investigated “various superconducting gradiometers (the sensing part of the magnetic sensor) working at liquid nitrogen temperatures and tested our new Superconducting QUantum Interference Device (SQUID)and we expect the completed design by next year.”

Low field NMR is the more recent of their activities and they hope to introduce or adapt many of the concepts they have already developed for capacitive sensors to this field. “In addition,” explained Filloy-Corbrion “we will work on that project with another group in our lab, specialised on high temperature superconductors and Josephson junctions, the main element of a SQUID sensor.”

Further reading

This article is based on the Letter: Laser controlled superconducting switch for low field NMR applications (new window).

A PDF version (new window) of this feature article is also available.

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Cover of Electronics Letters, volume 50, issue 08

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