Laser‐controlled superconducting switch for low‐field NMR applications

Abstract

In the case of a low-field nuclear magnetic resonance (NMR) system, the sensible magnetic sensor can be screened or even damaged by the large magnetic field produced during the polarisation phase. A switch is then necessary to preserve this sophisticated sensor. A simple and efficient switch is described here. It is made of a twisted and wound wire heated by a laser beam, in which the alignment is simple by construction. The switch can be used without any additional contact in sensing coils, has a low inductance and flips within milliseconds. It is shown that the switch resistance in open conditions is sufficient for low-field NMR applications and that a memory behaviour can be exhibited.Introduction: In low-field nuclear magnetic resonance (NMR) and magnetic resonance imaging applications, very sensitive magnetic sensors are required, and usually superconductor quantum interference devices (SQUIDs) are used [1]. During the magnetisation phase of a lowfield NMR experiment, a large magnetic field is applied to the specimen in order to enhance the NMR signal by recruiting more spins [2], but this field can saturate and even damage the SQUID sensor. It is therefore necessary to turn the SQUID sensor off during this phase by an appropriate device. For this purpose, Josephson junction arrays are often used [3], but this switch is expensive, hard to include in the measurement system and cannot be controlled by an external signal. Other switch technologies use the properties of superconducting materials, whose state depends on temperature, magnetic field or current density. Under a given critical value of these parameters, the material is in the superconductive state, whereas in the above, the material flips to normal state by exhibiting a resistivity. Thermal or magnetic controlled switches can be set to any state at any time by simply controlling the temperature or the magnetic field at a given part of the superconducting material. For lowfield NMR applications, a thermal control is preferred because a magnetic control could induce spurious effects in the measuring system. A recent device, based on thermal control, was developed and has been commercialised by a German company [4]. It is essentially formed by a thin niobium layer encapsulated in an epoxy coating. The heating source is produced by the Joule effect when a current flows through an adjacent heating resistance. Using this method, an induced resistance of up to a few hundreds of ohms can be obtained in a few microseconds. This switch is cheap and controllable; however, it remains brittle to a cryogenic environment and could be damaged by fault current higher than a few tens of milliamps. Moreover, the leads used to heat up the device can be a significant source of noise because of electromagnetic interference. In addition, the insertion of the device in the measurement system requires additional contacts which can be a source of problems. In this Letter, we present a thermally controlled superconducting switch. Thermal energy is brought by a laser beam throu...