PTB News 3/2004 (149 kB) PTB News 3/2004, English edition, Issue January 2005PTB has developed easy-to-use highly sensitive current sensors with specially designed SQUIDs. Due to their high robustness they can be operated in interfering magnetic fields of the same order of magnetitude as the earth's field. Therefore, they are suitable for a wider range of technical measurement tasks.
SQUIDs (Superconducting QUantum Interference Devices) are generally applied for very sensitive magnetic-field measurements. They can also be used to measure smallest electrical currents in an excellent manner. They are outstandingly suited as pre-amplifiers for specific types of radiation detectors, such as micro-calorimeters or superconducting hot-spot-detectors, which – like the SQUIDs themselves – are operated at temperatures at or below liquid Helium temperature (4.2 K).
In a project funded by BMWA (German Federal Ministry for Economy and Labour) PTB has developed SQUID current sensors together with Vericold GmbH in Ismaning, and Magnicon GbR in Schenefeld. The new devices are specifically designed for applications in energy-dispersive x-ray spectroscopy. However, they can also be used for other technical measurement tasks, such as realization and dissemination of electrical units.
Usually, in a SQUID operated as a current sensor the current to be measured is fed through a coil. The coil is aligned so that its magnetic flux penetrates the SQUID. PTB has achieved excellent noise levels (<1 pA/=Hz for frequencies above 1 kHz) with this type of current sensors.
The circuits used for these devices were made using Niobium as superconducting material, produced by multilayer technology with structural widths of 2.5 µm. Recently, the first current sensor based on a new interference-insensitive design was tested successfully. Due to the gradiometric design and in spite of their high sensitivity these sensors can be cooled down unshielded in interfering magnetic fields with amplitudes in the order of the earth’s field (approx. 50 µT) and can be operated without degradation of their characteristics or current noise. Furthermore, the sensors can be mounted directly on to massive copper substrates, which usually lead to higher noise, without deteriorating their parameters. Such a mounting enables a good thermal contact down to milli-Kelvin temperatures and simplifies the cryogenic setup. In the latest version up to 16 SQUIDS are connected in series to increase the dynamic range.
The simultaneously furthered development to improve the electronics used to read out the sensors aims at extending the bandwidth from presently 5 MHz to 20 MHz. Interested users can acquire the new current sensors commercially through the company Magnicon GbR. The company also produces the PTB electronics under license and distributes them.