by Dr. Allard Schnabel, Physikalisch-Technische Bundesanstalt (PTB) Berlin
13.02.2020, 17:00 h, TF, Seminar Room C-SR 1
Environments where the magnetic field is far below the earth magnetic field of 50 µT are a prerequisite for many modern precision experiments. Active magnetic field compensation with coil systems can reduce the external fields by up to two orders of magnitude. Passive magnetic shielding enclosures out of highly permeable material (µr >> 1) can provide volumes of up to 1 m3 with less than 1 nT static magnetic field. In combination with very sensitive magnetic field detectors like SQUIDs and OPMs, numerous basic physics experiments as well as biological studies have been carried out. In the past the driving force for the development of magnetically shielded rooms was brain research due to the spatial and temporal resolution of the neuronal activity. Today the strongest requirements are from basic physics experiments e.g. the search for a finite electric dipole moment of the neutron. These experiments need a homogeneous field of a few µT which should not change by more than 10 fT within 100 s.
Starting from the basic principles of magnetic shielding, commercially available shields will be discussed before the limits of the strongest existing magnetically shielded rooms, like BMSR-2 at PTB, are presented. In praxis, a larger shielding factor is associated with several restrictions which limit the usage of such shields. The demagnetization process (degaussing), necessary to achieve a low static magnetic field inside the shield, will be discussed in detail. It will also be explained why an “equilibration” of the shielding material is needed to obtain a field stable in time when an additional magnetic field is switched on or used inside the chamber.