World Record High Field Magnet for Neutron Scattering

At the end of 2009 the pioneering split pair magnet was successfully tested in the beam line of the Spallation Neutron Source SINQ of the Paul Scherrer Institute (PSI) in Switzerland. In early 2010 the commissioning will take place at the Spallation Neutron Source SNS Oak Ridge USA. This novel magnet is a prerequisite for a new class of experiments using the scattering of polarized neutrons in high magnetic fields.

In their joint project for the split pair magnet PSI, SNS and Bruker BioSpin have defined three main goals. Firstly, to achieve the highest possible magnetic field within the given constraints of size and weight; secondly, an active shielding of the magnet’s stray field and thirdly, a unique magnet design for operation in symmetric or asymmetric field mode.

Up to now the highest magnetic field reached by superconducting split pair magnets for neutron scattering experiments was 14.9 Tesla. Extensive calculations and simulations revealed that a maximum field of 16 Tesla in symmetric field mode and within the given constraints should be feasible.

One prerequisite to reach this goal was the exploitation of the best available superconducting wires on the market. Beside the wires, the very large electromagnetic forces and the selection of adequate materials were especially big challenges.

Active shielding of the stray field was only available for low field split pair magnets up to now. The stray field often limits maximum field because of strong electromagnetic interferences between magnet and instrumentation. To implement the active shielding for the highest field split pair magnet is a big step forward and was possible because Bruker BioSpin could benefit from its long-term experience in NMR magnets, where active shielding is already an established technology.

The operation in asymmetric field mode is a requirement for experiments with polarized neutrons. In order to maintain the polarization of the neutrons in the incident and scattered beam, zero field nodes have to be avoided and the change rate of the field direction has to be limited along the neutron beam path.

This is achieved by means of a symmetric magnet layout with two independent but asymmetric current loops. If both current loops bear the same current the field profile is symmetric, but if only one current loop is charged, the resulting field profile is asymmetric and any zero field nodes will be shifted away from the neutron’s beam path. To control the forces and the quench behavior, innovative design ideas had to be simulated, tested and implemented.

In addition to the magnet, a new split pair cryostat, VTI (Variable Temperature Insert) and power supply was developed. The VTI offers a wide temperature range from 2 K to 300 K and was designed in collaboration with the Institut Laue-Langevin (ILL) in Grenoble, France.

With the recently performed tests at PSI it was successfully demonstrated that the goals of this pioneering and challenging project were fulfilled, combining a world record high field, asymmetric field mode and first active shielding of a high field split pair magnet.

Acknowledgement
We would especially like to thank all team members at PSI, SNS, ILL and Bruker BioSpin for the fruitful work and collaboration. We kindly acknowledge supporting and funding of this project by the following organizations: SER, State Secretariat for Education and Research, Switzerland; PSI, Paul Scherrer Institut; MaNEP, Materials with Novel Electronic Properties, Switzerland; SGN/SSDN, Swiss Neutron Scattering Society; SNS, Spallation Neutron Source, Oak Ridge, USA; Bruker BioSpin AG, Switzerland.

[ Scientific Highlight , page 2 ]