High homogeneity split pair magnet system for muon spin resonance (μSR) spectroscopy
This magnet system provides a high homogeneity, specific field profile in a horizontal room temperature bore tube, at 8 T (4.2 K) and 9.5 T (2.2 K). Field alignment is precisely made between the magnetic and mechanical axes within the bore.
A particular challenge of this magnet system was to be able to maintain homogeneity from very low fields right up to 9.5 T. This was achieved using independent shims in conjuction with custom-written software to set the shim currents automatically from the magnetic field being set by the magnet power supply.
To achieve the upper field without resorting to Nb3Sn conductors, a lambda point refrigerator (LPF) system was used. Since the main liquid helium bath is maintained at 4.2 K and atmospheric pressure, a pulse tube (PTR) recondensing system could be fitted to give zero liquid helium consumption during static conditions. Further, this removes the need for users to enter the beamline area to refill liquid nitrogen since the PTR cools the radiation shields.
The recondensing system has sufficient power to recirculate the LPF exhaust back into the magnet system, and reliquefy it, providing full recondensation with the magnet persistent at 2.2 K.