During February, the TransHELIOS project team [1] successfully cooled down and operated the 3T high-homogeneity solenoid that was donated to the project by Nottingham University. The superconducting magnet was originally built by Oxford Magnet Technology in 1990 and used for MRI research at Nottingham. At Daresbury, it was installed in the old SRS hall and cooled using liquid nitrogen and liquid helium down to a temperature of 4.2K (-269°C). Finally, under the supervision of engineers from Siemens Magnet Technology (now the owner of OMT), the magnet was energised with a current of 250A.
The magnet was ramped to a current of 125A and held there, proving that it can generate a central field of 1.5T. This is the minimum field level required for most experiments envisaged at the HIE-ISOLDE [2] facility at CERN, where the magnet will eventually be used. During the ramp up to full field (3T), a quench occurred at a field of about 2.9T within the magnet.
A quench can occur when part of the superconducting wire inside the magnet becomes normal conducting – i.e. resistive – and generates heat, turning neighbouring regions normal conducting. This leads to a chain reaction where all of the cryogenic liquid is rapidly boiled off. Such events (‘training’ quenches) are common for new superconducting magnets, where the large stresses present can lead to small movements within the wires, generating eddy currents and resulting in resistive regions. This magnet had operated stably for over 20 years, but had then been powered down and transported from Nottingham to Daresbury; this increased the risk of a quench. This risk had been judged to be acceptably small. In any case, the quench took place with almost the maximum current (250A) in the magnet, proving that it is still operable at high fields.
The TransHELIOS project team gained valuable expertise in operating this magnet, and the collaboration, led by David Jenkins at the University of York, will continue their work. Eventually this magnet is planned to be installed at the HIE-ISOLDE facility at CERN, and we have made several important steps in ensuring its success.
