An ultra-thin-walled superconducting solenoid for meson-decay physics

Abstract: A compact and extremely thin-walled superconducting warm-bore solenoid magnet, has been developed for the WASA detector setup at the CELSIUS accelerator and storage ring of the The Svedberg Laboratory. It provides a central axial magnetic field strength of 1.3 tesla, and can be operated in a persistent current mode during several weeks without a significant decline of the field strength. The overall wall thickness of the magnet is a record low 0.18 radiation lengths which is equivalent to 16 millimetres of aluminium.The WASA detector setup will be used for the measurement of inelastic protonnucleon reactions, in particular the production of light mesons and their subsequent decays. It is capable of identifying charged particles and photons over a solid angle close to 4π with a high measurement accuracy in energy and track coordinates. Some π0 and η rare decay processes that are accessible with the WASA detector setup can be used for probing the limits of the Standard Model. Accurate studies of such decays require a calorimeter for the energy measurement of photons and a magnet for the momentum measurement of charged particles. The thinwalled magnet is placed inside of the cylinder-shaped calorimeter and not outside, as it is traditionally done in comparable detector setups. Due to this arrangement, the calorimeter readout electronics do not have to operate in a magnetic field.The magnet design has been optimized for momentum measurements while the wall thickness was minimized in order to avoid electromagnetic showers and to maintain accurate energy measurements in the calorimeter. It is made as a split-coil solenoid with a central gap of 40 millimetres creating an opening for a tube guiding small frozen hydrogen pellets into the circulating beam. The coil is 9 millimetres thick with a length of 465 millimetres, a diameter of 554 millimetres and a mass of 20 kilograms. It has a glass-fiber support structure, a cryostat with 1 mm thick corrugated walls of aluminium and high-purity aluminium strips forimproved heat conduction. The cryostat has only one radiation shield and is cooled by a small helium refrigerator. A flexible diagnostics and control system has been developed. The calorimeter readout electronics are shielded from the magnetic field by an iron yoke.