CLAS12 Superconducting Magnets

CLAS12 contains two superconducting magnets, a 5 T solenoid and a 6-coil torus magnet with a 3.6 T peak field. The two magnets provide magnetic analysis of charged particles in the large-angle range and in the forward-angle range, respectively. The magnets are separated by about 1.5 m, and their respective fields are partially overlapping. Owing to its symmetry properties, the toroid field drops rapidly with distance and has virtually no impact on the solenoid magnet. In particular, since its field is zero on the beam axis, it does not affect the homogeneity of the solenoid magnet in the critical target region. The solenoid field drops more slowly with distance and exerts a measurable force on the coils of the torus magnet that must be taken into account in the mechanical design of that magnet. At the closest distance between the solenoid and torus magnets, the coil deflection is about 1.3 mm, which due to the cylindrically symmetric solenoid field, affects all torus coils in the same way.

The figure to the right shows the CLAS12 transverse magnetic field as a function of the distance z from the solenoid center (where the target will be located) for three polar angles of 10o, 20o, and 30o in the mid-plane of one toroid sector. The torus field is strongest at small polar angles and weakest at large angle, while the opposite is the case for the solenoid field. Note that the transverse field component is the one providing momentum analysis for charged tracks.

The CLAS12 Torus Magnet

The Torus magnet is based on six superconducting coils arranged symmetrically around the beam line to generate a field primarily in the azimuthal (φ) direction. The choice of this configuration leads to an approximate toroidal field distribution around the beam axis. It has been driven by the necessity of satisfying the precise requirements determined by the physics program. Some critical requirements are: The toroid configuration offers a field-free region around the beam axis and a magnetic field that is always transverse to the particle trajectory resulting in optimal momentum resolution for charged particles. In addition, since the φ pattern of the event is preserved in the toroidal magnetic field, the determination of the azimuthal angle is decoupled from the measurements of the polar angle and momentum. The six coils have been shaped to give the desired ∫Bdl, and therefore, the requested resolution as a function of θ. The coil operating temperature of 4.5 K is maintained by a forced flow of supercritical liquid helium. The table shows some of the main technical parameters of the magnet.

The CLAS12 Solenoid Magnet

Solenoid magnets provide an ideal field distribution for the analysis of particle trajectories in the central region, where the bending power of the solenoid field is at a maximum. In CLAS12 the choice of a 5-T strong solenoid field has been driven by the necessity of satisfying precise requirements determined by the physics program. These requirements include: The solenoid magnet provides a strong magnetic field needed for a dynamically polarized solid-state target. At the same time, the solenoid field is used for particle tracking and momentum analysis by measuring the trajectories of charged particles in the field with high-resolution tracking detectors.

Last modified: May 5, 2013
Daniel S. Carman