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.
- uniform coverage of a large momentum and angle range, and symmetry around the beam axis,
- an open structure that allows for long path lengths for charge and neutral particles resulting in good particle identification through precise
- low background from electromagnetic processes to reach high luminosity.
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.
- A large opening for charged and neutral particles in the forward hemisphere. High momentum forward-going particles will not experience high enough
transverse field components from the solenoid field and must be momentum-analyzed in the torus field located downstream of the solenoid magnet. An
opening in polar angle up to ±40o is required.
- Good charged particle momentum resolution in a limited radial space for polar angles from 35o to 125o and momentum range from
0.3 to 1.3 GeV.
- Operation of a dynamically polarized target; requires a high magnetic field with homogeneity over the target volume of ΔB/B0 < 10-4.
- High-luminosity operation, the solenoid field offers an effective shield of the sensitive tracking detectors and guides them into areas where they
can dump their energy in specially designed tungsten absorbers.
- Compensation of the solenoid magnetic stray field; the CLAS12 detector setup requires locating magnetic field sensitive detectors in areas surrounding
the solenoid magnet. The field drops only slowly with radial distance, and must be shielded with additional compensation coils built directly into the
same cryostat as the solenoid coils and powered in series with the main coils. The field in the sensitive areas can be compensated to values below 25 G.
Last modified: May 5, 2013
Daniel S. Carman