Electromagnetic and Mechanical Analysis of the Coil Structure for the CLAS12 Torus for 12 GeV Upgrade

Abstract

The torus magnet for the CLAS12 spectrometer is a 3.6 T superconducting magnet being designed and built as part of the Jefferson Lab 12 GeV Upgrade. The magnet consists of six coil case assemblies mounted to a cold central hub. The coil case assembly consists of an aluminum case and cover enclosing an epoxy vacuum impregnated coil pack. The coil pack consists of a 117 turn double-pancake winding wrapped with 2 layers of 0.635 mm thick copper cooling sheets. The coil case assembly is cooled by supercritical helium at 4.6 K. This report details the structural analysis of the coil case assembly and the assessment of the coil pack stresses. For the normal operation of the torus magnet, the coil case assembly was analyzed for cool down to 4.6 K and the Lorentz forces at normal operating current. In addition to the normal operating configuration, the coil case assembly was analyzed for Lorentz forces arising from coil misalignment and current imbalances. The allowable stress criteria for the magnet followed the approach of the ASME codes. Primary stresses were limited to the lesser of 2/3 times the yield strength or 1/3 times the ultimate tensile strength. Primary plus secondary stresses were limited to 3 times the primary stress allowable. The analysis was performed using ANSYS Maxwell to calculate the magneto-static loads and ANSYS Mechanical to calculate the stresses.

Magnetic Field Mapping of the CLAS12 Torus—A Comparative Study Between the Engineering Model and Measurements at JLab

“…The magnet design was carried out by Jefferson Laboratory [2,5] with each coil layer consisting of 117 turns of surplus Superconducting Super Collider dipole (SSC) NbTi Rutherford cable soldered into a stabilizing copper channel as shown in Fig 2(a) [4]. A typical coil structure is shown in Fig.…”

“…The cable was wrapped in E-glass and a layer of G10 sheet was used between coil layers to prevent interlayer shorts. Ground insulation on the edges of the coil consists of multiple layers of half-lapped E-Glass cloth and G10 strips directly adjacent to the cooling tube which is also wrapped in 2 layers of glass tape as shown in Fig 2(c) [2,6]. After coil winding was successfully completed, the coil underwent a 2-step epoxy (CTD101K) impregnation [6].…”

“…The torus magnet is based on six superconducting racetrack-type coils arranged around the beam line to provide uniform coverage of a large angular and momentum range of produced particles as shown in Fig. 1 [2,3]. The coil configuration produces a toroidal magnetic field that is approximately transverse to the particle trajectory, and a field free region around the target, allowing the operation of polarized targets.…”

“…These are double pancake coils vacuum impregnated with epoxy and assembled into an aluminum case. They are indirectly cooled by supercritical helium gas, producing what is referred to as the coil cold mass (CCM) [2,4]. Each coil within the aluminum case is conduction cooled via a series of thin copper sheets wrapped around each coil.…”

Overview of Torus Magnet Coil Production at Fermilab for the Jefferson Lab 12-GeV Hall B Upgrade

“…The magnet consists of 6 torus coils arranged in a single electrical circuit, and utilizes a single supercritical helium cooling circuit [4], [5]. Each of the magnet coil cold masses is housed in a vacuum vessel shell and supported at a central cold hub, as well as by cold columns at the outside perimeter creating a hexagonal torus as seen in Fig.…”