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Superconducting Spectrometer Magnet, G0 Lynchburg, VA-- A large toroidal superconducting magnet system is being assembled at the Lynchburg Magnet Facility of BWX Technologies. This magnet is slated for recoil spectrometer of a Photonuclear Physics experiment to be performed at Jefferson Lab in 2001. BWXT is under contract to the University of Illinois http://www.npl.uiuc.edu/exp/G0/G0Main.html to design and build the magnet system for this experiment. The magnet consists of 8 large superconducting coils arranged around the axis of a stainless steel and aluminum vacuum vessel, as shown in the scale model below.
The magnet system has a number of significant and novel features. BWXT was provided with the required functional specification for the cold excited current distribution in space, and had to back out a warm as constructed design that would meet those requirements. The resulting warm design has very tight tolerances on the conductor thickness, turn spacing, and coil shape that required detailed warm mapping of the field for verification prior to the start of the cold mass assembly. The coils have been completed and assembled, as shown below.
The cryo-cooled production target for the experiment is to be located at the center of the magnet. In order to sort the desired scattered particles, several tons of lead collimators block mounted on aluminum frames are interleaved with the coils. In the operational orientation, these collimators are clocked at all angles, resulting in complex static loads that must be compensated to preserve the sector field symmetry. The large attractive forces (ala tokamak toriodal coils) must in addition be bucked at the center. Extensive coupled structural-thermal and structural-electromagnet calculations were performed to validate the design basis during the final design process. This analysis included the conductor and turn, layer and ground insulation in addition to all structural components. The magnet was solid modeled from that resulting analysis basis and fabrication drawings created directly from the 3D solid models.
All of the coil structure is made of aluminum in order to preserve the symmetry of the distribution of current in space. The coils are conductivity cooled through internal channels bored in the aluminum bobbins. Cryogens are circulated in 4 thermal-siphon loops of two coils each. The coils are composed of two double pancakes on roughly rectangular bobbins. In general, conductors were place to within 2-3 mils of their intended locations during winding. The field symmetry requirements of the eight magnetic sectors are such that vessel materials and construction were also critical to the design solution. Materials with magnetic permeability greater than 1.003 were prohibited in the vessel end caps- resulting in structural aluminum as the construction material of choice, but difficult given the large size and required flat planar geometry. Materials with magnetic permeability greater that 1.002 were prohibited for use the vessel shell, but envelope constraints required minimum overall size. This was solved by developing a special stainless steel chemistry and welding techniques, that resulted the 42 foot circumference vessel having peak magnetic permeability's not greater that 1.007-1.009.
The final assembly of the magnet is now in progress and acceptance testing will be performed at UIUC in September. Final shipping to Jefferson Lab should occur in early 2001. For more information contact: Dr. Timothy A. Antaya |
