The G0 Spectrometer superconducting magnet system: From a challenging construction to reliable operations

We report on the design, fabrication, commissioning and operation of a large superconducting magnet system that is an important element of the 8 sector super conducting toroidal GO Spectrometer located at Jefferson Lab (JLAB) in Newport News, VA. The purpose of the GO experiment is the high precision measurement of polarized electron scattering by protons to isolate the strange quark content of normal baryonic matter by observing parity violation caused by the weak interaction. The GO spectrometer has been operating for three years and first results are submitted for publication [1]. The G0 SC torus is 4 meters long and 4 meters outside diameter and produces 3 Tesla in the 8 gaps that are accessible to particles. The realization of this 8 sector superconducting toroidal magnet required the development of a number of challenging large scale features including: large total open solid angle, high sector-sector field symmetry, the symmetry axis aligned perpendicular to gravity, the location of the liquid hydrogen (proton) target on axis in the magnet cryostat, and large surface area but thin titanium exit windows on one end of the cryostat. The cryostat consists of a super-alloy welded low permeability stainless steel shell (to minimize magnetization effects) and aluminum end caps. The 8 superconducting coils have unique characteristics including dry pancake wound copper stabilized NbTi conductors, encased in aluminum structure, mechanically preloaded and indirectly cooled by a set of parallel thermo siphon circuits. This magnet was built by BWXT under a fixed price performance contract that included fabrication to a defined ideal cold current spatial distribution. The commissioning and operations will be discussed.