Quench-Back Management for Fast Decaying Currents in SHMS Superconducting Magnets at Jefferson Lab

The Super High Momentum Spectrometer (SHMS) of Hall C, part of the 12-GeV upgrade at Jefferson Lab, was successfully commissioned in 2017. Early operation demonstrated that fast current dumps decays of the SHMS Q2/Q3 and dipole superconducting magnets triggered quenches, causing some level of operational difficulty. Tests and detailed assessments suggest that nonquench induced fast current decay could result in substantial ac loss in the conductor and subsequently trigger a quench-back effect. The OPERA/ELECTRA software package was used to calculate the amount of heat deposited in the copper stabilizer from a fast current decay. The resistances of external energy dump resistors were lowered to slow the fast dumping of the magnet's current to reduce or eliminate the quench-back effect. A worst-case adiabatic quench scenario was analyzed, assuming no external dump resistor and no liquid helium surrounding the coil, to ensure the safety of the magnets. The stress levels in the coil imposed by winding, collaring preload, Lorentz force, and temperature gradient during a quench were also examined. The Tsai-Wu material failure criterion was used to determine the acceptable combined stress level. Linear orthotropic analysis of the coil indicates that the magnets can be operated safely with appropriately sized dump resistors. Fast dump tests with the modified dump resistors have been planned to verify the performance and suitability.