Stabilization of superconducting dry solenoids

Premature quenches in superconducting solenoids wound with Formvar-coated NbTi conductors have been studied. Some model coils wound with various tensions were tested. The experimental results are discussed with attention given to the stress distribution for coil winding, cool-down to liquid helium temperature, and energization at 4.2 K. Some mechanisms of premature quench are classified according to the winding tension, and some stabilization methods are presented on the basis of these quench mechanisms. It is found that if a solenoid is wound loosely, macroscopic slips occur in inner layers due to low frictional force. Although such motions are removed if the coil is wound tightly, shear stress concentration at the interface between the winding and the bore tube then causes quenches by microslips. Inserted polymer films affect the sliding behavior of the conductor, which can improve the coil performance.<
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