Energy margins in a dry-winding superconducting test coil Part 1: Dissipation within the conductor

Abstract

To study the role of liquid helium trapped within the winding of ‘dry’ superconducting magnets, energy margins were measured in a dry-winding superconducting test coil. The test coil comprised five non-inductive layers, each wound with circular cross-section multifilamentary NbTi superconducting wire. A pulsed coil, applied only through the two middle layers of the test coil carrying a transport current in a background magnetic field, was used to simulate a transient disturbance induced within the conductor over a confined region of the magnet. Measured energy margins, with the test coil immersed in liquid helium or in gaseous helium, agree well with the conductor's enthalpy densities required to drive the conductor normal. Experimental results show that the liquid helium occupying the void space within the winding has no beneficial effect against transient disturbances induced internally in the conductor; it does, however, slow down quench propagation. Because of the presence of an insulation layer at the conductor surface, the winding is effectively adiabatic against internal disturbance pulses. The trapped liquid helium is expected to be beneficial against transient heating applied external to the conductor surface, as would be the case with mechanical disturbances.