ENERGY MARGINS IN A DRY-WINDING SUPERCONDUCTING TEST COIL .1. DISSIPATION WITHIN THE CONDUCTOR

To study the role of liquid helium trapped within the winding of 'dry' superconducting magnets, energy margins were measured in a dry-windin g superconducting test coil. The test coil comprised five non-inductiv e layers, each wound with circular cross-section multifilamentary NbTi superconducting wire. A pulsed coil, applied only through the two mid dle layers of the test coil carrying a transport current in a backgrou nd magnetic field, was used to simulate a transient disturbance induce d within the conductor over a confined region of the magnet. Measured energy margins, with the test coil immersed in liquid helium or in gas eous helium, agree well with the conductor's enthalpy densities requir ed to drive the conductor normal. Experimental results show that the l iquid helium occupying the void space within the winding has no benefi cial effect against transient disturbances induced internally in the c onductor; it does, however, slow down quench propagation. Because of t he presence of an insulation layer at the conductor surface, the windi ng is effectively adiabatic against internal disturbance pulses. The t rapped liquid helium is expected to be beneficial against transient he ating applied external to the conductor surface, as would be the case with mechanical disturbances.