A high current density bath-cooled superconducting solenoid with varnish-coated conductor

A solenoid with 20-cm inner diameter, 28.5-cm outer diameter, and 23.9-cm length of the winding has been designed and constructed to test a design using a varnish-coated conductor and narrow helium channels between layers for high-current-density magnets. The conductor used was a rectangular multifilamentary twisted NbTi composite with copper-to-superconductor ratio equal to 2. The magnet is a typical layer-wound solenoid with liquid-helium channels between layers. The testing showed that the magnet achieved about 90% of the short-sample performance without training. The recovery of the normal zone was observed up to the point where the G-value (heat generation per unit cooled surface) was 0.55 W/cm². The critical temperature margin was 0.7 K, the overall current density was 15 kA/cm², and the corresponding G-value was 6.1 W/cm². For higher magnet currents, the normal zone propagated with high velocity, indicating the magnet could be self-protecting.