Thermal diffusion and quench propagation in YBCO pancake coils wound with ZnO and Mylar insulations

Abstract

The thermal diffusion properties of several different kinds of YBCO\ninsulations and the quench properties of pancake coils made using these\ninsulations were studied. Insulations investigated include Nomex, Kapton, and\nMylar, as well as insulations based on ZnO, Zn2GeO4, and ZnO-Cu. Initially,\nshort stacks of YBCO conductors with interlayer insulation, epoxy, and a\ncentral heater strip were made and later measured for thermal conductivity in\nliquid nitrogen. Subsequently, three different pancake coils were made. The\nfirst two were smaller, each using one meter total of YBCO tape present as four\nturns around a G-10 former. One of these smaller coils used Mylar insulation\nco-wound with the YBCO tape, the other used YBCO tape onto which ZnO based\ninsulation had been deposited. One larger coil was made which used 12 total\nmeters of ZnO-insulated tape and had 45 turns. The results for all short sample\nand coil thermal conductivities were ~1-3 Wm-1K-1. Finally, quench propagation\nvelocity measurements were performed on the coils (77 K, self field) by\napplying a DC current and then using a heater pulse to initiate a quench.\nNormal zone propagation velocity (NZP) values were obtained for the coils both\nin the radial direction and in the azimuthal direction. Radial NZP values\n(0.05-0.7 mm/s) were two orders of magnitude lower than axial values (~14-17\nmm/s). Nevertheless, the quenches were generally seen to propagate radially\nwithin the coils, in the sense that any given layer in the coil is driven\nnormal by the layer underneath it.\n