Speaker
Description
$RE-Ba_2Cu_3O_{7−δ}$ (REBCO, RE = rare earth) coated conductors maintain high current densities in the presence of large magnetic fields, making REBCO a promising conductor for use in high magnetic field applications. Unfortunately, the manufacturing process produces defects, resulting in significant drops in the critical current, $I_c$, along the length of the conductor. Additional variations in $I_c$ can occur due to the anisotropic behavior of REBCO with respect to magnetic field orientation, which can reduce $I_c$ by a factor of 2 depending on field orientation at 77 K. Conductor on Round Core (CORC) cables contain multiple layers of helically wound REBCO tapes, producing a flexible, isotropic conductor that promotes current sharing between layers. This could allow current to bypass local drops in I_c and minimize the risk of hot spot formation. The current sharing capability of CORC cables depends on the tape-to-tape contact resistance, $R_c$, which can be quite large since the tapes in CORC cables are not generally soldered in order promote cable flexibility. Measurements were conducted on several CORC cables to achieve a better understanding of how different winding parameters, cable bending, and magnetic field affect $R_c$. We found that $R_c$ can take on values from 10 to over 1,000 $\mu\Omega\cdot cm^2$, resulting in a broad range of current transfer length between layers. Further experiments isolated 1 or 2 tapes with a significant drop in $I_c$ for detailed studies, from which the effects of current transfer were evident on the current-voltage transition data. We compare cables that permit current transfer around these $I_c$ drops to cables with insulation between tape layers, disabling current sharing, allowing us to clearly see the impact of current sharing on cable $I_c$.
