High-Current ac Losses in Large Superconducting Niobium Tubes

Loss measurements were made at 60 Hz on 6‐m lengths of 1‐ and 3‐cm‐diam superconducting niobium tubes used as the inner conductor of a coaxial line. These tubes carried transport currents up to 1700 and 5300 A, respectively. The losses were found to fit the relation $${\mathrm{P=fAE}}_{\text{c1}}{\mathrm{(H/H}}_{\text{c1}}{)}^n$$ , where f is the frequency, A is the surface area, E_c1 is the energy loss per unit surface area per cycle at H_c1, the lower critical magnetic field, and H is the peak magnetic field at the surface of the superconductor. The exponent n equals 3 for H≤H_c1 and n≥4 for H≥H_c1. The ac losses showed a 25% linear increase with temperature from 4.2 to 5 K. For the case of an eccentric coaxial line, where the current and magnetic field vary about the circumference of the superconductor, the ac losses were found to agree with those predicted from calculated values of the current and magnetic field distributions. In particular, when the peak surface field is less than H_c1 and R is the ratio of the maximum to minimum surface magnetic fields about the circumference of the superconductor, the losses were a factor (3R²+2R+3)/4R greater than those found at an equivalent current for a concentric configuration.