AC Losses in MgB₂ Wires and Tapes in Frequencies up to 18 kHz

We present a study of the ac losses in MgB₂ superconductors under operation mode typical of applications such as SMES and HVDC, utilizing large dc current superimposed with small, switching frequency ac current. The ac losses in two MgB₂ superconductors-a wire with 36 filaments in Monel sheath and a tape with titanium sheath-are compared in a wide range of ac amplitudes and frequencies up to 18 kHz, at different temperatures and dc current levels. Strong influence of the sheath material on the ac losses was found. The wire with Monel sheath shows a strong nonlinear contribution of the magnetic material to the losses. The losses reduce as the Monel approaches saturation at high dc current. In contrast, losses in the tape with Titanium sheath are practically independent of the dc level, and are smaller than losses in the Monel wire for the whole range of measured parameters. The results demonstrate the importance of further development of nonmagnetic low-loss MgB₂ wires and tapes for applications that involve exposure to ac ripple current in switching frequencies.