LOSSES IN HIGH-TEMPERATURE SUPERCONDUCTORS AS A FUNCTION OF APPLIED-FIELD AND FREQUENCY

We have measured the losses at 77K in aligned YBCO for frequencies from 10 Hz to 20 kHz with amplitudes of applied field up to 100 Oe. The losses have been measured in two ways. First, the usual method of measuring the in-phase component of dB/dt was employed using the expression below W-v = 1/4 pi closed integral H dB = integral(0)(T) H-0 cos (omega t) dB/dt dt In this formula, W-v is the loss per cycle per unit volume while H = H-0 cos omega t and B are the applied field and resulting flux density, respectively. T is the period of the applied field. The components of dB/dt are measured with a phase sensitive detector (PSD). The second technique follows a suggestion by Gomory. He notes that, for a symmetric hysteresis loop, one can trace out the loop by measuring the output voltage of a PSD as a function of the phase angle between the applied field and the detector setting. From numerical integration of the first equation above, we can measure the loss per cycle independently. We find that the two methods give, within experimental error, identical results over the frequency range we have covered. The loss per cycle is roughly independent of frequency and at low applied fields, varies as the cube of the amplitude of the field. This is characteristic of the critical state model. Looked at more closely, however, the loss is seen to decrease at low (<100 Hz) and high frequencies (> 10 kHz). The first effect is consistent with thermally assisted flux flow (TAFF) coupled with the predictions of an extended critical state model while the second is in accord with this model that includes the viscous inhibition to flux flow.