Influence of surface layer and trapped flux on minimum A.C. losses in superconductors

The influence of a surface layer trapped magnetic flux on low frequency (115 Hz) losses in Nb - 50% Ti superconducting alloy were investigated in the presence of a static and an alternating magnetic field, both parallel to the long side of the sample. A minimum in the a.c. loss versus the bias magnetic field was observed at a constant value of the magnetic field amplitude which exceeded the first penetration field. It is shown that the minimum of the a.c. loss is strongly influenced by trapped flux in the sample. The depth of a. c. loss minimum is decreased as the trapped magnetic flux increases. The position of the minimum as a function of the static magnetic field is studied for different amplitudes and the trapping field.