Alternating current losses of a 10 metre long low loss superconducting cable conductor determined from phase sensitive measurements

The ac loss of a superconducting cable conductor carrying an ac current is small. Therefore the ratio between the inductive (out-of-phase) and the res istive (in-phase) voltages over the conductor is correspondingly high. In v ectorial representations this results in phase angles between the current a nd the voltage over the cable close to 90 degrees. This has the effect that the loss cannot be derived directly using most commercial lock-in amplifie rs due to their limited absolute accuracy. However, by using two lock-in am plifiers and an appropriate correction scheme the high relative accuracy of such lock-in amplifiers can be exploited. In this paper we present the res ults from ac-loss measurements on a low loss 10 metre long high temperature superconducting cable conductor using such a correction scheme. Measuremen ts were carried out with and without a compensation circuit that could redu ce-the inductive voltage. The 1 mu V cm(-1) critical current of the conduct or was 3240 A at 77 K. At an rms current of 2 kA (50 Hz) the ac loss was de rived to be 0.6 +/- 0.15 W m(-1). This is, to the best of our knowledge, th e lowest value of ac loss of a high temperature superconducting cable condu ctor reported so far at these high currents.