HP Labs Technical Reports
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AC Loss in Nb(sub 3)Sn Superconducting Cable - Mechanisms and Measurements
Larson III, John D.; Mitchell, Roger; Good, Jeremy A.; Lopez, Jose R.
Abstract: The AC dissipation of Nb(sub 3)Sn superconducting cable, formed in a Maxwell pair circuit configuration and operated in temperatures of 8 degrees K or 12 degrees K, is measured. The circuit is driven by an AC current, ranging in frequency from 10 Hz to 10 kHz, and at selected peak amplitudes up to 50 Amperes. The test circuit reactance, dissipated power, and drive current are the measured data. Combining these into a complex impedance Z, the resistive part R(sub ac) (of the order of mircoOhms) is found to be quite small compared to the inductive part Omega-L (of the order of 1 milliOhm), which makes R(sub ac) difficult to measure with conventional impedance meters. A new calorimetric measurement technique, sensitive primarily to the resistive loss, is described. Precision phase angle measurements, critical lead placement, and inductive compensation are thus eliminated. The calorimeter measures dissipated AC power in the Maxwell pair, by substitution of DC power in two nearby auxiliary heater coils. The accuracy is set by the system stability, typically (Delta P)/P of the order of plus or minus 1%, during an experiment. Several superconducting cables were measured. The result is data for the equivalent series resistance R(sub ac) vs. frequency. In addition, the effect of AC drive current amplitude, operating temperature, and applied magnetic flux intensity B(sub 0) on R(sub ac) is experimentally evaluated. A simplified theory of AC loss mechanisms in superconductors is developed. Two mechanisms, hysteresis loss in the superconductor and inductive-Ohmic loss in the metal jacketing, account for the observed resistance R(sub ac). A series resistance R(sub H), increasing linearly with frequency, and a shunt conductance G, emerge from the theory as good descriptors of the measured data.
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