Impact of Stator Wire Width on Output of a Dynamo-Type HTS Flux Pump

Superconducting flux pumps enable dc supercurrents to be injected into a superconducting coil without direct connection to an external current supply. Here, we report experimental data from a dynamo-type flux pump employing a high-T c superconducting stator. This device employs a mechanical rotor that rotates outside the cryogenic envelope, and excites the superconducting circuit through the cryostat wall via a magnetic circuit formed between the rotor and stator. We show that the width of the stator wire employed in this device has a significant effect on its output performance. At low frequencies, both the short circuit current, I sc , and the open-circuit voltage, V oc increase with stator width, and we obtain a maximum value of I sc > 340 A using a stator wire width of 46 mm. However at higher operating frequencies, we observe that I sc reaches a maximum value before dropping with any further increase in stator width. We attribute this to thermally dissipative effects due to circulating eddy currents in these wide superconducting stators. At frequencies above ~400 Hz, we observe a sharp rise in the internal resistance of the stator wire that we attribute to a local quench due to the eddy currents driven by the local electromotive force (EMF) beneath the rotor magnet. This paper emphasizes the importance of frequency dependence when defining optimum operating parameters of a high-T c superconductor (HTS) flux pump. Our results also offer the promise that very high current flux pumps may be developed through maximizing the ratio of the stator wire width to the peak width of the rotor field profile.