Anomalous open-circuit voltage from a high-Tc superconducting dynamo

Abstract

We report on the behavior of a high-T c superconducting (HTS) homopolar dynamo which outputs a DC open-circuit voltage when the stator is in the superconducting state, but behaves as a conventional AC alternator when the stator is in the normal state. We observe that this time-averaged DC voltage arises from a change in the shape of the AC voltage waveform that is obtained from a normal conducting stator. The measured DC voltage is proportional to frequency, and decreases with increasing flux gap between the rotor magnet and the HTS stator wire. We observe that the DC output voltage decreases to zero at large flux gaps, although small differences between the normal-conducting and superconducting waveforms are still observed, which we attribute to screening currents in the HTS stator wire. Importantly, the normalised pulse shape is found to be a function of the rotor position angle only. Based on these observations, we suggest that the origin of this unexpected DC effect can be explained by a model first proposed by Giaever, which considers the impact of time-varying circulating eddy currents within the HTS stator wire. Such circulating currents form a superconducting shunt path which "short-circuits" the high field region directly beneath the rotor magnet, at those points in the cycle when the rotor magnet partially overlaps the superconducting stator wire. This reduces the output voltage from the device during these periods of the rotor cycle, leading to partial rectification of the output voltage waveform and hence the emergence of a time-averaged DC voltage.

Open circuit voltage generated by dragging superconducting vortices with a dynamic pinning potential

“…In this case, the peaks occur when the voltage increment produced by the synchronous vortices is equal to the voltage decrement caused by the laggard vortices. The lower panel of figure 3(d) shows that at high sliding velocity all vortices slip out from the dragging potential and arrange themselves forming dynamic vortex chains in the fast pinning potential channels (see supplemental movies for figures 3(c)-(e) 8 ). With further increasing magnetic field, as shown in figure 3(e), the interstitial vortices can be observed between dynamic vortex chains.…”

“…An analogous outcome can be expected by dragging the vortex lattice with a sliding potential as the one created by an inhomogeneous magnetic field. This approach has been recently proposed by Bumby et al [8] to generate an open-circuit voltage from a high-T c superconducting dynamo. A similar coupling has been explored in other bi-layer systems, such as the coupling between vortex lattice and an electron gas in superconductor-semiconductor hybrids [9], the Coulomb interaction in two coupled electronic layers (electron-drag effect) [10,11], and the interplay between the vortex lattices and corotating optical lattices [12].…”

A Rotating Flux Pump Employing a Magnetic Circuit and a Stabilized Coated Conductor HTS Stator

“…Previous works have already demonstrated mechanically rotating barrel-type flux pumps which employ a CC HTS stator and a rotor comprising arrays of rotating permanent magnets, and these flux pumps have been used to inject currents into HTS coils [3][4][5][6][7][8]. In practical applications of a rotating flux pump, a preferred arrangement would allow the rotor to be located at room temperature, whilst the HTS stator is connected to an HTS coil that is located within the cryogenic environment.…”

“…Superconducting flux pumps enable large currents to be injected into a superconducting magnet system without the requirement for copper current leads which bridge between the cryogenic environment and room temperature [1][2][3][4][5][6][7][8]. Previous works have already demonstrated mechanically rotating barrel-type flux pumps which employ a CC HTS stator and a rotor comprising arrays of rotating permanent magnets, and these flux pumps have been used to inject currents into HTS coils [3][4][5][6][7][8].…”

“…1(a) in which the flux pump is considered to be a DC voltage source V oc with an effective internal resistance, R d which is due to Dynamic resistance [4,6,9,10]. Both V oc and R d are proportional to magnet-crossing frequency in the flux pumps [4,[6][7][8]. In a previous work, Hoffmann et al pointed out the usage of a stabilized CC stator in their barrel-type rotating flux pump causes non-linear V oc − f relationship due to eddy currents in the stabilizer layers in the CC.…”