Mechanism of a high-Tc superconducting flux pump: Using alternating magnetic field to trigger flux flow

Abstract

High-Tc Superconducting (HTS) magnets operating in persistent current mode suffer a current decay due to flux creep of superconductor and joint resistance. Flux pumps are able to inject direct current into superconducting circuit to compensate the current decay, without the thermal loss caused by current leads. In this work, we proposed a flux pumping mechanism for HTS coils, with an experimental verification and an analytical model. The basic principle we have used is that flux flow can be triggered when the superconductor carrying a direct current is subjected to a perpendicular AC magnetic field. Low frequency alternating current is induced in a loop of YBCO tape using an AC field. A portion of the tape which we refer to as the “bridge” shorts a superconducting coil. A high frequency AC field is applied perpendicular to the bridge tape when alternating current in the tape reaches one polarity. This triggers a net flux flow and results in a current increase in the coil. The proposed flux pump has clear physics and is easily controllable, which may make it promising in practical use.

Flux pumping for non-insulated and metal-insulated HTS coils

“…In contrast, the R c _ NI Coil is only1µΩ, which means the load impedance is very low. When an AC field is applied to the bridge, dynamic resistance is generated, which is dependent on the bridge current i B when the applied field magnitude is not much higher than the threshold value [13,19,23].The dynamic resistance is in parallel with the load impedance, sharing a current proportional to the load impedance. In this case, with the same i 2 , i B is much lower for the NI coil than for MI coil or INS coil, and the dynamic resistance value is also lower.…”

Feedback Control of a Rectifier Type HTS Flux Pump: Stabilizing Load Current With Minimized Losses

“…In our recent work, we proposed two transformer-rectifier flux pumps. One is based on automatically driving the superconductor into flux flow region [12], and the other is based on AC field triggered flux flow [13], [14]. In the following, we mainly focus on the latter.…”

“…The net flux flow direction is only determined by the direction of i B . Therefore, during each cycle of i 2 , if the field is always applied when i B is positive, the net flux flow will accumulate in the load, ending up with a high direct current in the load [13].…”

“…Hoffmann [9] developed a rotating magnets based flux pump, which has been followed by J. Geng [13], [14]. A transformer induces high alternating current in its secondary winding and an HTS bridge.…”

“…Our previous work focuses on the principle [13] and operational characteristics [14] of the flux pump, which is actually open loop operation. To make the flux pump suitable for application, it is important to achieve closed-loop control to adjust the load current as required.…”