Output During Continuous Frequency Ramping of a Dynamo-Type HTS Flux Pump

Flux pumps can inject large dc currents into superconducting coils, without requiring physical connections between the cryogenic superconducting circuit and the room-temperature environment, thus significantly lowering the heat load on the cryogenic cooling system. Here, we study a dynamo-type flux pump employing a 2G ReBCO coated conductor wire at 77 K with multiple spinning Nd-Fe-B magnets and a soft ferromagnetic iron yoke, with all moving parts situated outside of the cryogenic environment. Dynamo-type flux pumps can be modeled as a dc voltage source with an internal resistance, where all output parameters are functions of the speed (frequency) of the spinning magnets. In this work, we focus on the frequency dependence of the dc output voltage. Frominstantaneous traces of the generated voltage waveform, we observe three distinct frequency regimes of behavior. We classify them into regions of low, mid, and high magnet frequency, depending on the magnitude of the dc open-circuit voltage and the shape of the voltage waveform. The voltage-generation mechanism is qualitatively understood at low magnet frequencies, where the dc voltage increases proportional to frequency. However, at mid and high magnet frequencies, the dynamo output deviates from this accepted model. In the mid frequency region, the dc output voltage is roughly constant with frequency, but is unstable. At high frequencies, the dc voltage drops with increasing frequency following an approximately 1/f dependence. We have also studied the effects of ramping acceleration rate, noting a high degree of reproducibility in the low- and high-frequency regions, but complex unstable behavior in the intermediate region. We discuss our results in the context of self-heating due to EMF-driven eddy currents, leading to partial-quenching of the coated conductor wire. This becomes the dominant limiting factor in flux pump output at high frequencies, eventually suppressing the dc output altogether.