Abstract Body

High temperature superconducting (HTS) electromagnets consisting of one or more HTS tapes wound into a coil are becoming the go to technology for applications that require high magnetic fields [1]. These coils require high current supplies which are generally large Copper (Cu) based power supplies with Cu leads/bus bars that feed directly into the cryogenic environment, resulting in a large thermal load [2]. An alternative to Cu power supplies and their large leads are superconducting power supplies, also known as ‘flux pumps’[3]. These superconducting power supplies have the advantage of being constructed from superconducting materials allowing them to be smaller in volume and weight and to exist in the same cryogenic environment as the HTS magnet. What’s more, the feedthroughs required to power these superconducting power supplies are small, leading to a greatly reduced thermal load on cryogenic systems when compared to Cu current leads. Finally, the ohmic losses incurred from Cu leads are non-existent in these superconducting supplies, meaning much lower driving voltages are required to maintain high currents in a HTS load coil. Superconducting power supplies are separated into two categories: Dynamo and Transformer rectifier. Paihau - Robinson Research Institute is a world leader in the research of both types.

In this presentation the findings of a recently published letter [4] in Applied Physics Letters will be exhibited, in which we introduced a dynamo capable of outputting DC currents greater than 1 kA into a superconducting load. This dynamo utilizes a continuous cylindrical stator and a single permanent magnet on a rotor to generate this current. The continuous stator design of this dynamo results in an absence of edge effects which usually dominate the output voltages in single and multiple tape dynamos, thus allowing for direct observations of the driving mechanism. The stator temperature and rotor frequency responses were studied over the ranges of 30 – 100 Kelvin and 9 – 25 Hz respectively. Additionally, the dynamo was simulated with finite element modeling using T-A formulation based around the experimental apparatus dimensions and the measured characteristics of the stator material for both temperature and applied perpendicular magnetic field (B_⊥). These findings show that the output voltage of dynamos is due to the circulating currents in the superconductor interacting with the non-linear resistance of type-II superconductors in the presence of a moving B_⊥, providing further experimental evidence in support of the findings made by Matiara et al. a few years prior[5, 6]. Additionally, this work shows that as the stator temperature of a dynamo decreases, so too does its output voltage due to flux screening, demonstrating that multi kA dynamos should be operated well above the flux screening temperature of the stator. This working design is now being implemented into an ‘L-zero’ dynamo for space applications, addressing the problem of rotational inertia whilst supplying a DC output voltage. This is being done by utilizing two identical rotors which rotate in opposite directions at the same speed combined with two cylindrical continuous stators connected in parallel to the load.

This presentation will also touch on the breadth of dynamo research currently being pursued at Paihau – Robinson Research Institute by showcasing two further studies. One, a comparative study where open circuit voltages from various commercially available ReBCO tapes are compared directly using a rotor bathed in liquid nitrogen. The results of this study will influence future dynamo design by highlighting the differences between HTS manufacturer offerings when using their products as dynamo stators. Two, a HTS wafer dynamo where a B_⊥ is moved in a circular motion about the centre of a disk of HTS material attached to a sapphire plate. This disk is then connected to a superconducting load via HTS tapes soldered to the centre and rim of the disk.

References

1. Zhou, Y.-H., D. Park, and Y. Iwasa, Review of progress and challenges of key mechanical issues in high-field superconducting magnets. National Science Review, 2023. 10(3).

2. Geng, J., C.W. Bumby, and R.A. Badcock, Maximising the current output from a self-switching kA-class rectifier flux pump. Superconductor Science and Technology, 2020. 33(4): p. 045005.

3. Wen, Z., H. Zhang, and M. Mueller, High Temperature Superconducting Flux Pumps for Contactless Energization. Crystals, 2022. 12(6): p. 766.

4. Venuturumilli, S., et al., Temperature dependent behavior of a kA-class superconducting flux pump with a continuous cylindrical stator. Applied Physics Letters, 2023. 123(20): p. 202601.

5. Mataira, R.C., et al., Origin of the DC output voltage from a high-Tc superconducting dynamo. Applied Physics Letters, 2019. 114(16): p. 162601.

6. Mataira, R., et al., Mechanism of the High-$T_c$ Superconducting Dynamo: Models and Experiment. Physical Review Applied, 2020. 14(2): p. 024012.

Acknowledgment

This work was supported by funding from the Ministry of Business, Innovation and Employment, New Zealand under Contract Nos. RTVU2004 and RTVU1707.

Figure 1. Continuous stator dynamo. (a) A CAD model showing the superconducting circuit inclusive of the dynamo rotor and magnet. (b) The stator and lap joint HTS pieces tinned and ready to be soldered together mounted on a sapphire cylinder former. (c) The completed system constructed and mounted to a cold head.

Keywords: HTS power supply, Flux Pump, Dynamo, High current