Charging 2G HTS Double Pancake Coils With a Wireless Superconducting DC P ower Supply for Persistent Current Operation

We have designed and built a novel high-temperature superconducting (HTS) flux pump device to charge a second-generation (2G) HTS double pancake coil (DPC). This device generates dc-biased ac travelling wave on sections of 2G HTS coated conductor (superconducting stator), which pumps flux into the superconducting closed loop formed by the DPC and superconducting stator. In this paper, we discuss the importance of the dc-biased field, ac travelling wave, and the travelling direction in the flux pump process, and how to control the magnetization of an HTS coil with the above factors. We have found that a pure ac travelling wave can only transport net flux into the superconducting closed loop in the presence of dc-bias field. The magnetization direction of the DPC can be reversed by reversing the dc-bias direction of the dc background field or travelling direction of the ac travelling wave. This paper shows the versatility and flexibility of this novel flux pump device in controlling the magnetization of the HTS coils, which will work as a wireless dc power source to help persistent current mode operation of a small-scale MRI magnet built by HTS DPCs later in this project.