Levitation performance of an onboard high-temperature superconducting bulk unit with cryocooler direct cooling

Abstract

High-temperature superconducting (HTS) maglev systems show the intrinsic advantage of self-stabilizing levitation without external control, therefore, they have great potential to be a new type of rail transit. At present, the cryogenic condition (77 K) of HTS bulks is maintained by pouring liquid nitrogen (LN2) into the dewar and supplementing LN2 at regular intervals, which cannot be maintained for a long period and in a controllable cryogenic environment. This cooling method is easy to realize but not the best approach for future commercial high-speed maglev. It is therefore necessary to try other cooling methods. In this paper, we try the direct cooling method. A cryocooler was used to directly cool a cryogenic unit with eight bulk superconductors which are designed to be close to the practical application. The levitation performance of the cryogenic unit with superconductors is tested on SCML-01. The temperature stability of the cryogenic unit was verified and measurements of levitation force versus temperature and levitation force versus field cooling heights were performed at temperatures from 50 to 90 K. The experimental results show that the HTS bulk unit can obtain a stable and controllable cryogenic environment. By analyzing the experimental data, it is found that decreasing temperature is beneficial to the levitation performance of HTS bulks and as the temperature decreases, the rate of the levitation force increase steps down. Moreover, according to the application environment of the HTS maglev cryogenic unit, 60 K is recommended as its operating temperature. The experimental results provide valuable references for the design of HTS maglev units in the future.