Current control and tuning in a closed HTS coil

Up to today, a coil made of high temperature superconductor (HTS) usually needs a soldered joint to form a closed circuit. It is very difficult to have the joint fully superconducting. The Ohmic joint will cause Joule loss when the coil carries a quasi-persistent current. As a result, the electromagnetic energy stored in an HTS coil declines with time. We propose an approach to reduce the Joule loss of an HTS coil during the energy storing stage. The principle of the approach is to tune the current of the HTS coil smaller by introducing an iron core into the coil to increase the inductance of the coil. With this approach, the Joule loss on the HTS coil will be significantly reduced and the initially stored energy in the coil will be kept with little attenuation in a relatively long energy storing period. When a larger current is required for some functions, the current can be almost returned to the initial charged value by removing the iron core from the HTS coil. In this paper, we report our results in analytical deduction and experimental verification of this principle. Besides, the current value can be tuned to any value in a certain range by controlling the position of the iron core inside the HTS coil. This feature may be useful for some applications in which the current or magnetic field needs to be adjusted.