Impact of switch overcritical regime on the output voltage and charging current in an AC field-controlled transformer-rectifier HTS flux pump

Abstract High-temperature superconducting (HTS) flux pumps eliminate the need for bulky current leads and high-current power supplies when charging superconducting devices. In such systems, superconducting switches are essential for optimal performance. Typically, the switch critical current is designed above a certain threshold, such as twice the load critical current, to prevent operation in the overcritical regime where the switch current exceeds its critical current. However, due to limitations in the fabrication process and potential performance degradation during operation, the switches may fail to meet this condition, leading to changes in the performance of the flux pumps. This paper experimentally investigates the impact of the switch overcritical regime on the performance of an AC field-controlled transformer-rectifier HTS flux pump, revealing that how the output voltage and charging current vary with key operating parameters, including the amplitude and frequency of the transformer primary current, as well as the amplitude, frequency, and duration of the switch electromagnet (EM) current. No-load voltage tests show that when the switch enters the overcritical regime, the output voltage decreases as the transformer secondary current increases, due to the generation of flux flow resistive after the switch is switched off. Charging tests revealed that once the switch current reaches its critical current, the charging process halts prematurely. Afterward, increasing the EM current slightly boosts load current, while increasing the transformer current significantly reduces it. Additionally, we observed that the load was charged when only the switch was powered, suggesting the possibility of eliminating the transformer power supply. Based on experimental results, it is predicted that the switch critical current must be at least 1.66 times the load critical current to fully charge the load in our flux pump.