Study on Electromagnetic and Loss Characteristics of NI HTS Coil During Charging Process

No-insulation high temperature superconducting (NI HTS) coils are widely used in magnetic resonance imaging (MRI), particle accelerators and maglev trains due to their excellent electrothermal stability and self-protection characteristics. However, charging delay and charging loss will occur during the coil charging process, which will affect the stable operation of superconducting equipment. How to improve the charging efficiency of NI HTS coils and further reduce energy losses will become critical. This paper is to study the electromagnetic and loss characteristics of two kinds of NI coils charging process. The first coil is only directly wound with REBCO tape, and the other is solder impregnation treatment on the basis of it, which reduces the turn-to-turn contact resistance of the coil. Firstly, the equivalent circuit model is used to calculate the circumferential current, radial current, voltage and central magnetic field of the coil, and the influence of different factors on the charging process is discussed. Secondly, the equivalent circuit model is used to calculate the turn-to-turn loss of the coil, and the circumferential current is introduced into the finite element model (FEM) to calculate the magnetization loss. The charging loss characteristics of the two coils under different external field conditions are analyzed, and the effective measures to reduce the charging loss are obtained. The results show that the charging time decreases with the increase of the charging rate. However, excessive charging rate will have an adverse effect on the coil. In addition, reducing the current amplitude and increasing the turn-to-turn contact resistance can effectively accelerate the charging process. The turn-to-turn loss of the NI coils during charging is much higher than the magnetization loss. The charging loss can be reduced by increasing the turn-to-turn contact resistance and reducing the charging rate.