Method for generating linear current-field characteristics and eliminating charging delay in no-insulation superconducting magnets

No-insulation (NI) rare-earth barium copper oxide (REBCO) magnets are promising for high field or high temperature superconducting magnets because they simplify quench protection. However, the turn-to-turn leakage current path induced by the absence of insulation introduces nonlinearities into the magnetic fieldcurrent characteristic and significant delay in reaching the desired field. This paper shows that active feedback control can mitigate both the nonlinearity and the charging delay. To verify our approach, simulations and tests were performed with an NI REBCO magnet made of 13 double-pancake coils. A proportional and integral (PI) feedback control of the power supply was adopted which allowed determination of the appropriate PI gains using dynamic simulations of the equivalent circuit of the NI magnet. Feedback control tests were then performed in liquid nitrogen at 77 K. The time to reach 99.5% of the target magnetic field to become essentially steady-state was reduced by more than 2000 times from 850 s without control to 0.4 s with control. The results demonstrate a potential that one of the most significant perceived disadvantages of an NI magnet can essentially be removed by active feedback control of the power supply current.