I'm reading a paper on a superconducting magnetic energy storage (SMES) power control system. One large superconducting inductor (L1 in the figure below) is charged up by a 10kA power supply. Once charged, L1 is isolated from the power supply (Sw1a & Sw1b are switched off) and connected to the DC chopper circuit with switches Sw2a & Sw2b. The chopper circuit transfers the energy to SMES inductor L2 with a switching frequency of 300Hz. The energy is then stored in L2 or transferred back to L1 again with the chopper circuit. This serves as a demonstration of transferring energy between SMES systems.
I’m confused with how the DC chopper operates specifically. I’ve tried to picture different switching scenarios where one or two IGBTs are off and the others are open. What is the order of how the switches S1, S2, S3, and S4 are operated in the attached figure? Is the purpose of the chopper is to split up the DC output and isolate L1 from L2 so all the energy from L1 can be transferred to L2? For reference the paper is "Energy Transfer Experiments between 3 and 4 MJ Pulsed Superconducting Magnets" by T. Onishi, 1985.