Bipolar magnet power supplies using switching techniques are commonly used to provide stable excitation current for correction magnet coils. Specification of the Shenzhen Superconductive Soft-X-ray Free Electron Laser (S³FEL) project requires enhanced magnetic field stability of correction magnets. However, when switching power supplies are used for correction magnets at low currents, the very narrow pulse width of the power switch leads to poor stability in the output current.

This study aims to propose a method involving a series controllable bidirectional impedance circuit to improve the stability of low current output from the correction magnet power supply.

Firstly, the characteristics of a controllable bidirectional impedance was identified when connected in series from the power supply to the correction magnet, and a Bode diagram was applied to analyzing its features. Then, a controllable bidirectional impedance circuit was designed on the basis of simulation analysis. Finally, experimental validation was conducted using the correction magnet and power supply from Shanghai Synchrotron Radiation Facility (SSRF).

The experimental results demonstrate that serially connecting controllable bidirectional impedances not only improves low current stability but also allows for smooth switching between MOSFETs and controllable bidirectional impedances.

The circuit design proposed in this study is simple and proves to be effective to improve the stability of bipolar magnet power supply under low current condition.