Determination of microwave instability threshold of storage ring based on longitudinal bunch asymmetry characterization

In this study, a novel method is proposed to measure the microwave instability (MWI) threshold of the storage ring by means of the ultrafast photon detection streak camera system. It provides an additional and complementary diagnostic tool for determination of the MWI threshold of the current storage ring. Compared to other conventional measurement methods, this scheme can realize the more accurate measurement of the MWI threshold current with the remarkable advantages of simple optical architecture, good robustness, and high accuracy. In the experiment, the MWI threshold current of the Hefei Light Source-II (HLS-II) is obtained as 12.77 ± 0.54 mA corresponding to 123.6 kV of the nominal radio-frequency (RF) voltage by measuring the current dependence of the bunch asymmetry factor, under single-bunch operation mode. With a further adjust RF voltage to 100.2, 110.2, 130.3, and 140.2 kV, the turbulent threshold currents are acquired to be 11.11 ± 0.90, 11.74 ± 0.47, 13.21 ± 0.68, and 13.78 ± 0.82 mA, respectively. Additionally, the applicability, feasibility, and reliability of this method are further verified by experimentally observing the beam characteristic behaviors of the energy spread broadening and horizontal beam size widening. From experimental verification and comparative analysis, it is proved that this reported approach is valid for measuring the MWI threshold of the storage ring. This method is importance for the study of the beam instability mechanisms and related longitudinal beam dynamics in storage ring, which is also useful for machine debugging and beam status analysis.