Estimations of Longitudinal Impedances Based on Temporal Beam Characteristic Parameter Extraction Method in Storage Ring Light Source

In this study, a simple suitable temporal beam characteristic parameter extraction (T-BCPE) method is developed to measure the longitudinal coupling impedance of the Hefei Light Source-II (HLS-II). In the experiment, a streak camera is applied to observe the potential-well distortion (PWD) under single-bunch operation mode and then accurately measure the longitudinal impedance of machine. According to the theoretical analytical model and in combination with the measured longitudinal bunch behaviors, the inductive, resistive, and longitudinal broadband impedance of the HLS-II storage ring are obtained as 107 ± 28 nH, 1430 ± 242 $\Omega$ , and 1.009 ± 0.013 $\Omega$ , respectively, for the normal operating radio frequency (RF) accelerating voltage of 122.8 kV. It is concluded that the effective longitudinal coupling impedance of HLS-II is calculated to be 3.05 ± 0.80 $\Omega$ . Additionally, the bunch lengthening factor $B_{l}$ and asymmetry factor $A$ of longitudinal bunch distribution are also measured experimentally in various RF voltages so as to further research nonlinear longitudinal beam dynamics and beam evolution associated with bunch behaviors. Then, the applicability, feasibility, and reliability of this proposed method are also verified. When the RF voltage is adjusted to 100.4, 110.3, 130.4, and 140.5 kV, the inductive impedance of machine is estimated to be 115 ± 13, 107 ± 17, 105 ± 31, and 108 ± 28 nH, respectively. And the according resistive impedance is 1320 ± 310, 1380 ± 214, 1360 ± 230, and 1300 ± 260 $\Omega$ , respectively. Through a series of experimental measurements, it has demonstrated the validity and accuracy of the measured longitudinal impedances of HLS-II, of which agree well with the simulated results.