Impedance estimations from bunch lengthening in the Pohang light source

Bunch lengthening in the Pohang Light Source storage ring has been investigated. With the three kinds of impedance models, the impedances of the storage ring are estimated by fitting the solutions of the Haïssinski equation and the measured bunch lengthening. The ring impedances estimated by a series R + L impedance model indicate an inductance of L = 13.7 ± 3.5 nH and a resistance of R = 743 ± 84 Ω. The behaviors of the measured bunch lengthening are compared with those obtained from the estimated impedances. These results show a good agreement. The threshold beam current is estimated to be 7.4mA. The scaling law for the bunch lengthening above microwave instability threshold is obtained as σ_x = 0.437I ^1/3.35. Using the Keil-Schnell criterion, longitudinal broadband impedance of the ring is estimated to be 0.24 Ω.