Experimental investigation on coupled-bunch instability and suppression for the upgrade of Beijing Electron-Positron Collider (BEPCII)

After touching the design collision luminosity of 1.0 x 10(33) cm(-2) s(-1) in 2016, this target has not been reached again for the upgrade of Beijing Electron-Positron Collider during physical collision operation in subsequent years due to various factors, such as the collision background, noise, and equipment stability under high power operation. One of the most serious factors was the beam instability, especially the coupled-bunch instability, whose sideband distribution was clearly displayed on the spectrum analyzer when the feedback system was OFF. Experimental measurements on coupled-bunch instability and the damping from the feedback system were carried out over the past two years. These measurements were conducted by transiently turning off/on the feedback system, and the bunch-by-bunch oscillations were recorded to obtain the mode distribution and growth and damping rates. The experimental results demonstrate that the feedback damping is strong enough to suppress instability, and precise adjustments to the feedback system during physical collision operation can increase the collision luminosity, which has been verified in last year's physical run.