H. Fukuma email@example.com
KEK, Oho 1-1, Tsukuba-shi
Ibaraki-ken, 305 Japan3mm Z. Y. Guo firstname.lastname@example.org
IHEP, Beijing 100039, China3mm A series of experiment on the beam photo-electron instability (PEI) is in progress at the Beijing Electron Positron Collider (BEPC) in a collaboration of IHEP and KEK.
The possibility of this instability was proposed by K. Ohmi to explain the vertical coupled bunch instability observed in a positron beam in the Photon Factory(PF) at KEK. The observation shows a low threshold current of the instability (15-20 mA) and a broad distribution of the vertical betatron sidebands in the range of about 100 MHz. This broad distribution is not observed in a electron beam. In Ohmi's model the photo-electrons are produced by the synchrotron light hitting a vacuum chamber. The photo-electrons experience an attractive force from the positron beam and are eventually absorbed in the vacuum chamber. In multi-bunch operation the continuous production of photo-electrons makes a stationary distribution of the photo-electrons because the absorption is balanced by the production. If a positron bunch has a vertical displacement the stationary distribution of the electrons is disturbed and gives a kick to the trailing bunches. Treating the kick as a wake force of the transverse dipole mode, growth rates are calculated by the conventional coupled bunch theory. The calculated growth rate in the PF is 1000-10000 and very high compared to the radiation damping time of 120 . This can explain the low threshold current of the instability. The broad distribution of the betatron sidebands can be explained by the short-range feature of the wake force. The calculated wake force affects only several trailing bunches. The PEI may have a harmful effect on the performance of B and Tau-charm factories because all of them will be operated with high beam current and many bunches.
The first PEI experiment in the BEPC was done in June 1996. The BEPC has a circumference of 240 m, RF frequency() of 200 MHz and harmonic number of 160. The vertical radiation damping time at 1.55 GeV is 50 ms. In the experiment at 1.55 GeV the vertical betatron side bands were observed around while filling a positron beam with 158 bunches. The sidebands distributed in the range of about 50 MHz. The threshold current of the instability was about 9 mA. The broad distribution of the sidebands in positron-filling was not observed in electron-filling. The instability observed in this experiment has very similar characteristics to that in the PF, i.e., the low threshold current, the broad distribution of the betatron sidebands and the manifestation only in a positron beam. A simulation based on Ohmi's computer code shows that the growth rate of the PEI in this experimental condition is about 100 , which is comparable to the radiation damping rate of 20 . This instability will be further studied in this winter.