H. Fukuma fukuma@kekvax.kek.jp
KEK, Oho 1-1, Tsukuba-shi
Ibaraki-ken, 305 Japan3mm
Z. Y. Guo guozy@bepc3.ihep.ac.cn
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[1] to
explain
the vertical coupled bunch instability observed in a positron beam in the
Photon Factory(PF) at KEK[2]. 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.