Study on beam instabilities at electron-positron colliders (factories)

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Study of fast-ion instability (FII) and electron cloud instability (ECI), which are most serious instabilities at high-performance, two-ring electron-positron colliders (factories) were investigate by using KEKB at KEK and BEPC at IHEP Beijing.In an electron ring, the growth of vertical dipole oscillation amplitued along a bunch train was observed by the use of a transient recorder. Since the oscillation frequencies coincided with those of ion oscillations, we believe that FII was detected.In a positron ring, not only dipole oscillations but also vertical beam blowup were observed. We measured the tune of each bunch along a bunch train and found that the tune increased along the train and became saturated at some value and that the increase of the tune was proportional to the line charge density of the bunch train. This strongly suggests that electron clouds around the beam excited intra-bunch oscillation and caused the beam blowup.In JFY2000 we wound solenoids over the KEKB positron ring over 1200 m. These solenoids clearly suppressed the beamblowup ; however, the suppression was not as strong as simulations predicted. The reason yet to be made clear. By continued operation of KEKB with these solenoids on, the beam blowup has become weaker. By the end of March 2001, we got a luminoity of 3.2×10^<33>cm^<-2>s^<-1>, which is close to the world record of 3.3×10^<33>cm^<-2>s^<-1> recorded at PEP-II.Some scrubbing effect is suspected.