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The effects of electron clouds on positively-charged beams have been an active area of research in recent years at particle accelerators around the world. Transverse beam-size blow-up due to electron clouds has been observed in some machines, and is considered to be a major limiting factor in the development of higher-current, higher-luminosity electron-positron colliders. The leading proposed mechanism for beam blow-up is the excitation of a fast head-tail instability due to short-range wakes within the electron cloud. We present here observations of betatron oscillation sidebands in bunch-by-bunch spectra that may provide direct evidence of such head-tail motion in a positron beam.The development of clouds of electrons in positivelycharged-beam storage rings has been observed at several machines, including the KEKB Low Energy Ring (LER), a 3.5 GeV positron storage ring which is part of the KEK B-Factory. Observations at the KEKB LER of betatron tune shifts along a bunch train via gated tune meter [1], and of transverse bunch size along the train via highspeed gated camera [2] and streak camera [3], show a characteristic increase of transverse tune shifts and beam size starting near the head of the train, reaching saturation at some point along the train. Simulations of electron cloud density due to photo-electrons being drawn towards the positron beam have shown a similar build up of cloud density along the train, reaching saturation at some point [4,5]. Electrons from the cloud have also been measured directly via electrode [6]. Solenoids have been wound around approximately 95% of the drift space in the LER, with a maximum field at the center of the beam pipe of 45 Gauss [7]. The beam size blow-up has been observed to occur above a threshold average bunch current of ∼ 0.35 mA/bunch at 4-rf bucket spacing between bunches with the solenoids off; this threshold is raised when the solenoids are powered on [8]. The beam blow-up has been found to reduce the specific luminosity of the affected bunches [8].One proposed mechanism for the beam blow-up due to the presence of electron clouds is a strong head-tail instability caused by wake fields created by the passage of the bunch particles through the cloud [9]. Attempts have been made to observe this head-tail motion directly via streak camera [3], but have been unsuccessful, possibly due to a lack of sufficient light intensity. A vertical sideband peak has been reported for a proton beam at the CERN SPS which could be an indication of head-tail motion [10], though no clear signature has yet been reported at a positron machine. We report here on observations of a sideband peak, above the betatron tune, which may provide direct evidence of such a coupled-mode spectral peak in a positron beam. The sideband peak first appears near the bunchcurrent threshold of beam blow-up -the sidebands cannot be seen when the average bunch current is below the beam-blow-up threshold, and can be seen when the average bunch current is over the threshold. In addition, the presence of the...

High-speed digital filter systems for bunch-by-bunch feedback systems have been developed at KEK. A two-tap finite impulse response filter with simple hardware realizes the functions of a 90 ± phase shift, suppression of the static component and digital delay of up to a few hundred turns for the KEKB rings. Difficulties in the circuit board, such as the trimming of the timing skews or the problem of long-term reliability, have been solved using custom GaAs large-scale integrated circuits which demultiplex and multiplex fast parallel-digital data coming from the analog-to-digital converter and going to the digitalto-analog converter. Two major applications of the filter board, the bunch current monitor and the bunch oscillation recorder with 20 MB memories for transient-domain analysis of the instabilities, are also described.