New Simulations of Positron-beam Electron Cloud Build- up and Electron-beam Coupled-bunch Instabilities for Kekb and Super-kekb

I summarize recent electron-cloud and coupled-bunch instability simulation studies for KEKB and SuperKEKB. The possible suppression of the electron-cloud build up inside quadrupole magnets by means of 4 permanent dipoles or by a super-imposed solenoid is investigated for the LER positron beam. Also studied is the electron accumulation in three different types of solenoidal fields, namely for a uniform field and for a series of short separated solenoids with either equal or alternating polarity, considering 4, 2, and 1 rf bucket spacing. No evidence for a ‘resonant’ build up is found. Simulations for solenoid fields in the Super KEKB upgrade are discussed next. The effective residual electron density in the present KEKB can be estimated from the measured persistent tune shift along the train. The interplay of electron-cloud wake and pinch with the beam-beam interaction in KEKB is studied by a fewparticle model, which reveals a potentially large synergy. The vertical coupled-bunch instability growth rate of the HER electron beam is calculated analytically for the resistive-wall impedance and it is found to be a factor 3 smaller than observed. Electron-beam multi-bunch instabilities driven by either resistive-wall or CO ions are also explored in simulations. In the latter case, the effects of a feedback and of beam intensity changes on the growth rates and on the mode spectrum are also investigated. Except for the vertical growth rate, the simulation results may be compatible with observations.