HOM Effects in Vacuum Chamber with Short Bunches

High luminosity in electron-positron factories requires high currents of very short bunches. SLAC PEP-II and KEKB B-factories are progressively increasing currents gaining more and more luminosity [1-2]. Simultaneously the interaction of high currents and vacuum chamber elements becomes more important for operation of the rings. High Order Modes excited by short intense bunches are propagating along the vacuum chamber, penetrating and dissipating inside vital vacuum elements, like shielded bellows, vacuum valves and vacuum pump. As a result these elements get large temperature rise or temperature oscillations. Often HOM heating has a resonance character. HOM heating of vacuum pumps leads to increasing of the vacuum pressure. High frequency modes “check” the quality of vacuum chamber: they detect small gaps, weak RF screens or feed-through. Smooth tapers and collimators become the source of HOM production. We will discuss the physical nature of these exciting HOM effects.