Non-linear pressure rise with beam current in BEPC II positron ring

It was found that the pressure-current curves were linear in electron ring, while non-linear in positron ring, especially in arc sections of positron ring. It might be since that the synchrotron photoelectrons transversely kicked by the positron beam bunches hit the vacuum chamber surface and the hit produces secondary electrons, which cause beam-induced multipacting (BIM) and large outgassing under some conditions. To find these conditions, bunch filling patterns of positron beams were changed to derive the corresponding non-linear pressure-current curves. The result is consistent with that from KEK-B low energy ring, which shows the non-linearity exhibits a threshold behavior. The non-linearity will take place only when the single bunch current and the quantity of the successive bunch in one train are both sufficiently large. With the total current increasing, BEPC II positron beam should take certain bunch filling patterns of smaller single bunch current and larger bunch number to avoid BIM. (authors)