Discharge phenomena in the button electrodes of the beam position monitors of TRISTAN MR

In the TRISTAN Main Ring, when bunch intensity exceeds about 1 mA, abnormal burst signals appear in the position monitor electrodes. The burst pulses appear after about 10 nsec from the bunch signal. The polarity of the pulses are mostly negative. They occur mainly in each position-monitor-electrode of RF cavity regions. We found that we can suppress them by a biasing voltage of about 290 volts. Moreover, we verified that they don't appear in the new button-electrode which has a wide gap between the button and the chamber. We think that the cause of the abonormal signal is ''Multipacting discharge'' excited by higher order modes of the RF cavity