Speaker
Description
The J-PARC Main Ring (MR) has achieved a high extraction efficiency above 99.5% during 30 GeV slow extraction at the current beam power of 92 kW (8.1x10^13 ppp). However, at beam powers above 30 kW, we observed ring-wide beam loss due to transverse beam instability associated with vacuum pressure rise and electron cloud, believed to be triggered by longitudinal microwave structure in the beam. To achieve stable operation, we implemented phase offset injection into RF buckets and a two-step RF voltage reduction technique for debunching, in addition to chromaticity control. The generation of the longitudinal microwave structure and coherent transverse oscillation by the electron cloud during debunching has been investigated using a newly developed simulation code. The simulation predicts the generation of the longitudinal microwave structure under known ring coupling impedances. The code models the electron cloud–beam interaction as a transverse impedance derived from an assumed neutralization factor. The simulation results will be compared with measurements from a wall current monitor and a beam position monitor. Through this study, we will explore further instability mitigation strategies toward higher beam intensity operations planned for the future.
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