Abstract
Bunch phase shift along the train due to a bunch gap transient is a concern in high-current colliders. In KEKB operation, the measured phase shift along the train agreed well with a simulation and a simple analytical form in most part of the train. However, a rapid phase change was observed at the leading part of the train, which was not predicted by the simulation or by the analytical form. In order to understand the cause of this observation, we have developed an advanced simulation, which treats the transient loading in each of the cavities of the three-cavity system of the accelerator resonantly coupled with energy storage (ARES) instead of the equivalent single cavities used in the previous simulation, operating in the accelerating mode. In this paper, we show that the new simulation reproduces the observation, and clarify that the rapid phase change at the leading part of the train is caused by a transient loading in the three-cavity system of ARES. KEKB is being upgraded to SuperKEKB, which is aiming at 40 times higher luminosity than KEKB. The gap transient in SuperKEKB is investigated using the new simulation, and the result shows that the rapid phase change at the leading part of the train is much larger due to higher beam currents. We will also present measures to mitigate possible luminosity reduction or beam performance deterioration due to the rapid phase change caused by the gap transient.
Highlights
In a high-current multibunch storage ring, a gap with empty buckets is introduced in the bunch train to allow for the rise time of a beam abort kicker
We have presented an investigation of the bunch gap transient effect using an advanced transient simulation, which treats transient loading in the three-cavity system of the accelerator resonantly coupled with energy storage (ARES) cavities
The result clarifies that the rapid phase change at the leading part of the bunch train observed in the KEKB operation is attributed to the parasitic modes (0 and π-modes) excitation in the ARES cavities
Summary
In a high-current multibunch storage ring, a gap with empty buckets is introduced in the bunch train to allow for the rise time of a beam abort kicker. The longitudinal synchronous position is shifted bunch-by-bunch along the train, which shifts the collision point of each bunch in a collider. This can result in luminosity degradation or even degradation of the beam performance due to beam-beam effects. One serious concern for high-current storage rings is the coupled-bunch instability caused by the accelerating mode of the cavities. This issue arises from the large detuning of the resonant
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