Microwave Instability at Transition Crossing: Experiments and a Proton-Klystron Model

Abstract

Longitudinal bunch shapes in the KEK proton synchrotron were measured by a fast bunch-monitor system, which showed the rapid growth of the instability at the frequency of ϳ1 GHz and significant beam loss just after transition energy. Temporal evolution of the microwave instability is explained for the first time with a proton-klystron model. [S0031-9007(96)02288-0] PACS numbers: 29.27.Bd

Methods of observing the microwave instability above and below transition

“…In fact in all cases in real accelerators measurements are unavoidably done in the presence of resistive impedances. Indeed "switching off' space charge in simulations does not change results [4] obtained for the resonant impedance…”

“…However it seems that strong µw signals were observed just after transition crossing in accelerators both with capacitive (space charge) [4] and inductive type [13] of low frequency impedance, suggesting the possibility of another explanation. In fact in all cases in real accelerators measurements are unavoidably done in the presence of resistive impedances.…”

“…In this special case the azimuthal motion in the phase space is practically frozen and particles are moving along momentum axes under the influence of external (RF) and induced voltages. It is noticed that strong microwave signals appear just above transition (AT) [4,13,15] which can be interpreted as a difference in bunch stability above and below transition (BT). In Fig.…”

“…In proton machines µw instability is often observed at transition crossing [4,13,14,15]. In this special case the azimuthal motion in the phase space is practically frozen and particles are moving along momentum axes under the influence of external (RF) and induced voltages.…”

“…which varies from tens of Ohms in old accelerators to fractions of an Ohm in modern. Comparison of the slope of the bunch lengthening curve "before" and "after" gives a good indication about the global success of impedance reduction programmes (like in [3], [4]). …”