Beam Behavior Due to Crab Cavity Breakdown

Crab cavities were installed in KEKB in 2007. The function of the cavity is to tilt the bunch of the beam in the longitudinal direction. But if the RF phase gets out of control, the cavity kicks the beam like a steering magnet. To avoid this unwanted kick, the RF phase must be controlled well. In the beam operation, some disturbances may occur such as a discharge, a quench, etc. When such disturbances occur, it is very difficult to control the RF phase precisely. We can’t trust measured RF phase at that time. In KEKB, beam is aborted quickly when a disturbance is detected. Beam behavior before detecting the disturbances has been investigated. MEASUREMENT OF BEAM BEHAVIOR KEKB has digital bunch-by-bunch feedback systems to suppress beam instability [1]. They measure bunch position continuously to calculate feedback signal. The beam positions in 4096 turns of every bunches are stored in the feedback system. The data acquisition of the recorder is triggered by a beam loss. The sensitivity of the beam position measurement depends on the bunch current. The sensitivity is about 0.01mm/(count·mA), and 127 count is a full range for each direction. When the abort system works and the beam is dumped actually, the transition of the cavity voltage, phase and RF power to feed cavity are recorded by an oscilloscope. Both data may not be taken due to the different trigger signals. In most cases where the RF systems of crab cavities abort are required an abort, beam losses were accompanied before aborts. So, the behavior of the beam positions and the cavity parameters just before abort can be investigated. MACHINE PARAMETERS OF KEKB The related machine parameters of KEKB are shown in Table 1 [2] [3]. In KEKB, the horizontal betatron tunes are set very close to the half integer. Even if the crab cavity phase shifts by π/2, the beam can be maintained from the view point of physical aperture. However, in case of colliding beams of high beam currents, some beam losses occur with this size of COD and the beams are aborted. The Figure 1 shows the COD, when the cavity phase shifts by π/2. The COD in the Figure 1 is a difference between the cases of the crab phases of π/2 and zero. In the usual operation, the cavity phase is adjusted at 0.35 rad to suppress horizontal oscillations of the colliding beams [4]. The net kick ∗ [email protected] voltage caused by the phase offset is compensated by steering magnets. The revolution periods of the beam for both rings are 10 μsec. The decay times of the cavity voltage are much longer than the revolution periods of the beams. The beam abort timing and the behavior of the crab cavity voltage are shown in Figure 2, when the crab cavity RF system makes an abort signal. It shows that the cavity voltage is turned off in about 100 μsec, and after a 200 μsec further delay, the beam is dumped. Table 1: Typical values of related machine parameters of KEKB Ring LER HER unit β∗ x 80 80 cm β x 21 39 m β x 73 162 m νx 44.506 45.511 Ibunch 1.0 0.75 mA Vcrab 0.95 1.45 MV ψcav 0.35 0.35 rad τcrab 130 84 μsec Figure 1: The COD which was appeared when the crab cavity phase shifts π/2 is shown. This orbit was observed at 1MV kick voltage when the kick voltage and phase were calibrated. The maximum displacement was about 5mm. BEHAVIOR OF THE BEAM