Abstract
Many particle accelerators are proposing the use of crab cavities to correct for accelerator crossing angles or for the production of short bunches in light sources. These cavities produce a rotation to the bunch in a well-defined polarization plane. If the plane of the rotation does not align with the horizontal axis of the accelerator, the bunch will receive a small amount of spurious vertical bunch rotation. For accelerators with small vertical beam sizes and large beam-beam effects, this can cause significant unwanted effects. In this paper we propose the use of a 2nd smaller crab cavity in the vertical plane in order to cancel this effect and investigate its use in numerical simulations.
Highlights
Particle colliders often employ a finite crossing angle between colliding bunches
In this paper we look at the theory of such a device and numerically model an anticrab cavity for use in the International Linear Collider (ILC)
For the ILC crab cavities a 10 mrad roll misalignment on one cavity can cause the loss of 30% of the luminosity
Summary
Particle colliders often employ a finite crossing angle between colliding bunches. The associated loss of luminosity for long thin bunches can be recovered by rotating the bunches prior to collision, such that the bunches will have an effective head-on collision. These devices are deflecting mode cavities in which the cavity phase is set such that the head and tail of the bunch receive equal and opposite kicks, and the center of mass receives zero kick This will impart a transverse momentum to the bunch which causes the bunch to rotate as it travels along the beam line. If the polarization of the operating mode of these crab cavities is not perfectly aligned with the horizontal axis of the accelerator, a small amount of the crabbing effect will take place in the beams’ vertical plane This will cause the bunches to collide with a small vertical crossing angle. We simulate the use of this anticrab cavity in the beam delivery system of the ILC, using the particle tracking code PLACET [7] together with the beam-beam code GUINEA-PIG [8]
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