Coherent Beam-Beam Instability in Collisions with a Large Crossing Angle

Area of Science:

• Particle Physics
• Accelerator Physics

Background:

• The crab-waist collision scheme is increasingly adopted for future circular electron-positron colliders.
• Previous studies on beam-beam effects in this scheme primarily used weak-strong simulations.

Purpose of the Study:

• To investigate beam-beam effects in crab-waist collisions using strong-strong simulations.
• To identify and characterize novel instabilities that could impact future collider performance.

Main Methods:

• Implementation of strong-strong simulations to model beam-beam interactions.
• Analysis of the "cross-wake force" to understand the instability mechanism.
• Eigenmode analysis of beam-beam head-tail modes.

Main Results:

• Discovery of a strong coherent head-tail instability in crab-waist collisions.
• Identification of the "cross-wake force" as the underlying mechanism.
• Demonstration that this instability can limit collider performance.

Conclusions:

• The identified instability poses a potential challenge for all colliders utilizing the crab-waist scheme.
• Experimental verification at SuperKEKB is proposed during its commissioning phase II.