# Effect of Crab cavity HOMs on the coupled-bunch stability of HL-LHC

## TL;DR

This paper analyzes how high order modes in crab cavities impact the coupled-bunch stability in HL-LHC, revealing challenges in suppression methods and quantifying impedance limits for stable operation.

## Contribution

It develops an analytical model and simulation tool to study HOM effects on stability, providing new insights into impedance thresholds and mitigation strategies for HL-LHC.

## Key findings

- Instability involves multiple azimuthal intra-bunch modes with similar growth rates.
- Traditional damping methods are ineffective against this instability.
- Limiting transverse shunt impedance to 1 MΩ/m is crucial for stability.

## Abstract

High frequency High Order Modes can significantly affect the coupled-bunch stability in a circular accelerator. With a large enough shunt impedance they may drive a coupled-bunch transverse instability. We have developed an analytical model and implemented it in the NHT Vlasov solver. The results show that the instability is characterized by the excitation of many azimuthal intra-bunch modes, which have similar growth rates, that make the traditional remedies such as a flat resistive feedback and chromaticity inefficient in suppressing it. For the High Luminosity Large Hadron Collider this could result in a significant increase of the stabilizing Landau octupole current, up to $\sim 100$ A ($\sim 20\%$ of the maximum available current). In order to limit the increase below 10 A ($\sim 2\%$), the transverse shunt impedance has to be kept below 1 M$\Omega$/m.