Comment on "Fast-slow mode coupling instability for coasting beams in the presence of detuning impedance"

TL;DR

This paper critically examines a recent study on beam mode coupling, arguing that its core equations are invalid and that its simulation results are misleading, thereby challenging the original conclusions.

Contribution

The paper provides a detailed critique of the equations and simulation results in the referenced study, highlighting fundamental inaccuracies and questioning its validity.

Findings

Key equations are shown to be unacceptable as approximations.

The claimed agreement with simulations is likely illusory.

The original study's conclusions are therefore unreliable.

Abstract

In this comment we show untenability of key points of the recent article of N. Biancacci, E. Metral and M. Migliorati [Phys. Rev. Accel. Beams 23, 124402 (2020)], hereafter the Article and the Authors. Specifically, the main Eqs. (23), suggested to describe mode coupling, are shown to be unacceptable even as an approximation. The Article claims the solution of this pair of equations to be in "excellent agreement" with the pyHEADTAIL simulations for CERN PS, which is purportedly demonstrated by Fig. 6. Were it really so, it would be a signal of a mistake in the code. However, the key part of the simulation results is not actually shown, and the demonstrated agreement has all the features of an illusion.