Synchrotron oscillation damping due to beam-beam collisions

TL;DR

This paper reports the first observation of synchrotron oscillation damping due to beam-beam collisions with a large crossing angle in a collider, attributed to Landau damping from tune spread.

Contribution

It demonstrates that beam-beam collisions with a crossing angle can induce synchrotron damping via tune spread, a novel insight in collider physics.

Findings

Observation of synchrotron damping during collisions with crab waist scheme

Shift of approximately 600Hz in residual synchrotron sidebands

Damping explained by Landau damping from tune spread

Abstract

In DA{\Phi}NE, the Frascati e+/e- collider, the crab waist collision scheme has been successfully implemented in 2008 and 2009. During the collision operations for Siddharta experiment, an unusual synchrotron damping effect has been observed. Indeed, with the longitudinal feedback switched off, the positron beam becomes unstable with beam currents in the order of 200-300 mA. The longitudinal instability is damped by bringing the positron beam in collision with a high current electron beam (~2A). Besides, we have observed a shift of \approx 600Hz in the residual synchrotron sidebands. Precise measurements have been performed by using both a commercial spectrum analyzer and the diagnostics capabilities of the DA{\Phi}NE longitudinal bunch-by-bunch feedback. This damping effect has been observed in DA{\Phi}NE for the first time during collisions with the crab waist scheme. Our explanation is that beam collisions with a large crossing angle produce a longitudinal tune shift and a longitudinal tune spread, providing Landau damping of synchrotron oscillations.