Wakefield effects of the bypass line in LCLS-II

TL;DR

This paper analyzes the resistive wall wakefields in LCLS-II's 2.5 km stainless steel pipe, demonstrating their role in beam dechirping and exploring methods to optimize wake effects for improved beam quality.

Contribution

It provides detailed calculations of the longitudinal and transverse wakefields in LCLS-II's bypass line, assessing their impact and proposing ways to enhance wake-induced dechirping.

Findings

Resistive wall wakefields effectively dechirp the beam before the undulator.

The linearity of the induced chirp can be improved through specific modifications.

Transverse wake effects are significant and require careful management.

Abstract

In LCLS-II, after acceleration and compression and just before entering the undulator, the beam passes through 2.5 km of 24.5 mm (radius) stainless steel pipe. The bunch that passes through the pipe is extremely short---with an rms of 8 um for the nominal 100 pC case. Thus, even though the pipe has a large aperture, the wake that applies is the {\it short-range} resistive wall wakefield. The bunch distribution is approximately uniform, and therefore the wake induced voltage is characterized by a rather linear voltage chirp. It turns out that the wake supplies needed dechirping to the LCLS-II beam before it enters the undulator. In this note we calculate the wake, discuss the confidence in the calculation, and investigate how to improve the induced chirp linearity and/or strength. Finally, we also study the strength and effects of the transverse (dipole) resistive wall wakefield.