Effects of beam-tube roughness on x-ray free electron laser performance

Gennady Stupakov,Dieter Walz,Ruth Ellen Thomson,Roger Carr

doi:10.1103/physrevstab.2.060701

Abstract

In an x-ray free electron laser like the Linear Coherent Light Source being designed at SLAC, electron bunches about $70\ensuremath{\mu}\mathrm{m}$ FWHM long are sent into a beam tube only 5 mm in internal diameter and more than 100 m in length. Because of the surface roughness of the beam tube, wakefields can be generated that catch up to the bunch and interact with it, causing energy spread and emittance growth. The strength of this effect depends on the details of the roughness of the surface. We present here a study in which the roughness of the beam tube was measured and the longitudinal impedance of the tube was calculated. Our result shows that a commercially available beam tube can be made smooth enough so the resulting wakefield effects are within the tolerance determined by the requirement that the induced relative energy spread of the beam be less than $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$.

Highlights

At SLAC and DESY, x-ray free electron lasers are being designed to produce very high brightness x-ray beams at 0.15 nm wavelengths and below
If permanent magnets are used for the undulator, the allowable internal diameter of the beam tube is about 5 mm
The transverse rms beam size is about only 30 mm, and its relative energy spread is of the order of 0.1%

Summary

INTRODUCTION

At SLAC and DESY, x-ray free electron lasers are being designed to produce very high brightness x-ray beams at 0.15 nm wavelengths and below. The emittance growth of the beam due to the roughness transverse wakefield imposes looser tolerances than the longitudinal effect This impedance is generated by scattering of the beam field on the disturbances of the conducting surface of the tube. Assuming a Gaussian bunch of rms length sz, it is easy to find the energy spread of the beam at the end of the undulator generated by the impedance (1) In this paper we report the results of measurements of the surface roughness of a tube similar to the one that will be used for the fabrication of the undulator beam tube Based on these measurements, we calculate the impedance of the surface and compare it with the tolerance given by Eq (3)

MEASUREMENTS AND OBSERVATIONS

THEORY

Findings

SUMMARY