Comparison of the coherent radiation-induced microbunching instability in a free-electron laser and a magnetic chicane

S Reiche,J B Rosenzweig

doi:10.1103/physrevstab.6.040702

Abstract

A self-amplified spontaneous emission free-electron laser (SASE FEL) is a device which is based on the creation of a very intense, relativistic electron beam which has very little temperature in all three phase planes. The beam in this system is described as having ``high brightness,'' and when it is bent repetitively in a magnetic undulator, undergoes a radiation-mediated microbunching instability. This instability can amplify the original radiation amplitude at a particular, resonant wavelength by many orders of magnitude. In order to obtain high brightness beams, it is necessary to compress them to obtain higher currents than available from the electron source. Compression is accomplished by the use of magnetic chicanes, which are quite similar to, if much longer than, a single period of the undulator. It should not be surprising that such chicanes also support a radiation-mediated microbunching interaction, which has recently been investigated, and has been termed coherent synchrotron radiation (CSR) instability. The purpose of this paper is to compare and contrast the characteristics of the closely related FEL and CSR microbunching instabilities. We show that a high-gain regime of the CSR instability exists which is formally similar to the FEL instability.

Highlights

A linear accelerator consists mainly of nominally straight accelerating structures and focusing elements, and often relies on sections in the beam line where the electron beam is transversely deflected
To enhance the performance of an free-electron laser [4] (FEL), bunch compressors are often incorporated in the beam line to shorten the bunch length, increasing the driving beam current and beam brightness, and the FEL gain
Chicane can degrade the electron beam quality [5], reducing the performance of a succeeding FEL employed on the compressed beam

Summary

INTRODUCTION

A linear accelerator consists mainly of nominally straight accelerating structures and focusing elements, and often relies on sections in the beam line where the electron beam is transversely deflected. In the process of bending the beam in the chicane, broadband synchrotron radiation is emitted The amplitude of this radiation at wavelengths similar to, or longer than, the bunch dimension can be greatly enhanced by constructive interference. To enhance the performance of an FEL, bunch compressors are often incorporated in the beam line to shorten the bunch length, increasing the driving beam current and beam brightness, and the FEL gain It is explicitly sought in an undulator or wiggler, the interaction of the electron bunch with the coherent, spontaneous radiation within a magnetic. Chicane can degrade the electron beam quality [5], reducing the performance of a succeeding FEL employed on the compressed beam On their curved trajectory in bend magnets, the electrons emit radiation, which again moves forward in the beam, because the radiation propagates faster, and in a straight line. We extend the model of CSR instability to the high-gain limit, noting similarities in the theoretical analysis and underlying physics of both CSR and FEL cases

MICROBUNCH INSTABILITY IN A FEL

A MAGNETIC CHICANE

THE CHICANE MODEL

THE LOW-GAIN MODEL

LR2 s2

VIII. CONCLUSIONS