Modeling of the microbunching instability

M Borland

doi:10.1103/physrevstab.11.030701

Abstract

We show that, through careful control of noise sources, it is possible to determine the microbunching gain curve for the FERMI@ELETTRA linac using the particle tracking code elegant. In addition to using a sufficiently large number of particles ($60\ifmmode\times\else\texttimes\fi{}{10}^{6}$), use of a low-pass filter is very helpful in controlling noise and providing convenient intrabin interpolation. Gains of up to 1500 are seen for modulation wavelengths down to $25\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Because of the high gain, very small initial modulations are needed to avoid saturation, which further motivates the use of a large number of particles. We also show, for the first time, how the density modulation evolves in detail inside the dipoles of a multichicane system.

Highlights

That coherent synchrotron radiation (CSR) generated by short bunches in magnetic bunch compressors and other bending systems may degrade beam quality has been known for more than a decade [1]
Simulations [4,5] with the code ELEGANT [6] predicted in addition a CSRdriven microbunching instability that may have a dramatic impact on free-electron lasers (FELs) performance
In addition to CSR, it was discovered [7] that longitudinal space charge (LSC) can lead to a potentially more serious microbunching instability when combined with bunch compression and CSR

Summary

Introduction

That coherent synchrotron radiation (CSR) generated by short bunches in magnetic bunch compressors and other bending systems may degrade beam quality has been known for more than a decade [1]. Magnetic bunch compression is a common feature of linacs designed as drivers for free-electron lasers (FELs), for example, the Linac Coherent Light Source (LCLS) [2] and FERMI [3] projects. It was thought that the most important effect of CSR was to increase the projected beam emittance and energy spread. Simulations [4,5] with the code ELEGANT [6] predicted in addition a CSRdriven microbunching instability that may have a dramatic impact on FEL performance. In addition to CSR, it was discovered [7] that longitudinal space charge (LSC) can lead to a potentially more serious microbunching instability when combined with bunch compression and CSR. Various suppression mechanisms were identified, such as use of a superconducting wiggler [2] or ‘‘laser-undulator beam heater’’ [8] to increase the slice energy spread of the beam

Methods

Results

Conclusion