Reversible beam heater for suppression of microbunching instability by transverse gradient undulators

Abstract

The microbunching instability driven by beam collective effects in a linear accelerator of a free-electron laser (FEL) facility significantly degrades the electron beam quality and FEL performance. A conventional method to suppress this instability is to introduce an additional uncorrelated energy spread by laser-electron interaction, which has been successfully operated in the Linac Coherent Light Source and Fermi@Elettra, etc. Some other ideas are recently proposed to suppress the instability without increasing energy spread, which could benefit the seeded FEL schemes. In this paper, we propose a reversible electron beam heater using two transverse gradient undulators to suppress the microbunching instability. This scheme introduces both an energy spread increase and a transverse-to-longitudinal phase space coupling, which suppress the microbunching instabilities driven by both longitudinal space charge and coherent synchrotron radiation before and within the system. Finally the induced energy spread increase and emittance growth are reversed. Theoretical analysis and numerical simulations are presented to verify the feasibility of the scheme and indicate the capability to improve the seeded FEL radiation performance.

Shaping Micro-Bunched Electron Beams for Compact X-ray Free-Electron Lasers with Transverse Gradient Undulators

“…We will assume the form K(x) = K 0 (1 + αx). The motion of a particle in the presence of such a field was analyzed in [26], in which they found that the motion could be described using a linear transfer matrix R with the key non-trivial elements being…”

“…We begin with simulations using ideal beams, meaning beams generated simply by explicitly applying the transformations given in Equations ( 23)- (26). We again consider three modulation amplitudes: A = 16.5, A = 34, and A = 50.…”

Mitigation of microbunching instability in x-ray free electron laser linacs

“…The longitudinal phase space smearing brought up by the coupling terms in BC1 can greatly smooth the upstream density and energy modulation, thus suppresses further instability amplification in the following transport system. Meanwhile, due to the existence of energy chirp before BC1, additional slice energy spread will also be induced by the transverse to longitudinal coupling terms, which is given as [21,25]:…”

“…However, for seeded FELs it could degrade the FEL performance due to the increase of final electron beam energy spread [16]. Recently several new methods based on transverse-to-longitudinal coupling were proposed to suppress the microbunching instability [17][18][19][20][21]. These schemes are attractive because of the "reversible feature," which means the transverse emittance and slice energy spread can be recovered meanwhile effectively suppresses the microbunching instability [22].…”

X-ray free electron laser linear accelerator without a laser heater

“…The electron beam passing through the laser heater chicane attains extra uncorrelated energy spread from the laser induced energy modulation. Such a device provides an effective control of the electron beam uncorrelated energy spread and was widely adopted in most modern linear accelerator based x-ray FEL light sources [15,16,17,18,19] even though several different but less mature methods were proposed to mitigate the microbunching instability [20,21,22,23,24,25].…”