Abstract
The transverse gradient undulator (TGU) has important application in the short-wavelength high-gain free electron lasers (FELs) driven by laser-plasma accelerators. However, the usual transversely tapered TGUs need special design and manufacture, and the transverse gradient cannot be tuned arbitrarily. In this paper we explore a new and simple method of using the natural transverse gradient of a normal planar undulator to compensate the beam energy spread effect. In this method, a vertical dispersion on the electron beam is introduced, then the dispersed beam passes through a normal undulator with a vertical off-axis orbit where the vertical field gradient is selected properly related to the dispersion strength and the beam energy spread. Theoretical analysis and numerical simulations for self-amplified spontaneous emission FELs based on laser plasma accelerators are presented, and indicate that this method can greatly reduce the effect of the beam energy spread, leading to a similar enhancement on FEL performance as the usual transversely tapered TGU, but with the advantages of economy, tunable transverse gradient and no demand of extra field for correcting the orbit deflection induced by the field gradient.Received 21 December 2016DOI:https://doi.org/10.1103/PhysRevAccelBeams.20.020707Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsSynchrotron radiation & free-electron lasersAccelerators & Beams
Highlights
In recent years, laser plasma accelerators (LPAs) have made dramatic progress in producing high-energy and high-brightness electron beams, and have been pursued to drive compact x-ray free electron lasers (FELs) [1,2,3,4]
It is generally required that the FEL bandwidth broadening due to the energy spread should be smaller than the intrinsic bandwidth, which is proportional to the FEL parameter ρ for high gain case and to the reciprocal of undulator period number for low gain case
We explore a new and simple method that a normal undulator doubles as a transverse gradient undulator (TGU) for compensating the beam energy spread effect by using the natural transverse field gradient
Summary
Laser plasma accelerators (LPAs) have made dramatic progress in producing high-energy and high-brightness electron beams, and have been pursued to drive compact x-ray free electron lasers (FELs) [1,2,3,4]. The concept of using a “transverse gradient wiggler (undulator)”, which was initially proposed to reduce the effects of large beam energy spread in low gain FELs [5,6], has been reconsidered to improve the performance of short-wavelength FELs driven by LPAs [3,7]. It is proposed using transverse gradient undulators (TGUs) to enhance the harmonic bunching in the seeded FEL regime [8], to suppress the microbunching instability [9], to improve the performance of ultimate storage rings based FEL [10], and to generate largebandwidth x-ray FEL [11,12].
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