Normal planar undulators doubling as transverse gradient undulators

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.

Generation of coherent two-color pulses at two adjacent harmonics in a seeded free-electron laser

“…where γ is the Lorentz factor of the electrons, λ 0 is the wavelength of the seed laser, and K, λ u are the strength parameter and period of the radiator, respectively. However, in the proposed scheme, a transverse gradient undulator (TGU) is adopted as the radiator whose normalized field parameter K has a linear x dependence [32,33],…”

Features and futures of X-ray free-electron lasers

“…57 Moreover, the phase-merging effects of the TGU can also be fulfilled by utilizing a wavefront-tilted seed laser, 59 magnetic dipole, 60 or by using the natural transverse gradient of a normal undulator. 61,62 The requirement for FEL amplification of the beam energy spread makes it impossible to generate hard X-rays via single-stage HGHG or EEHG. There-fore, the multi-stage harmonic generation schemes have been proposed in which an FEL pulse generated by an intermediate radiator is used as the seed laser for the harmonic generation stage via the "fresh bunch" technique.…”

“… 57 Moreover, the phase-merging effects of the TGU can also be fulfilled by utilizing a wavefront-tilted seed laser, 59 magnetic dipole, 60 or by using the natural transverse gradient of a normal undulator. 61 , 62 …”

Dispersion effects on performance of free-electron laser based on laser wakefield accelerator

“…Here, only the linear dispersion is considered in our simulation. It is noted that the beta function is the one before introducing the dispersion [24, 26] . As we discuss in the following part, only the electrons near the beam center achieve the radiation.…”

“…However, the transversely tapered TGUs require a special design and manufacturing process, and the transverse gradient cannot be tuned arbitrarily. A similar scheme using the natural transverse gradient of a normal planar undulator (PU) doubling as a TGU was recently proposed [26] , where the vertical dispersion of the beam is introduced.…”