Impact of Non-Gaussian Electron Energy Heating upon the Performance of a Seeded Free-Electron Laser

Abstract

Laser-heater systems have been demonstrated to be an important component for the accelerators that drive high gain free electron laser (FEL) facilities. These heater systems suppress longitudinal microbunching instabilities by inducing a small and controllable slice energy spread to the electron beam. For transversely uniform heating, the energy spread augmentation is characterized by a non-Gaussian distribution. In this Letter, we demonstrate experimentally that in addition to suppression of the microbunching instability, the laser heater-induced energy distribution can be preserved to the FEL undulator entrance, significantly impacting the performance of high-gain harmonic generation (HGHG) FELs, especially at soft x-ray wavelengths. In particular, we show that the FEL intensity has several local maxima as a function of the induced heating caused by the non-Gaussian energy distribution together with a strong enhancement of the power at high harmonics relative to that expected for an electron beam with an equivalent Gaussian energy spread at an undulator entrance. These results suggest that a single stage HGHG FEL can produce scientifically interesting power levels at harmonic numbers m ≥ 25 and with current seed laser technology could reach output photon energies above 100 eV or greater.

Laser heater commissioning at an externally seeded free-electron laser

“…These results are similar to the improvements in the LCLS output pulse energy due to LH heating [8]. A very detailed study concerning the effective extension of the harmonic upshift range for FERMI's FEL-1 due to non-Gaussian energy spread heating effects of the LH was recently presented in Ferrari et al [40]. That paper (i.e., Figs.…”

“…FERMI's FEL-1 [15] and FEL-2 [16] are externally seeded, single stage, harmonic upshift FELs based on the principle of high gain harmonic generation [39]. As previously reported [10,40] beneficial effects of increased energy spread from the LH have been observed on the FEL-1 output radiation pulse energy over the full current operating range in wavelength from 65 nm down to 17 nm. These results are similar to the improvements in the LCLS output pulse energy due to LH heating [8].…”

Optical Klystron Enhancement to Self Amplified Spontaneous Emission at FERMI

“…Increasing the laser heater energy provides additional energy spread that depletes the OK enhancement, as observed also in [40], and its maximum value is obtained for smaller values of R 56 (see gray triangles and blue diamonds curves in Figure 3). Since, in the present scheme, the first undulator is three meters long, which means slightly more than two in gain length, the spontaneous radiation induced a very small energy modulation in comparison to the uncorrelated energy spread.…”

Transverse space-charge induced microbunching instability in high-brightness electron bunches

“…Indeed, the energy distribution in the simulated beam, including the effect of the laser heater, has a markedly non-Gaussian profile. For an example of reduced microbunching due to non-Gaussian beam distribution in a different but related context see [23]. Another limitation of the analytical model, of course, is the assumption that one can approximate the microbunching instability induced energy modulation by an effective purely sinusoidal modulation.…”