Enhanced seeded free electron laser performance with a “cold” electron beam

FERMI is a seeded free electron laser (FEL) based on the high gain harmonic generation (HGHG) scheme and has been generating intense and fully coherent extreme ultra-violet and soft x-ray pulses for several years. The high-degree performance of the FEL leans on a high brightness electron beam, with small transverse emittance ($\sim 1\mu \mathrm{m}$) and a peak current of about 700-800 A. The main constraint on lasing at high harmonics of the seed is low electron time-slice energy spread. The optimization of the photoinjector and of some linac parameters has allowed a reduction of the relative slice energy spread to the level of few times of ${10}^{-5}$. With these new conditions, the FEL can be operated without the need of a laser heater to suppress micro bunching instabilities and this “cold” beam has allowed generation of extreme UV pulses with pulse energy exceeding a mJ, and with peak power of about 10 GW. We describe the electron beam characterization and the FEL performance improvement, including the extension of the range of harmonics of the seed which can be amplified, up to the twenty-fifth harmonic, i.e., 10 nm.

Figure 1

FEL pulse energy at 31.9 nm versus the dispersive section $R_{56}$, for different values of the laser heater energy. Inset reports, for comparison, the FEL normalized pulse energy at 32.1 nm versus $R_{56}$ from previous experiments [33].

Figure 2

Measured time-slice energy spread obtained from Eq. (1) applying the FEL versus $R_{56}$ scan as reported in Fig. 1. The error bars refer to a confidence interval of 68% in evaluating the optimum value of $R_{56}$.

Figure 3

The maximum ${\delta }_{\mathrm{FEL}}$ versus the FEL output wavelength calculated for FERMI FEL-1 undulators tuned in circular and linear polarization, according to Eq. (2). The standard FERMI case (${\sigma }_E=100\mathrm{keV}$) and the actual condition (${\sigma }_E=40\mathrm{keV}$) are indicated with horizontal lines, assuming a beam energy of 1.32 GeV.

Figure 4

FEL performance versus laser heater for a standard electron beam in 2018 (a,b,c) and for the current one (d,e,f). FEL intensity (blue curve) and laser heater intensity (red curve) (a,d) are plotted as a function of the LH attenuator angle. FEL spectra versus LH attenuator angle are reported in (b,e) in false color scale (normalized for the maximum intensity value). Few spectra for the standard case (c) and for the actual case (f) have been randomly selected for the minimum value of the LH ($\sim 0.2\mu \mathrm{J}$).

Figure 5

Characterization of FEL pulses at $h=12$ ($\sim 21\mathrm{nm}$) in linear horizontal polarization. Random selection of individual spectra (a) and statistical analysis over 1800 shots showing energy per pulse (b) central wavelength (c) and rms spectral bandwidth evolution (d). Few shots ($\sim 10\%$) have been removed from the analysis due to very different e-beam parameters.

Figure 6

Characterization of FEL pulses at $h=18$ (i.e., $\sim 14\mathrm{nm}$) in linear horizontal polarization. Random selection of individual spectra (a) and statistical analysis over 1800 shots showing energy per pulse (b) central wavelength (c) and rms spectral bandwidth evolution (d). Few shots ($\sim 10\%$) have been removed from the analysis due to very different e-beam parameters.

Figure 7

Average energy per pulse (blue dots) measured for FEL-1 in circular polarization from $h=7$ ($\sim 35\mathrm{nm}$) down to $h=25$ ($\sim 10\mathrm{nm}$). Error bars correspond to the standard deviation over hundred of shots. Red line corresponds to the standard guaranteed FERMI FEL-1 energy as reported in [53].

Figure 8

FEL gain curve for an highly compressed beam ($\sim 1.3\mathrm{kA}$) at $h=7$ ($\sim 37\mathrm{nm}$) in circular polarization. (a) FEL energy growth as a function of the number of resonant undulators (blue curve) and applied tapering (red curve). (b) Normalized spectra along the gain curve.