Multicolor High-Gain Free-Electron Laser Driven by Seeded Microbunching Instability

Laser-heater systems are essential tools to control and optimize high-gain free-electron lasers (FELs) working in the x-ray wavelength range. Indeed, these systems induce a controllable increase of the energy spread of the electron bunch. The heating suppresses longitudinal microbunching instability which otherwise would limit the FEL performance. Here, we demonstrate that, through the action of the microbunching instability, a long-wavelength modulation of the electron beam induced by the laser heater at low energy can persist until the beam entrance into the undulators. This coherent longitudinal modulation is exploited to control the FEL spectral properties, in particular, multicolor extreme-ultraviolet FEL pulses can be generated through a frequency mixing of the modulations produced by the laser heater and the seed laser in the electron beam. We present an experimental demonstration of this novel configuration carried out at the FERMI FEL.

Figure 1

FERMI FEL-1 experimental setup. Electrons extracted by a photoinjector (Gun) pass through a laser-heater system (LH) and then a bunch compressor (BC1). They are then accelerated up to 1.0–1.5 GeV or, alternatively, can be sent to a spectrometer (SPBC1) where their energy distribution can be measured on an intercepting YAG (Yttrium aluminium garnet) screen. Following final acceleration to 1.0–1.5 GeV, they are bent through an achromatic spreader to the FEL-1 undulators. At the end of the undulators, they are dumped into the main beam dump (MBD).

Figure 2

Electron beam energy spectra in the spectrometer (SPBC1) after the BC1 in the case of (a)–(c) energy spread modulation and (d)–(f) uniform heating. (a),(d) Measured laser-heater pulse shape and sketched overlap position with the electron bunch. (b),(e) Spectrometer images with energy on the horizontal axis and vertical coordinate on the vertical axis. (c),(f) Electron beam energy spectra. When the electron bunch is overlapping the beating region of the laser heater, a strong modulation in the energy spectrum is visible (c). The parameters of linac 1 are $\varphi =117°$, $k_{\mathrm{rf}}=2\pi /0.1{\mathrm{m}}^{-1}$, $E_i=98\mathrm{MeV}$, and $E_{L1}=188\mathrm{MeV}$.

Figure 3

(a) Amplitude of the modulation in the energy spectrum versus the compression factor in BC1. (b) COTR signal at the end of the FEL-1 undulators versus the laser-heater intensity in the case of an energy spread modulation (red line) and a uniform heating (black line). The COTR signal in the case of uniform heating is multiplied by a factor 100. The COTR signal is acquired using a germanium sensor from UDT Instruments in the wavelength range 800–1800 nm.

Figure 4

(a) FEL emission spectra versus radiator resonance in the case of an energy spread modulation. The color scale corresponds to the spectrum intensity. (b) FEL spectra in the case of a uniform heating (black line) with a LH energy of $2.8\mu \mathrm{J}$ and in the case of an energy spread modulation (red line) with a LH energy of $7.3\mu \mathrm{J}$ (average over 20 shots). The FEL spectra are acquired using the energy spectrometer PRESTO (pulse-resolved energy spectrometer: transparent and online) [41].

Figure 5

FEL spectrum versus compression factor of BC1. The color scale (a) corresponds to the spectral intensity of the two spectral components. As the compression factor is changed, the wavelength of the sideband is shifted. (b) Compression factor deduced from the upper sideband (red dots) and the lower sideband (blue dots) in (a), by using Eq. (4). In that range of compression, the pyro detector signal is proportional to the compression factor (linear fit, black line).