Stable Operation of a Free-Electron Laser Driven by a Plasma Accelerator

The breakthrough provided by plasma-based accelerators enabled unprecedented accelerating fields by boosting electron beams to gigaelectronvolt energies within a few centimeters [1–4]. This, in turn, allows the realization of ultracompact light sources based on free-electron lasers (FELs) [5], as demonstrated by two pioneering experiments that reported the observation of self-amplified spontaneous emission (SASE) driven by plasma-accelerated beams [6,7]. However, the lack of stability and reproducibility due to the intrinsic nature of the SASE process (whose amplification starts from the shot noise of the electron beam) may hinder their effective implementation for user purposes. Here, we report a proof-of-principle experiment using plasma-accelerated beams to generate stable and reproducible FEL light seeded by an external laser. FEL radiation is emitted in the infrared range, showing the typical exponential growth of its energy over six consecutive undulators. Compared to SASE, the seeded FEL pulses have energies 2 orders of magnitude larger and stability that is 3 times higher.

Figure 1

Experimental setup. The driver (D) and witness (W) electron bunches, produced by the photoinjector, are focused by a triplet of PMQs in a 3 cm-long capillary containing the plasma produced by ionizing hydrogen gas with a high-voltage discharge. The accelerated witness is extracted by the second triplet of permanent-magnet quadrupoles (PMQs) and transported by using six electromagnetic quadrupoles. A magnetic chicane allows one to displace the beam and to insert an in-vacuum metallic mirror to inject the IR laser along the beam path. A dipole spectrometer is used to measure its energy with a scintillator screen installed on a 14° beamline. The FEL beamline consists of six planar undulators with tunable gaps and five quadrupoles in between. The emitted FEL radiation is collected by an in-vacuum high-reflective metallic mirror and measured with an imaging spectrometer equipped with a diffraction grating and a cooled intensified camera (iCCD).

Figure 2

Witness bunch acceleration in plasma. (a) Snapshot of the driver (D) and witness (W) spectrum with plasma turned off (top) and on (bottom). The red line shows the projected energy spectrum. The decelerated driver energy spectrum is obtained by merging the images obtained with different currents of the magnetic spectrometer. (b) Numerical simulation of the plasma wakefield acceleration. The snapshot shows the two bunches moving through the plasma background. The white dashed line shows the axial accelerating field along the longitudinal coordinate $\xi$.

Figure 3

Simulated FEL pulse energy (blue crosses) downstream of the last undulator as a function of the seed laser wavelength. The red dashed line shows the Gaussian fit of the theoretical data centered at $826.6\pm 4\mathrm{nm}$. The green circle shows the expected energy for the seed wavelength used in the experiment.

Figure 4

Exponential growth of the radiation. Energy gain obtained with the seeded laser on and off along the undulators (red crosses and blue diamonds, respectively). The dashed lines show the exponential fit of the experimental data. The resulting FEL simulations (green and purple stars) are also reported. The error bars are computed as the standard deviation of the signal amplitudes measured at each point.

Figure 5

Spectral analysis of the amplified light energy in the seeded configuration. (a) Comparison of 100 shots acquired in SASE and seeded FEL configurations with the seed laser turned off and on, respectively. (b) Spectral distributions of the 10 seeded FEL shots with the largest energies. The shot number is reported adopting the correspondent spectra color.