Experimental Demonstration of Longitudinal Beam Phase-Space Linearizer in a Free-Electron Laser Facility by Corrugated Structures

Removal of the undesired time-energy correlations in the electron beam is of paramount importance for efficient lasing of a high-gain free-electron laser. Recently, it has been theoretically and experimentally demonstrated that the longitudinal wakefield excited by the electrons themselves in a corrugated structure allows for precise control of the electron beam phase space. In this Letter, we report the first utilization of a corrugated structure as a beam linearizer in the operation of a seeded free-electron laser driven by a 140 MeV linear accelerator, where a gain of $\sim 10000$ over spontaneous emission was achieved at the second harmonic of the 1047 nm seed laser, and a free-electron laser bandwidth narrowing by 50% was observed, in good agreement with the theoretical expectations.

Figure 1

Moveable corrugated plates, where a rectangular coordinate system is centered on the axis. For the case in this Letter, the separation between the two aluminum plates can be adjusted from $a=1$ to 25 mm, with parameters $p=0.6\mathrm{mm}$, $\delta =2.0\mathrm{mm}$, and $g=0.3\mathrm{mm}$, and plate lengths of 0.3 m and plate widths of 30 mm.

Figure 2

Schematic layout of experiment at the SDUV-FEL. In the experiment, two correctors attached at the two edges of the modulator are first fine tuned for beam trajectory control in the corrugated structures, while the signals from UNPRF2 and BPM1 serve as the reference monitor of the bunch charge, thus to make sure that the electron beam passes through the corrugated chamber without much loss. Then, screens UNPRF0 and UNPRF1 are used to spatially synchronize the electron and laser in the modulator. The FEL radiation properties were investigated with a spectrometer, a photodetector, and a charge-coupled device.

Figure 3

Simulated phase space of the electron beam, where the blue and red lines represent the phase space before and after the corrugated device, respectively, with the phases of $A3$ and $A4$ being (a) 0°, (b) $-25°$, and (c) 25°.

Figure 4

FEL spectra from the simulation and experiment for two corrugated plane gaps (red at $a=1.0\mathrm{mm}$ and blue at $a=3.0\mathrm{mm}$). (a) Simulated radiation spectra. (b) Typical spectra from experiment. The measurements were performed with a commercial spectrometer (model TRIAX-550, Jobin Yvon) with a focal length of 550 mm, a grating of $600\text{lines}/\mathrm{mm}$, and a calibrated resolution of 2.7 nm with the front slit open. (c) Sequence of eight consecutive shots of FEL spectra from experiment.

Figure 5

The beam energy distribution measurement with on-crest acceleration of $A3$ and $A4$ cavities, in which the corrugated linearizer with the separation parameter $a=5.0$, 3.0, and 1.0 mm, respectively, was used.