SASE FEL experiment at SPARC (2009)
In the SASE FEL experiment at SPARC_LAB the intensity of the FEL radiation has been observed and measured after each of the six undulator modules and the exponential growth has been well reproduced even not reaching saturation. The beam used in the SASE FEL experiment was obtained with a longitudinally flattop laser pulse, with a length of 6–8 ps (FWHM) with about 400 pC of charge and a beam energy of 150 MeV. The reconstructed longitudinal phase space, measured by means of the RF deflector, is shown in Fig. 1. A complete list of electron beam parameters is detailed in Table 1 .
|Beam energy||150 MeV|
|Relative energy spread (rms)||9 x 10-4|
|Slice energy spread (rms)||2 x 10-4|
|Bunch length (rms)||2.6 ps|
|Electron charge||400 pC|
|Beam current||53 A|
|Repetition rate||10 Hz|
|Vertical emittance||2.9 mm mrad|
|Horizontal emittance||2.5 mm mrad|
Table 1 – Electron beam parameters for the SASE FEL experiment at SPARC_LAB
Such a beam was injected in the SPARC_LAB undulator tuned at a resonant wavelength of 500 nm, with an undulator strength $K_u$=2.07. The undulator focusing was reached by imposing a periodic condition to the transverse beam over the focusing-defocusing (FODO) and by imposing that the transverse average Twiss βx,y coefficients in the two directions are equals.
Spectra of the FEL radiation have been measured together with the pulse energy, as a function of the position in the undulator, just turning off the FEL interaction by progressively suppressing the amplification process, by detuning one by one the undulator modules starting from the begin of the line. This procedure allowed the measure of the spectrum generated in the remaining modules, with an increasing number of undulators participating in amplification, while keeping unchanged the geometry of the radiation detection.
The most relevant parameters for the FEL radiation are summarized in Table 2. The evolution of the spectral profile and pulse energy is shown in Fig. 2. Also the third harmonic at 166 nm has been detected.
Results for the fundamental ad third harmonic have been compared with the most used numerical codes in the field, showing a very good agreement with simulations.
|Undulato parameter K||2.07|
|Gain length||0.7 m|
|Pierce parameter (effective)||1.8 x 10-3|
|Pule energy||0.01 mJ|
|Spot size (rms)||1 mm|
Table 2 – Measured parameters of the output FEL radiation.
“Self-amplified spontaneous emission for a single pass free-electron laser”, L.Giannessi et al, Phys. Rev. ST Accel. Beams vol. 14 (2011), pag. 060712 doi: 10.1103/PhysRevSTAB.14.060712