F. Villa Laboratori Nazionali di Frascati - LNF On behalf of Sparc_lab Generation, measure and FEL radiation of ultrashort electron beams at Sparc_lab F. Villa Laboratori Nazionali di Frascati - LNF On behalf of Sparc_lab

Sparc_lab Compton, LWFA THz, diagnostic studies FLAME 300 TW laser FEL Photoinjector

Ultrashort bunch generation: Gun Laser: 300 fs FWHM, 0.3 mm diameter (flat top) 264 nm central wavelength Electron bunch charge: 20 pC Injection phase: 38° (optimizes the energy at the gun exit, 5 MeV)

Ultrashort bunch generation: velocity bunching Emittance compensating solenoids (300G) Velocity bunching On crest Energy: 113.9 MeV Energy spread: 0.2 MeV (0.1%)

Electron longitudinal phase space Pixel size = 33.5 um Pixel value= 22 fs Beam Energy (MeV) Energy Spread (%) Bunch rms duration (fs) Charge (pC) Long. Emit. (keV mm) 113.93 (0.01) 0.146 (0.004) 32 (1) 20.00 (0.25) 2.62 (0.09) Actual peak current: 625 A

Compression scan resolution

Electron envelope and emittance

FEL: undulators FEL diagnostics: Fiber spectrometer (850-350 nm) Linac Electron diagnostics and matching 5 undulator sections FEL diagnostics: Fiber spectrometer (850-350 nm) In vacuum spectrometer (NIR-UV)

FEL radiation: single spike Central wavelength 𝜆 𝐹𝐸𝐿 = 𝜆 𝑢 2 𝛾 2 1+ 𝐾 2 2 =740 𝑛𝑚 Single spike condition Fiber spectrometer In vacuum spectrometer GENESIS 1.3 simulation 𝐿 𝑒 <2𝜋 𝐿 𝑐 = 𝜆 𝐹𝐸𝐿 2 3 𝜌 𝐿 𝑒 =32 𝑓𝑠⋅𝑐; 𝐿 𝑐 =37 𝑓𝑠⋅𝑐 FEL parameter: K = 1.8 ρ = 5.3 10-3

SASE fluctuation Experimental data GENESIS 1.3 simulations (with e- shot noise only) Due to other jitters (Charge, positions, energy, emittance…)

FEL spectrum bandwidth 15 20 25 30 35 40 45 50 55 3,5 4,0 4,5 5,0 5,5 6,0 6,5 Dl (nm) L(fs) FEL spectrum bandwidth Experimental points GENESIS 1.3 simulation Analytical formula 𝜎 𝜆 ∝𝜌∝ 𝜎 𝑧 − 1 3 at saturation

Summary Short electron bunch FEL radiation Velocity bunching Single spike condition Electron length vs. FEL spectral bandwidth