# Laser system

Detail of the laser amplifiers

The SPARC_LAB photocathode laser system is based on a Titanium-Sapphire laser at 800 nm of about 150 fs and its harmonics. The laser system is synchronized with the RF system and based on the Chirped Pulse Amplification (CPA) of the pulse (the same scheme awarded by the 2018 Nobel prize) with a layout similar to the FLAME laser and a compressed energy of about 50 mJ. We split the laser pulses from the oscillator to two CPA systems, one used for the photocathode and one for FEL seeding and diagnostics. The photocathode pulses are then up-converted via harmonic generation to the 3rd harmonic at 266 nm, the wavelength required for electron extraction out of copper cathode. We developed a system capable of generating a longitudinal train of pulses used for the Comb Beam, needed in two color FEL or plasma acceleration experiments. We use the imaging of an iris to obtain a quasi-flat-top laser profile on the cathode (like the image below). A transport line delivers the IR laser from the laser rooms to the FEL and diagnostic (in particular to the Electro-Optic Sampling) along the electron beamline.

UV profile on the cathode

Laser system parameters
Laser central wavelength (nm) 795
Laser oscillator rep rate (MHz) 79.3
Laser FWHM pulse length @ oscillator exit (fs) 150
Laser FWHM pulse length @ cathode (ps) 0.2 – 10
Max laser pulse energy (IR) for the photocathode system (mJ) 50
Max laser pulse energy (IR) for the seeding/diagnostics system (mJ) 5
Max laser pulse energy (UV) @ cathode (mJ) 1
Repetition rate (Hz) 10
Rms energy jitter in UV (shot-to-shot) (%) 1 – 5
Rms pointing stability on cathode (µm) 15

# RF gun

SPARC_LAB RF gun

The photocathode is composed by a 1.6 cell RF gun operated at S-band (2.856 GHz, of BNL/UCLA/SLAC type) with a high peak field on the cathode (120 MV/m) generating 5 MeV electron beam. It has two laser access points: one at 70° of incidence from viewports beside the photocathode and one at 3° of incidence with an off-axis mirror inside a vacuum chamber at about 0.5 m from the cathode. Quantum efficiency is typically in the order of $10^{-5}$ for our copper cathode and we measured intrinsic emittance of 0.7 mm mrad /mm. A four-coil solenoid assembly approximately 20 cm in length is inserted after the RF gun. This element is crucial for emittance minimization to have high brightness electron beam, in a configuration where a double minimum in emittance is generated along the line. After the solenoid there is a small diagnostic station for beam characterization in transverse dimension, energy, charge and emittance.

Surface of the polycrystalline oxigen-free copper cathode

Photocathode system parameters
Maximum peak field on the cathode 120 MV/m
Repetition rate 10 Hz
Operating vacuum 10^-9 Torr
Quantum efficiency max (typical) 10^-5 (5 10^-6)
Maximum gun solenoid field 0.3 T
Laser central wavelength 263 nm
RF frequency 2.856 GHz
RMS intrinsic emittance 0.7 mm mrad/mm
Beam charge 10-1000 pC
Number of cells 1.6

Publication highlights:

• “Laser comb with velocity bunching: Preliminary results at SPARC”, M. Ferrario et al., Nucl. Instr. Meth. Phys. Res. A vol. 637 (2011) pag. S43, doi: 10.1016/j.nima.2010.02.018
• “Laser pulse shaping for multi-bunches photoinjectors”, F. Villa et al., Nucl. Instr. Meth. Phys. Res. A vol. 740 (2014) pag. 188, doi: 10.1016/j.nima.2013.11.060
• “Laser pulse shaping for high gradient accelerators”, F. Villa et al., Nucl. Instr. Meth. Phys. Res. A vol. 829 (2016) pag. 446, doi: 10.1016/j.nima.2016.01.010
• “RF and Magnetic Measurements on the SPARC Photoinjector and Solenoid at UCLA”, J.B. Rosenzweig et al., PAC conference 2005 doi: 10.1109/PAC.2005.1591206
• “Direct measurement of the double emittance minimum in the beam dynamics of the SPARC high-brightness photoinjector”, M. Ferrario et al., Phys. Rev. Lett. vol. 99 (2007) pag. 234801, doi: 10.1103/PhysRevLett.99.234801
• “Nano-machining, surface analysis and emittance measurements of a copper photocathode at SPARC_LAB”, J. Scifo et al., Nucl. Instr. Meth. Phys. Res. A vol. 909 (2018) pag. 233, doi:10.1016/j.nima.2018.01.041

Contact people:

Cathode: J. Scifo, jessica.scifo@lnf.infn.it, tel. (+39) 069403 2281

Laser: F. Villa, fabio.vila@lnf.infn.it, tel. (+39) 069403 2347