Free Electron Lasers (FEL’s) Toon Coenen 07-04-2009

Outline Introduction FEL’s Laser characteristics Power output calculation Research Conclusion Photon Physics Laser presentation 07-04-2009

Introduction Bound electrons Population inversion Coherent radiation Free electrons in vacuum Photon Physics Laser presentation 07-04-2009

FEL’s Photocathode Accelerator Relativistic speeds Undulator Periodic magnetic field Oscillations Photon Physics Laser presentation 07-04-2009

FEL’s Point dipoles Low energy (electron frame)→ High energy (lab frame) Blue shift Kinetic energy electron Tune emission wavelength Photon Physics Laser presentation 07-04-2009

FEL’s Conventional design Laser Cavity Standing wave Impossible for λ<150nm Mirrors Mirror Undulator Photon Physics Laser presentation 07-04-2009

FEL’s Single pass FEL’s SASE Micro-bunching Hard x-ray laser XFEL in Hamburg ~3.4 km long Operational in 2015 Photon Physics Laser presentation 07-04-2009

Laser Characteristics Silfvast European XFEL Laser wavelengths 248 nm to 8 mm 0.06 to 0.4 nm Laser gain medium length 1–25 m 100-200 m Pumping method Electron beam Electron beam pulse length 2 ps to cw 200 fs Electron beam energy 200 keV to 1 GeV 17.5 GeV Electron beam peak current 0.1-800 A 5 kA Undulator magnet period 5 mm to 0.2 m 26 mm Magnetic field strength 0.02 to 1T 0.28 to1.24T Output power Up to GW(pulsed), 10W (cw) 20 to 150 GW (pulsed) Photon Physics Laser presentation 07-04-2009

Power Output Stanford FEL Power within the cavity: PC = N2*P/(1-(1-T)1/2)2 N = 2.94*107 electrons P = 3.24 *10-14W T = 1.5 % Output power: Pout = PCT = 7.4 kW Properties Values Wavelenght 2.8 μm Electron pulse length 0.13 cm Pulse spacing 84.6 ns Electron energy 43.5 MeV Beam radius 0.03 cm Magnetic Field period 3.22 cm Field strength 0.24 T Interaction length 5.2 m Cavity length 12.7 m Photon Physics Laser presentation 07-04-2009

Single shot ultrafast diffraction λ =32nm tp=25 fs Photon Physics Laser presentation 07-04-2009

Conclusion Tune device to match needs Wavelenghts up to x-rays High output power Short pulses Sources: [1] W. T. Silfvast, Laser Fundamentals 2nd edition 2004, chapter 14.4 [2] Technical design report European XFEL [3] Y.W. Chan, W.L. Tsui, On the output power of a magnetic free electron laser, Physics letters, October 1979 Photon Physics Laser presentation 07-04-2009

Questions? Photon Physics Laser presentation 07-04-2009