LCLS Introduction to Free Electron Lasers Zhirong Huang

LCLS Introduction to Free Electron Lasers Zhirong Huang

Outline Ø What is FEL Ø What is SASE FEL Ø Dependence on e-beam properties Ø Recent SASE experiments Ø Accelerator issues

Free Electron Lasers • Produced by the resonant interaction of a relativistic electron beam with a photon beam in an undulator • Tunable, Powerful, Coherent radiation sources • 1977 - First operation of a free-electron laser at Stanford University • Today – 22 free-electron lasers operating worldwide – 19 FELs proposed or in construction – More info at http: //sbfel 3. ucsb. edu/www/vl_fel. html

FEL oscillators Single pass FELs (SASE or seeded)

Three FEL modes

Undulator Radiation l 1 lu forward direction radiation (and harmonics) undulator parameter K = 0. 93 B[Tesla] u[cm] LCLS undulator K = 3. 5, u = 3 cm, e-beam energy from 4. 3 Ge. V to 14 Ge. V to cover 1 = 1. 5 nm to 1. 5 Å Can energy be exchanged between electrons and copropagating radiation pulse?

FEL principles Ø Electrons slip behind EM wave by 1 per undulator period Vx. Ex>0 Ø Due to sustained interaction, some electrons lose energy, while other gain energy moduation at 1 Ø Electrons losing energy slow down, and electrons gaining energy catch up density modulation at 1 (microbunching) ØMicrobunched beam radiates coherently at 1, enhancing this process exponential growth of radiation power

Self-amplified spontaneous emission (SASE)

X-ray FEL requires extremely bright beams Ø Power grows exponentially with undulator distance z only if “slice” energy spread << 10 -3 slice emittance power gain length local peak current Ø FEL power reaches saturation at ~ 20 LG Ø SASE performance depends exponentially on ebeam qualities

Slippage and FEL slices Ø Due to resonant condition, light overtakes e-beam by one radiation wavelength 1 per undulator period Interaction length = undulator length optical pulse electron bunch z Slippage length = 1 × undulator period (100 m LCLS undulator has slippage length 1. 5 fs, much less than 200 -fs e-bunch length) Ø Each part of optical pulse is amplified by those electrons within a slippage length (an FEL slice) Ø Only slices with good beam qualities (emittance, current, energy spread) can lase

SASE temporal spikes • Due to noisy start-up, SASE has many intensity spikes • LCLS spike ~ 1000 1 ~ 0. 15 m ~ 0. 5 fs! • From one spike to another, no phase correlation Each spike lases indepedently, depends only on the local (slice) beam parameters LCLS pulse length ~ 200 fs with ~ 400 SASE spikes ~ x-ray energy fluctuates 5% 1 % of X-Ray Pulse Length from H. -D. Nuhn

SASE Demonstration Experiments at Longer Wavelengths • IR wavelengths (1998 -1999) UCLA/LANL ( = 12 , G = 105) LANL ( = 16 , G = 103) BNL ATF/APS ( = 5. 3 , G = 10, HGHG = 107 times S. E. ) • Visible and UV (2000 -2006) LEUTL (APS): Ee 400 Me. V, Lu = 25 m, 120 nm 530 nm VISA (ATF): Ee = 70 Me. V, Lu = 4 m, = 800 nm TTF (DESY): Ee < 300 Me. V, Lu = 15 m, = 80– 120 nm SDL (NSLS): Ee < 200 Me. V, Lu = 10 m, = 800– 260 nm TTF 2 (DESY): Ee ~ 700 Me. V, Lu = 27 m, = 13 nm All Successful, TTF 2 (FLASH) is in user operation mode

LEUTL FEL A B C st (ps) 0. 19 0. 77 0. 65 I (A) 630 171 184 en ( m) 8. 5 7. 1 sd (%) 0. 4 0. 2 0. 1 (nm) 530 385 Observations agree with theory/ computer models (S. Milton et al. , Science, 2001)

Nonlinear Harmonic Radiation at VISA* Nonlinear Harmonic Energy vs. Distance Fundamental 2 nd harmonic April 20, 2001 3 rd harmonic Associated gain lengths * A. Tremaine et al. , PRL (2002) Energy Comparison Mode (n) Wavelength (nm) Energy ( J) % of E 1 1 845 52 2 421 . 93 1. 8 3 280 . 40 . 77

TTF FEL at 98 nm* Statistical fluctuation Transverse coherence after double slit * V. Ayvazyan et al. , PRL (2002); Eur. Phys. J. D (2002) after cross

Operational experience and recent results from FLASH (VUV FEL at DESY) E. Saldin, E. Schneidmiller and M. Yurkov for FLASH team FLS 2006, May 16, 2006 • Milestones • Parameters of FEL radiation • Beam dynamics: consequences for machine operation • Tuning SASE: tools and general remarks • Main problems • Lasing at 13 nm 14 -29. 01. 2005

LCLS must extend FEL wavelength by another two orders of magnitude from 13 nm 1 Å 6 Me. V z 0. 83 mm 0. 05 % 4. 30 Ge. V z 0. 022 mm 0. 71 % 13. 6 Ge. V z 0. 022 mm 0. 01 % Linac-X L =0. 6 m rf= -160 Linac-0 L =6 m rf gun 250 Me. V z 0. 19 mm 1. 6 % 135 Me. V z 0. 83 mm 0. 10 % b , -a L 0 Linac-1 L 9 m rf -25° . . . existing linac 21 -1 b, c, d DL 1 L 12 m R 56 0 Linac-2 L 330 m rf -41° X BC 1 L 6 m R 56 -39 mm Commission in Jan. 2007 21 -3 b 24 -6 d BC 2 L 22 m R 56 -25 mm Linac-3 L 550 m rf 0° 25 -1 a 30 -8 c Commission in Jan. 2008 SLAC linac tunnel undulator L =130 m DL 2 L =275 m R 56 0 research yard

Slice emittance >1. 8 mm will not saturate e. N = 1. 2 mm P = P 0 e. N = 2. 0 mm P P 0/100 courtesy S. Reiche electron beam must meet brightness requirements

Accelerator issues • RF photocathode gun – 1 m normalized emittance, reasonable peak current • Emittance preservation in linacs (SLC experiences) • Bunch compression – coherent synchrotron radiation – microbunching instability (mitigated by a laser heater) • Machine stability – energy jitter (wavelength jitter) – bunch length and charge jitters (FEL power jitter) – transverse jitters (power and pointing jitters) • Undulator – straight trajectory to m level (beam-based alignment) – undulator parameter tolerance (e. g. , DK/K ~ 10 -4)

Future upgrade possibilities • More undulator lines (different wavelength coverage) • Shorter x-ray pulses (200 fs 1 fs and below) • Enhanced performance (optical buncher, FEL buncher) • Better temporal coherence (some forms of seeding) • …