Introduction to Free Electron Lasers Zhirong Huang

1. LCLS Introduction to Free Electron LasersZhirong Huang

3. Outline • What is FEL • What is SASE FEL • Dependence on e-beam properties • Recent SASE experiments • Accelerator issues

4. Produced by the resonantinteraction 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://sbfel3.ucsb.edu/www/vl_fel.html Free Electron Lasers

5. FEL oscillators Single pass FELs (SASE or seeded)

6. Three FEL modes

7. Undulator Radiation l1 lu forward direction radiation (and harmonics) undulator parameter K = 0.93 B[Tesla] lu[cm] LCLS undulator K = 3.5, lu = 3 cm, e-beam energy from 4.3 GeV to 14 GeV to cover 1 =1.5 nm to 1.5 Å Can energy be exchanged between electrons and co-propagating radiation pulse?

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

9. Self-amplified spontaneous emission (SASE)

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

11. Slippage and FEL slices • Due to resonant condition, light overtakes e-beam by one radiation wavelength 1 per undulator period Interaction length = undulator length optical pulse optical pulse z electron bunch electron bunch Slippage length = 1× undulator period (100 m LCLS undulator has slippage length 1.5 fs, much less than 200-fs e-bunch length) • Each part of optical pulse is amplified by those electrons within a slippage length (an FEL slice) • Only slices with good beam qualities (emittance, current, energy spread) can lase

12. 1 % of X-Ray Pulse Length from H.-D. Nuhn SASE temporal spikes • Due to noisy start-up, SASE has many intensity spikes • LCLS spike ~ 1000 1 ~ • 0.15 mm ~ 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%

13. • IR wavelengths (1998-1999) UCLA/LANL (l = 12m, G = 105) LANL (l = 16m, G = 103) BNL ATF/APS (l = 5.3m, G = 10, HGHG = 107 times S.E.) • Visible and UV (2000-2006) LEUTL (APS): Ee 400 MeV, Lu = 25 m, 120 nm  l  530nm VISA (ATF): Ee = 70 MeV, Lu = 4m, l = 800 nm TTF (DESY): Ee < 300 MeV, Lu = 15 m, l = 80–120 nm SDL (NSLS): Ee < 200 MeV, Lu = 10 m, l = 800–260 nm TTF2 (DESY): Ee ~ 700 MeV, Lu = 27 m, l = 13 nm All Successful, TTF2 (FLASH) is in user operation mode SASE Demonstration Experimentsat Longer Wavelengths

14. LEUTL FEL Observations agree with theory/ computer models (S. Milton et al., Science, 2001)

15. Nonlinear Harmonic Radiation at VISA* Nonlinear Harmonic Energy vs. Distance Energy Comparison Fundamental 2nd harmonic April 20, 2001 3rd harmonic Associated gain lengths * A. Tremaine et al., PRL (2002)

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

17. Operationalexperience and recent resultsfromFLASH(VUVFELatDESY) FLS2006, 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 E. Saldin, E. Schneidmiller and M. Yurkov for FLASH team 14-29.01.2005

18. Commission in Jan. 2008 Commission in Jan. 2007 LCLS must extend FEL wavelength by another two orders of magnitude from 13 nm  1 nm  1 Å 250 MeV z  0.19 mm   1.6 % 4.30 GeV z  0.022 mm   0.71 % 13.6 GeV z  0.022 mm   0.01 % 6 MeV z  0.83 mm   0.05 % 135 MeV z  0.83 mm   0.10 % Linac-X L =0.6 m rf= -160 Linac-0 L =6 m rf gun L0-a,b Linac-3 L 550 m rf  0° Linac-1 L 9 m rf  -25° Linac-2 L 330 m rf  -41° 25-1a 30-8c 21-3b 24-6d ...existing linac 21-1 b,c,d undulator L =130 m X BC1 L 6 m R56 -39 mm BC2 L 22 m R56 -25 mm DL1 L 12 m R56 0 DL2 L =275 m R56  0 SLAC linac tunnel research yard

19. eN = 1.2 mm P = P0 eN = 2.0 mm PP0/100 courtesy S. Reiche Slice emittance >1.8 mm will not saturate electron beam must meet brightness requirements

20. RF photocathode gun 1 mm 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 mm level (beam-based alignment) undulator parameter tolerance (e.g., DK/K ~ 10-4) Accelerator issues

21. More undulator lines (different wavelength coverage) Shorter x-ray pulses (200 fs  10 fs  1 fs and below) Enhanced performance (optical buncher, FEL buncher) Better temporal coherence (some forms of seeding) … Future upgrade possibilities