Introduction to Free Electron Lasers Zhirong Huang LCLS Introduction to Free Electron Lasers Zhirong Huang

Outline What is FEL What is SASE FEL Dependence on e-beam properties Recent SASE experiments 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://sbfel3.ucsb.edu/www/vl_fel.html

Single pass FELs (SASE or seeded) FEL oscillators

Three FEL modes

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?

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

Self-amplified spontaneous emission (SASE)

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

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

SASE temporal spikes 1 % of X-Ray Pulse Length from H.-D. Nuhn 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%

SASE Demonstration Experiments at Longer Wavelengths • 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

LEUTL FEL A B C st (ps) 0.19 0.77 0.65 I (A) 630 171 184 en (mm) 8.5 7.1 sd (%) 0.4 0.2 0.1 l (nm) 530 385 Observations agree with theory/ computer models (S. Milton et al., Science, 2001)

Nonlinear Harmonic Radiation at VISA* Energy vs. Distance Energy Comparison Mode (n) Wavelength (nm) Energy (J) % of E1 1 845 52 2 421 .93 1.8 3 280 .40 .77 Fundamental 2nd harmonic April 20, 2001 3rd harmonic Associated gain lengths * A. Tremaine et al., PRL (2002)

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)

Operational experience and recent results from FLASH (VUV FEL at DESY) E. Saldin, E. Schneidmiller and M. Yurkov for FLASH team 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 14-29.01.2005

LCLS must extend FEL wavelength by another two orders of magnitude from 13 nm  1 nm  1 Å 6 MeV z  0.83 mm   0.05 % 135 MeV z  0.83 mm   0.10 % 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 % Linac-X L =0.6 m rf= -160 Linac-0 L =6 m rf gun L0-a,b Linac-1 L 9 m rf  -25° Linac-2 L 330 m rf  -41° Linac-3 L 550 m rf  0° 25-1a 30-8c ...existing linac 21-3b 24-6d 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 Commission in Jan. 2007 Commission in Jan. 2008 SLAC linac tunnel research yard

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

Accelerator issues 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)

Future upgrade possibilities 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) …