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

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

4. 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

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

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. 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

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. 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%

13. SASE Demonstration Experiments at Longer Wavelengths
• IR wavelengths ( )
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 ( )
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

14. 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)

15. 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)

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. 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

18. 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  mm
  0.71 %
13.6 GeV
z  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

19. 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

20. 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)

21. 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) …