1. LCLS FEL Parameters Heinz-Dieter Nuhn, SLAC / SSRL April 23, 2002
SASE Introduction
Overview of Main LCLS Components
Gun
Linac (Bunch Compression)
Undulator (Structure, Focusing)
Baseline Parameters
FEL Performance
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

2. SASE FELs SASE FEL theory well developed and verified by simulations
FEL radiation starts from noise in spontaneous radiation
Transverse radiation electric field modulates the energy and bunches the electrons within an optical wavelength
Exponential build-up of radiation along undulator length
Saturation
Exponential Gain Regime
Undulator Regime
1 % of X-Ray Pulse
Electron Bunch Micro-Bunching
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

3. Operational Range
Operation will take place at any wavelength in the range
1.5 Å – 15 Å
This will be accomplished by an adjustment of electron beam acceleration to values in the range GeV – 4.54 GeV
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

4. Relevance of Slice Emittance
Primary FEL interaction between electrons and radiation field is local.
Each electron is affected only by radiation field at electron’s current location.
New radiation field is produced at electron’s location and travels with the electron.
“Slippage” between electrons and radiation field in the undulator extend interation interval.
“Slippage” causes the electrons to fall behind one optical wavelength for every undulator traveled
Electrons are affected by radiation produced earlier by electrons closer to the tail of the bunch
Maximum interaction interval not larger than maximum “slippage” in undulator length Lu
Lslip = ( Lu / lu ) × lr (0.5 mm at 1.5 Å, 5 mm at 15 Å)
Interaction interval reduced In exponential gain regime to cooperation length LC, i.e., to “slippage” in power gain length LG
LC = ( LG / lu ) × lr (0.02 mm at 1.5 Å, 0.2 mm at 15 Å)
Amplitude of earlier radiation becomes neglegible compared to more recent radiation.
FEL performance for an interaction interval (or slice) only dependent on slice parameters
Slice emittance, Slice energy spread, Peak Current in a slice, Average slice energy
Distant slices might produce different radiation characteristics.
Slices will saturate over the length of the undulator if slice parameters are sufficient.
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

5. Goal Values for LCLS Beam Parameters
Parameter Location LCLS Goal Values*
Normalized Slice Emittance Injector MeV) 1.0 mm mrad (RMS)
Undulator Entrance 1.2 mm mrad (RMS)
Normalized Projected Emittance Injector MeV) 1.2 mm mrad (RMS)
Undulator Entrance 1.5 mm mrad (RMS)
Slice Energy Spread Injector MeV) <0.01 % (RMS)
Undulator Entrance <0.01 % (RMS)
Projected Energy Spread Undulator Entrance 0.06 % (RMS)
*At a peak current of 3400 A at the undulator
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

6. LCLS: System Components
SLAC linac tunnel
Undulator Hall
Linac-0
L =6 m
Linac-1
L =9 m
Linac-2
L =330 m
Linac-3
L =550 m
BC-1
BC-2
L =22 m
DL-2
L =66 m
DL-1
L =12 m
Undulator
L =121.8 m
7 MeV
z  0.83 mm
  0.2 %
150 MeV
  0.10 %
250 MeV
z  0.19 mm
  1.8 %
4.54 GeV
z  mm
  0.76 %
GeV
  0.02 %
...existing linac
new RF
Gun
25-1a
30-8c
21-1b
21-1d X
Linac-X
L =0.6 m
21-3b
24-6d
Beam
Dump
Exp Halls
1.5 Å
8 GW
15 Å
17 GW
=> Talk by P. Emma
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

7. => Talks in Breakout Session for Subgroup 1./2.
RF Photo-Cathode Gun
“Half”
Cell
Laser
Port
Normalized Slice Emittance: 1 mm rad (rms)
Max Bunch Charge: 1 nC
Bunch Length: 0.8 mm
=> Talks in Breakout Session for Subgroup 1./2.
Full
Cell
Electron
Beam
Exit
Photocathode
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

8. Linac Acceleration and Magnetic Bunch Compression
Initial Energy: 7 MeV
Final Energy: 4.54 – GeV
Initial Bunch Length: 0.8 mm
Final Bunch Length: mm
Number of Compressors: 2
Total Compression Factor: 35
DE/E z
2sz0
DE/E z
DE/E z
2sz
Under-compression
Over-compression
=> Talk by P. Emma
V = V0sin(wt)
RF Accelerating
Voltage
Dz = R56DE/E
Path Length-Energy
Dependent Beamline
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

9. => Talk by E. Gluskin Breakout Session for Subgroup 2.
LCLS Undulator
Space for X-Ray Diagnostics QD QF
Beam Position Monitor (BPM)
Undulator Segment Length m
Vacuum Pump
Vacuum Chamber 5 mm ID / 5.6 mm OD 121 m Length
Electron an X-Ray Beams
Magnet Gap 6 mm
=> Talk by E. Gluskin Breakout Session for Subgroup 2.
Va Permendur
NdFeB
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

10. Basic Undulator Parameters
Undulator Type planar hybrid
Magnet Material NdFeB
Gap 6 mm
Period Length 3 cm
Peak On-Axis Field 1.32 T
K 3.71
Segment Length 3.42 m
Number of Segments 33
Segment Break Lengths m
Undulator Magnet Length m
Undulator Device Length (incl. Breaks) m
Undulator Filling Factor 93 %
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

11. LCLS Optimum b-Function at Short Wavelength
Energy = GeV
Operational <b > = m
lr = 1.5  Å
lu = 3.0 cm
Bu = 1.32 T
Optimum Beta-Function
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

12. LCLS Average b-Function <b>
< b > predicted for LCLS
< b > for minimum saturation length
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

13. LCLS Optimum b-Function at Long Wavelength
Energy = 4.54 GeV
Operational <b > = 7.3 m
lr = 15.0  Å
lu = 3.0 cm
Bu = 1.32 T
Optimum Beta-Function Reduced at the Longer Wavelength End
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

14. => Talk by E. Gluskin and Undulator Breakout Session
FODO Optics
Break Lengths
S S L S S L
=> Talk by E. Gluskin and Undulator Breakout Session
Energy GeV
<b > = m
bmax = 21.9 m
bmin = 14.0 m
(bmax- bmin)/(bmax+ bmin)= 0.22
Superperiod
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

15. FODO Optics Energy 4.54 GeV <b > = 7.3 m bmax = 12.4 m
Break Lengths
S S L S S L
Energy GeV
<b > = m
bmax = 12.4 m
bmin = m
(bmax- bmin)/(bmax+ bmin)= 0.66
Superperiod
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

16. Start-to-End Tracking Simulations => Talks by P. Emma and S. Reiche
LCLS Simulations
No Undulator Wakefields
Including Undulator Wakefields
Energy = GeV
Wavelength = 1.5  Å
Start-to-End Tracking Simulations
=> Talks by P. Emma and S. Reiche
Parmela
Elegant
Genesis
space-charge
compression, wakes, CSR, …
SASE FEL with wakes
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

17. Selected LCLS Baseline Design Parameters
Fundamental FEL Radiation Wavelength Å
Electron Beam Energy GeV
Normalized RMS Slice Emittance mm-mrad
Peak Current kA
Bunch/Pulse Length (FWHM) fs
Relative Slice Energy Entrance < %
Saturation Length m
FEL Fundamental Saturation Exit GW
FEL Photons per Pulse
Peak Undulator Exit *
Transverse Coherence Full Full
RMS Slice X-Ray Bandwidth %
RMS Projected X-Ray Bandwidth %
* photons/sec/mm2/mrad2/ 0.1%-BW
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

18. LCLS Working at the Short Wavelength End
Extendable Undulator Length
Available Undulator Length
2.0 mm mrad
1.7 mm mrad
1.2 mm mrad
Nominal Working Point
Saturation predicted 30 m before undulator end
Space for Undulator Extension Available if needed.
Length of Dogleg m
Length of Extension Space m
Length of Undulator m
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

19. LCLS Working at the Long Wavelength End
Nominal Working Point
Strongly reduced requirements for electron beam parameters to achieve saturation before end of undulator.
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL

20. Summary
A consistent reference set of LCLS parameters is the basis of the CDR.
The LCLS reference parameters describe a light source that exceeds existing devices by many order of magnitude in brilliance and that produces x-ray pulse with sub-picosecond pulse length.
LCLS DOE Review, April 23, 2002
Heinz-Dieter Nuhn, SLAC / SSRL