1. Accelerator Physics Challenges of X-Ray FEL SASE Sources
Paul Emma
Stanford Linear Accelerator Center

2. Why a Linac-Based Free-Electron Laser (FEL*) ?
Longitudinal emittance from linac is much smaller than ring
Bunch length can approach 100 fsec with small energy spread
Potential for 1010 brightness increase and 102 pulse length reduction
Much experience gained from linear collider operation and study (SLC, JLC, NLC, TESLA, CLIC)
At SLAC, the linac is available
at DESY, XFEL fits well into TESLA collider plans
Use SASE** (Self-Amplified Spontaneous Emission)  no mirrors at 1 Å
* Motz 1950; Phillips 1960; Madey 1970
** Kondratenko, Saldin 1980; Bonifacio, Pellegrini 1984

3. SASE Saturation Results
LEUTL
APS/ANL
385 nm
September 2000
XFEL  0.1 nm
(or 1 Å)
Just 20 months ago:
SASE saturation not yet demonstrated
Since September 2000:
3 SASE FEL’s demonstrate saturation
TTF-FEL
DESY
98 nm
VISA
ATF/BNL
840 nm
March 2001

4. Proposed/Planned SASE X-Ray FEL’s
TESLA-XFEL at DESY ( Å)
LCLS at SLAC ( Å)
LCLS
INFN/ENA FEL in Roma (15 Å)
Fermi FEL at Trieste (12 Å)
SCSS at Spring-8 (36 Å)
4GLS FEL at Daresbury (SXR)
SASE-FEL at BESSY ( Å)

5. Peak Brilliance of FEL’s
1 Å
X-Ray
~109
photons per phase-space volume per band-width
103 by e- quality, long undulators
106 by FEL gain
courtesy T. Shintake

6. TESLA XFEL at DESY 0.85-60 Å X-FEL Integrated into linear collider
user facility Å
3 compressors
multiple undulators
X-FEL Integrated into linear collider

7. X-FEL based on last 1-km of existing SLAC linac
LCLS at SLAC Å
LCLS
2 compressors
one undulator
X-FEL based on last 1-km of existing SLAC linac

8. SASE FEL Electron Beam Requirements
radiation wavelength
(~1.5 mm realistic goal)
transverse emittance:
eN < 1 mm at 1 A, 15 GeV
peak current
undulator period
energy spread:
<0.08% at Ipk = 4 kA, K  4, lu  3 cm, …
undulator ‘field’
beta function
FEL gain length:
20Lg > 100 m for eN  1.5 mm
Need to increase peak current, preserve emittance, and maintain small energy spread, all simultaneously
AND provide stable operation

9. ‘Slice’ versus ‘Projected’ Emittance
For a collider…
…collision integrates over bunch length — emittance ‘projected’ over the bunch length is important
For an FEL…
e- slips back in phase w.r.t. photons by lr per period lu
Nlr  0.5 mm lr
…FEL integrates over slippage length: ‘slice’ emittance (and E-spread) is important

10. Slice Emittance is Less Sensitive
HEAD
<1%sz
however, centroid shifts of slices can be important
transverse wakefield effect
TAIL
…as can b variations along bunch
emittance of short ‘slice’ not affected by transverse wakes
…also true for quad-misalignments, CSR, and RF kicks

11. RF Photo-Cathode Gun
rapid RF-acceleration to avoid space-charge dilution
Ipk  A
Q  1 nC
eN  1 mm
laser beam
electron beam
emittance compensation solenoid
courtesy J. Rossbach

12. Thermionic pulsed high-voltage gun
…at Spring-8 SCSS ( nC), CeB6 cathode and sub-harm. bunchers
T. Shintake, TUPRI116

13. Emittance Results from Gun Test Facility at SLAC
‘projected’ emittance at reduced charge levels
Parmela
gaussian bunch at GTF limits gex
Measurements
1 mm at 1 nC possible, but not demonstrated yet
C. Limborg, TUPRI041, TUPRI042, TUPRI043
courtesy S. Gierman, J. Schmerge

14. Slice ey Measurements at BNL
weak quad setting
Slice ey Measurements at BNL
dump
75 MeV
5 MeV
undulators
linac
linac (off)
DUVFEL
medium quad setting
projected gey  3.0 mm
strong quad setting
200 pC, 50 A, 75 MeV
care needed with image processing
Data from SDL at BNL: W. Graves, et al.

15. Magnetic Bunch Compression
DE/E z
DE/E z
‘chirp’
DE/E z sz
…or over-compression
under-compression
sz0
sE/E
V = V0sin(wt)
RF Accelerating
Voltage
Dz = R56DE/E
Path Length-Energy
Dependent Beamline

16. LCLS Linac Parameters for 1.5-Å FEL
single bunch, 1-nC, 120-Hz
SLAC linac tunnel
FFTB hall
Linac-0
L =6 m
Linac-1
L =9 m
rf = -38°
Linac-2
L =330 m
rf = -43°
Linac-3
L =550 m
rf = -10°
BC-1
R56= -36 mm
BC-2
L =22 m
R56= -22 mm
DL-2
L =66 m
R56 = 0
DL-1
L =12 m
R56 0
undulator
L =120 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 %
14.35 GeV
  0.02 %
...existing linac
new rf
gun
25-1a
30-8c
21-1b
21-1d X
Linac-X
L =0.6 m
rf=180
21-3b
24-6d
Two stages of bunch compression
(RF phase: frf = 0 at accelerating crest)

17. Coherent Synchrotron Radiation
Power
coherent power
N  6109
vacuum chamber cutoff
~l-1/3
incoherent power sz
Wavelength

18. Coherent Synchrotron Radiation (CSR)
Powerful radiation generates energy spread in bends
Energy spread breaks achromatic system
Causes bend-plane emittance growth (short bunch worse)
bend-plane emittance growth e– R
coherent radiation for l > sz
DE/E = 0 sz l  L0 s
DE/E < 0 Dx
Dx = R16(s)DE/E
overtaking length: L0  (24szR2)1/3
CSR wake is strong at very small scales (~1 mm) ~

19. CSR Microbunching* Animation
f(s)
DE/E0
gex
* First observed by M. Borland (ANL) in LCLS Elegant tracking

20. …Energy Profile also modulated
DE/E vs. z
energy profile
Next set of bends will magnify this again…
 ‘slice’ effects
current profile

21. CSR Microbunching Gain vs. l
gex=1 mm
‘cold’ beam
see also E. Saldin, Jan. 02, and Z. Huang, April 02
Initial modulation wavelength prior to compressor
gex=1 mm, sd=310-5
“theory”: S. Heifets et al., SLAC-PUB-9165, March 2002

22. Evolution of LCLS Longitudinal Phase Space
after L2
energy profile
phase space
time profile
after DL1
sz = 830 mm
sz = 190 mm
after L1
after BC2
sz = 830 mm
sz = 23 mm
Current spikes further drive CSR
after X-RF
after L3
sz = 830 mm
sz = 23 mm
after BC1
at und.
sz = 190 mm
sz = 23 mm

23. CSR Micro-bunching in LCLS
energy profile
long. space
temporal profile
CSR can amplify small current modulations:
micro-bunching
sd  310-6
230 fsec
Super-conducting wiggler prior to BC increases uncorrelated E-spread (310-6  310-5)
R. Carr
SC-wiggler damps bunching
sd  310-5
tracking with Elegant code, written by M. Borland, ANL

24. CSR in Chicane (animation through LCLS BC2)
f(s)
DE/E0
gex

25. CSR Projected Emittance Growth (simulated)
projected emittance growth is simply ‘steering’ of bunch head and tail
x-pos
0.5 mm
‘slice’ emittance is not altered
LCLS
Tracking using Elegant
z-pos

26. Energy Spectrum at TTF-FEL (DESY)
no SASE
T. Limberg,
P. Piot, et al.
TraFiC4 simulation

27. SPARC Project at INFN-LNF
1st stage bunch compression without bend magnets
122 MeV
(simulation)
SPARC
INFN-LNF
Collab. Among
ENEA-INFN-CNR-
Univ. Roma2-ST-
INFM
9.4 M€ funding
velocity compression with no bends
ge  0.6 mm
peak current
>500 A
~100 mm
L Serafini, WEYLA001
M. Ferrario, TUPRI056
C. Ronsivalle

28. Harmonic RF used to Linearize Compression
RF curvature and 2nd-order compression cause current spikes
Harmonic RF at decelerating phase corrects 2nd-order and allows unchanged z-distribution
1  -40°
x = p
Slope linearized
lx = ls/4
830 mm
avoid!
200 mm
3rd harmonic used at TTF/TESLA
4th harmonic used at LCLS
0.5-m X-band section for LCLS (22 MV, 11.4 GHz)

29. Diagnostics: Transverse RF Deflector
sz  20 mm,
E  5 GeV, V0  15 MV sx RF
‘streak’
off-axis screen
2.4 m
V(t)
S-band
transverse RF deflector e- sz
single-shot, absolute bunch length measurement
V0 = 0
LCLS simulation
V0 = 20 MV
resolves ~20-fsec structure
230 fsec
RF streak
R. Akre, THPRI097

30. Measurements with Deflector at SLAC
46 GeV
y = V0sin(f)
voltage was calibrated with BPM
12 mm
built in 1960’s (G. Loew…)
20 MV
Planned also for TTF at DESY
bunch length measurements in the SLAC linac
sz<500 mm

31. Machine Stability Simulations (M. Borland, ANL)
Track 105 particles with Parmela Elegant Genesis
Repeat 230 times with ‘jitter’ in gun, RF, magnets, etc.
Include wakefields and CSR Lg
Ipk
gex
Pout
<10%
<10%
Provides realistic estimate of operational stability and verifies machine ‘jitter budget’
<4%
~25%

32. Undulator Beam-Based Alignment (LCLS)
uncorrected undulator trajectory
sx > 500 mm
S/m
Undulator trajectory must be straight to ~5 mm level 
simulate beam-based alignment procedure with realistic errors: BPMs, quads, poles…
Quadrupole positions
Trajectory
BPM read-back
trajectory after 3rd pass of BBA
S/m
sx  1.5 mm
Df  82º
beam size: 30 mm
BBA also used for TESLA-FEL: DESY (B. Faatz)

33. Resistive-Wall Wakefields in the Undulator
Strong magnetic field in undulator requires small gap…
Need small radius pipe (r  2.5 mm, Lu  120 m: LCLS)
Wake changes ‘slice’ energy during exponential gain regime — more damaging than incoming ‘chirp’
X-ray pulse r
limit
gap due to wake
Courtesy S. Reiche
Need smooth copper pipe

34. FEL Output Power with Undulator Wakefields
~40% power loss due to wakefield
LCLS
Genesis 1.3, S. Reiche, NIM A 429 (1999) 242.

35. Emittance Exchange: Transverse to Longitudinal
transverse RF in a chicane… h k
ex0
ez0
Electric and magnetic fields
hk = 1
ex  ez0
ez  ex0
x0 , x0
z0 , d0
gex0 =5 mm
gez0 =1 mm
system also compresses bunch length
x, x
z, d
gex0 =1 mm
gez0 =5 mm
M. Cornacchia, P. Emma, SLAC-PUB-9225, May 2002

36. Final Comments Merci Beaucoup
For LCLS, slice emittance >1.8 mm will not saturate (TESLA ?)
courtesy S. Reiche
P = P0
eN = 1.2 mm
P = P0/100
eN = 2.0 mm
Merci Beaucoup
SASE FEL is not forgiving — instead of mild luminosity loss, power nearly switches OFF
electron beam must meet brightness requirements