1. Accelerator Physics Considerations for the LCLS Linac Paul Emma, SLAC April 23, 2002
Design of Compression and Acceleration Systems
Technical Challenges
Full System Simulations
LCLS
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

2. use 2 compressors, 3 linacs
Nominal System Design
1.5-Å SASE FEL Linac:
Requirements
Acceleration to 14.3 GeV (4.5 GeV min.)
Bunch compression to 3.4 kA (for gex,y  1.2 mm)
Emittance preservation (<20% ‘slice’, <100% ‘projected’)
Final energy spread (<0.02% ‘slice’, <0.1% ‘projected’)
Minimal sensitivity to system ‘jitter’
Diagnostics integrated into optics
Flexible operations (15 Å, low-charge, chirp, etc.)
use 2 compressors, 3 linacs
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

3. Nominal System Design Constraints 1 km
Use existing SLAC linac compatible with PEP-II operation
Use existing ‘FFTB’ hall for undulator
1 km
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

4. 2 Design Optimization Fast, computer model used to optimize design
Set final e- parameters (E, sE/E, Ipk, etc.),
minimize sensitivity (e.g., gun-timing  Ipk),
find best RF phases, chicane strengths and locations,
with reasonable constraints (RF power., optics, etc.)
|R56|i < lim.
|fRF|i < lim. N
sz0 E0 2
R56 (BC1, BC2)
fRF (L1, L2, L3)
Ei (BC1, BC2)
|dI/dt| < lim.
|dE/dQ| < lim.
explore alternate parameter sets (15 Å, low-charge, chirp, etc.)
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

5. Nominal LCLS Linac Parameters for 1.5-Å FEL
Single bunch, 1-nC charge, 1.2-mm slice emittance, 120-Hz repetition rate…
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
(RF phase: frf = 0 at accelerating crest)
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

6. after BC1 after X-RF after L1 after DL1 after BC2 after L3 at und.
energy profile
phase space
time profile
sz = 830 mm
sz = 190 mm
sz = 830 mm
sz = 23 mm
sz = 830 mm
sz = 23 mm
FINAL
sz = 190 mm
sz = 23 mm
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

7. Machine Flexibility
Operation at 0.2 to 1.0 nC and constant 87-m saturation length at 1.5 Å
in/output beam
linac setup
jitter
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

8. LCLS Technical Challenges Coherent Synchrotron Radiation in Bends
projected emittance growth
micro-bunching instability
Emittance Preservation in Linacs
transverse wakefields
misalignments & chromaticity
Machine Stability
jitter tolerance budget
simulation of budget
LCLS
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

9. Magnetic Bunch Compression
DE/E z
2sz0
DE/E z
DE/E
2sz
Under-compression
Over-compression
V = V0sin(wt)
RF Accelerating
Voltage
Dz = R56DE/E
Path Length-Energy
Dependent Beamline
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

10. Coherent Synchrotron Radiation (CSR)
Powerful radiation generates energy spread in bends
Induced energy spread breaks achromatic system
Causes bend-plane emittance growth (short bunch is worse)
coherent radiation for l > sz
bend-plane emittance growth sz l L0 s
DE/E = 0 Dx e– R 
DE/E < 0
overtaking length: L0  (24szR2)1/3
Dx = R16(s)DE/E
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

11. CSR Micro-bunching*
S. Heifets, S. Krinsky, G. Stupakov, SLAC-PUB-9165, March 2002
CSR can amplify small modulations on bunch current  Successive bend-systems cause micro-bunching  Growth of slice-emittance.
without
SC-wiggler
sd  310-6
avoid!
230 fsec
R. Carr
Super-conducting wiggler prior to BC2 increases uncorrelated E-spread (310-6  310-5)
SC-wiggler damps bunching
sd  310-5
* First observed by M. Borland (ANL) in LCLS Elegant tracking
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

12. CSR in BC2 Chicane (animation through chicane)
DE/E0
f(s)
gex
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

13. CSR Micro-bunching and Projected Emittance Growth
14.3 GeV at undulator entrance
x versus z without SC-wiggler
230 fsec
projected emittance growth is simply ‘steering’ of bunch head and tail
‘slice’ emittance is not altered
0.5 mm
x versus z with SC-wiggler
Workshop in Berlin, Jan to benchmark results ( – follow-up meeting in summer
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

14. X-band RF used to Linearize Compression (f = 11.424 GHz)
S-band RF curvature and 2nd-order momentum compaction cause sharp peak current spike
X-band RF at decelerating phase corrects 2nd-order and allows unchanged z-distribution
1  -40°
x = p
Slope linearized
lx = ls/4
avoid!
0.6-m section, 22 MV available at SLAC (200-mm alignment)
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

15. Transverse Wakefields and Component Misalignments
transverse wake effect on projected emittance
corrected trajectory y
correctable emittance growth x y
300 mm rf-structure
150 mm quad/BPM
experience at SLC…
gey  2 1 nC
beam-based align
correction bumps… x
L2 phase adv/cell optimized
sz = 195 mm
wakes off
wakes on
L3 phase adv/cell optimized
sz = 22 mm
wakes on
off
also misaligned quads/BPMs generate dispersion  De
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

16. Correction of Wakefield Emittance Growth
Measure emittance at end of linac
Launch b-oscillations of various amplitudes & phases
Empirically minimize gex,y
Well tested at SLC*
LCLS L2 sim. with Liar
SLC gey = nC
with 3-km linac and
1-mm bunch length
* J.T. Seeman et al., 15th International Conf. on High Energy Accelerators, Hamburg, Germany, July 1992.
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

17. Emittance and Energy Spread Diagnostics*
4 energy spread meas. stations (optimized for small bx)
5 emittance meas. stations designed into optics (Dyx,y)
slice measurements possible with transverse RF (L0 & L3)
3 prof. mon.’s
(Dyx,y = 60°)
...existing linac L0 rf
gun L3 L1 X L2
gex,y sE
* see also D. Dowell and P. Krejcik talks
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

18. Transverse RF deflector as diagnostic*sz
V(t)
S-band sx RF
‘streak’
V0 = 0
230 fsec
LCLS simulation
long. phase space
* see P. Krejcik talk
Built & used at SLAC in 1960’s
meas. bunch length & slice emittance
V0 = 20 MV
meas. longitudinal phase space
y = kt [mm]
x = hDE/E [mm]
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

19. Machine Stability
Final peak current & energy jitter caused by variations of…
bunch charge
gun-laser timing
RF phases
RF voltage
chicane supplies
Need feedback for
time scales >1 min.
Design is
optimized, but…
system jitter tol.-
budget is needed
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

20. Jitter Budget (<1 minute time-scale)
measured RF performance
klystron phase rms  0.07°
(20 sec)
klystron ampl. rms  0.06%
(60 sec)
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

21. Start-to-End Tracking Simulations
Track entire machine to evaluate beam brightness & FEL
Track machine many times with jitter to test stability budget (M. Borland, ANL)
Parmela
Elegant
Genesis
space-charge
compression, wakes, CSR, …
SASE FEL with wakes
LCLS
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

22. Initial Beam from Parmela Tracking (C. Limborg)
1 nC
10-psec FWHM
0.7-ps rise/fall
120 MV/m gun
getherm  0.3 mm
150 MeV
gex,y < 1 mm
gex
gey
2105 to 2106 macro-particles
z, DE/E
x, y
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

23. Sliced e- Beam to Evaluate FEL (Dz  0.7 mm)
After full system tracking
(also see talk by S. Reiche)
gex
gey
mismatch amplitude variation zx
slice 4D centroid osc. amplitude zy
Lg < 4 m
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

24. Machine Stability Simulations
Track LCLS 230 times with Parmela Elegant Genesis
Include wakes, CSR, etc. + jitter budget (M. Borland, ANL) Lg
Ipk
gex
Pout
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC

25. LCLS Summary Linac design well optimized for nominal 1.5-Å operation
Design is flexible to accommodate 15-Å, low-charge, & chirp
CSR growth of projected emittance – not slice
Much experience on SLAC linac with wakefield control
Beam diagnostics built into design
Full system tracking to…
Evaluate e- brightness preservation,
Calculate SASE gain,
Simulate pulse-to-pulse stability.
LCLS
Full tracking confirms saturation at 1.5 Å
LCLS DOE Review, April 23, 2002
Paul Emma, SLAC