1. Linac Design Update
P. Emma
LCLS DOE Review May 11, 2005
LCLS

2. Linac Design is Mature and Stable
Single bunch, 1-nC charge, 1.2-mm slice emittance, 120-Hz repetition rate…
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 rf
gun
new
Linac-1
L 9 m
rf  -25°
Linac-2
L 330 m
rf  -41°
Linac-3
L 550 m
rf  -10°
Linac-0
L =6 m
21-1b
21-1d X
21-3b
24-6d
25-1a
30-8c
undulator
L =130 m
...existing linac
BC-1
L 6 m
R56 -39 mm
BC-2
L 22 m
R56 -25 mm
DL-1
L 12 m
R56 0
LTU
L =275 m
R56  0
SLAC linac tunnel
research yard
(RF phase: frf = 0 is at accelerating crest)

3. New Since Last Review Optional low-charge operating point (0.2 nC)
Laser-heater low FEL gain “lock-in” detection
Bunch length & energy feedback simulations
Dark current, beam loss, and collimation study complete
Detailed linac tuning simulations done
Engineering constraints worked into design

4. Low Charge Operating Point (1)
Motivation:
Less charge  less wake
Same compression factor  ~same jitter
Lower gun current  lower emittance
Chosen Scaling:
Charge: 1 nC  0.2 nC
Gun Current: 100  30 A (10 ps  6.5 ps)
Sliced gun emittance: 1 mm  0.8 mm
Final current: 3 kA  2 kA (same Lsat)

5. Low Charge Operating Point (2)
gex,y (mm)
1.0 nC
rms errors:
300-mm struct.
200-mm quad
200-mm BPM
steer 10 seeds
gex,y (mm)
0.2 nC
Linac Alignment Eased

6. Low Charge Operating Point (3)
0.2 nC
1 nC
Bane/Stupakov AC-conductivity model

7. Low Charge Operating Point (4)
1012 photons
B. Fawley,
S. Reiche

8. Low Charge Operating Point - Summary
BC2 CSR De  1/5
Linac quad/BPM align. tol.’s  2
L2 transverse wake De  1/16
Peak current jitter  ½
X-ray pulse 80 fs (was 200 fs)
Weak undulator RW-wake
FEL power: 20 GW & ~1012 photons
1-nC operation still fully supported

9. Low FEL Gain ‘Lock-in’ Detection* (1)
Simulate LCLS (Linac + M. Xie) with:
Linac jitter (DQ, t0, f, V, etc.)
Large emittance (3 mm)  low gain
Spontaneous radiation background
Laser-heater modulated at 7 Hz
Use FFT to ‘lock-in’ on very weak FEL signal in spont. background
* Idea from K. Robinson and comments by J. Rossbach (FAC, April 2004)

10. Low FEL Gain ‘Lock-in’ Detection (2)sE
PFEL
at 13.6 GeV

11. Low FEL Gain ‘Lock-in’ Detection (3)
total signal
Gain of ~25 detectable
spontaneous signal
FEL signal
FFT of total signal
PAC’05 paper:
(P.E., Z. Huang, J. Wu)

12. Bunch Length and Energy Feedback
Feedback OFF
Feedback ON
Juhao Wu (SLAC)

13. Dark Current, Beam Loss, Collimation (1)
Model cathode dark current (Fowler-Nordheim and Parmela), scaling charge from GTF measurements
Add dark current in critical RF structures along linac, based on K. Bane work in NLC (a non-issue)
Track dark current through linac and through und.
Include aperture restrictions and collimators
Assess collimation scheme in terms of undulator protection and average power loss on collimators
Evaluate wakefield effect of each collimator
PE, J. Wu

14. Dark Current, Beam Loss, Collimation (2)
1 new BC2 E-coll.
36-mm (d = 10%)
2 new E-coll.
2.5 mm (d = 2%)
under
ground
BC1
BC2
undulator
4 existing x-coll.’s
4 existing y-coll.’s
1.6 & 1.8 mm
3 new x-coll.’s
3 new y-coll.’s
2.2 mm…
1 new BC1 E-coll.
45-mm (d = 20%)
2.4 pC/pulse
3.3 W (120 Hz,
11.3 GeV)
1.0 pC/pulse
1.6 W (120 Hz,
13.6 GeV)
0.2 pC/pulse
0.3 W (120 Hz,
13.6 GeV)
DE/E of 1 dropped klystron = -1.7%

15. Dark Current, Beam Loss, Collimation - Summary
Undulator is protected from gun and structure dark current
Maximum collimated beam power in ‘above-ground’ section is 0.3 W (well below safe level)
Results still look safe even for 10-times more dark current (but already used worst-case GTF)
Collimator wakefields should not be an issue (~0.5-mm alignment tolerances)
Shower calculations were done (20 W/coll. was assumed, now ~70-times smaller)

16. Linac Tuning Simulations (1)
Start with low-charge (0.2 nC, no CSR)
Use elegant to automate tuning; use only ‘real’ diagnostics
Add large errors to linac systems (e.g., magnets, RF, beam)
Assume rough corrections already made (see LCLS Commissioning Workshop, Sep. 2004)
Track through linac many times, each step simulating one particular correction (e.g., b-matching or RF phasing)
Use correction devices already built into design (e.g., BC2 correction quads, trajectory controls)
Evaluate final beam quality, correction convergence, dynamic range, problem areas, etc.

17. Linac Tuning Simulations (2)
Element
rms errors (Gaussian, 3-s cutoff)
value
unit
Quads
x and y misalignments
300 mm
z misalignments 5
relative gradient errors
0.5 %
roll angle errors 2
mrad
Bends
relative field errors
0.3
anomalous field gradients (BC’s) 3
tol.
BPMs
RF strucs.
phase errors (static)
deg
relative voltage errors (static) 1
e- beam
random charge error 10
initial beta mismatch in x and y
2.0 z

18. Linac Tuning Simulations (3)
0.2 nC
before tuning
gex,y  1.4 mm
zx = 3.3 !

19. Linac Tuning Simulations (4)
after tuning
gey= 0.91 mm
gex = 0.88 mm
zx,y  1

20. Linac Tuning Simulations (5)
DESIGN
DESIGN
DESIGN
TUNEUP
TUNEUP
TUNEUP
2 kA

21. Comments
System in mature, well studied, and reasonably ready for construction
Many more slides are available to answer questions