Transcript LCLS Longitudinal Feedback and Stability Requirements P. Emma LLRF Review November 23, 2005 LCLS 23 Nov.

LCLS Longitudinal Feedback
and Stability Requirements
P. Emma
LLRF Review
November 23, 2005
LCLS
23 Nov. 2005
LLRF Meeting
P. Emma
1
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Critical LCLS Accelerator Parameters
Final energy 13.6 GeV (stable to 0.1%)
Final peak current 3.4 kA (stable to 12%)
Transverse emittance 1.2 mm (stable to 5%)
Final energy spread 10-4 (stable to 10%)
Bunch arrival time (stable to 150 fs)
(stability specifications quoted as rms)
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FEL Power Sensitivity to e- Beam
12% DIpk/Ipk  20% DP/P
0.1% DE/E  0.2% Dlr/lr
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Electron Bunch Compression
d  DE/E
d
s zi
d
‘chirp’
z
z
z
sz
sdi
Dz = R56d
V = V0sin(kz)
RF Accelerating
Voltage
Path-Length EnergyDependent Beamline
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undercompression
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Compression Stability
d
d
Df
z
RF phase jitter becomes bunch length jitter…
Compression factor:
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Phase and Bunch Length Stability Example (not LCLS)
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Machine Schematic with Parameters
6 MeV
sz  0.83 mm
sd  0.05 %
250 MeV
sz  0.19 mm
sd  1.6 %
Linac-X
L =0.6 m
rf= -160
rf
gun
Linac-1
L 9 m
rf  -25°
Linac-0
L =6 m
21-1b
21-1d
...existing linac
DL1
L 12 m
R56 0
3 klystrons
13.6 GeV
sz  0.022 mm
sd  0.01 %
Linac-2
L 330 m
rf  -41°
Linac-3
L 550 m
rf  0°
21-3b
24-6d
25-1a
30-8c
X
BC1
L 6 m
R56 -39 mm
BC2
L 22 m
R56 -25 mm
1 X-klys.
1 klystron
26 klystrons
SLAC linac tunnel
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4.30 GeV
sz  0.022 mm
sd  0.71 %
135 MeV
sz  0.83 mm
sd  0.10 %
undulator
L =130 m
45 klystrons
LTU
L =275 m
R56  0
research yard
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Correlated or Uncorrelated Errors?
Suppose the mean RF phase of all 26 Linac-2 klystrons
changes by: 0.21°  |DIpk/Ipk|  12%
This may arise statistically with 26 random uncorrelated
phase errors with rms spread of: f21/2 = 0.21°261/2 = 1.07°,
or with 26 identical phase errors.
Since we don’t fully understand the correlations, we choose
the conservative (smallest) tolerance of 0.21° rms/klys. and
then reduce this by ~N, where N (=12) is the number of
major error sources.
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Phase, Amplitude, and Charge Sensitivities
parameter |DE/E0| = 0.1%
1.6
Dti
46
DQ/Q0
3.5
Df0
0.32
DV0/V0
0.32
Df1
0.29
DV1/V1
5.5
DfX
2.0
DVX/VX
0.54
Df2
1.1
DV2/V2
0.35
Df3
0.15
DV3/V3
|DI/I0| = 12%
4.4
5.2
0.65
0.24
0.17
0.25
1.4
1.2
0.21
1.0
24.8
5.7
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|Dtf| = 100 fs
1.5
24
5.9
0.95
1.0
0.78
7.6
6.3
0.084
0.13
15
8.6
unit
psec
%
deg-S
%
deg-S
%
deg-X
%
deg-S
%
deg-S
%
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Longitudinal Fast-Jitter Tolerance Budget
tolerances are rms values
laser timing (w.r.t. RF) 
laser energy 
0.50
mean phase of 2 klys. 
1 klys. 
1 X-klys. 
mean phase of 26 klys. 
mean phase of 45 klys. 
mean amp. of 2 klys. 
1 klys. 
1 X-klys. 
mean amp. of 26 klys. 
mean amp. of 45 klys. 
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X-
X-band
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Jitter Simulations (Particle Tracking)
0.09%
0.004%
Lg
96 fs
Pout
10%
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LCLS Longitudinal Beam-Based Feedback
(stabilizes beam for jitter frequencies < 10 Hz @ 120-Hz rep-rate)
gun
V0
s z1
d0
d1
1 V1
L1
DL1
BPM
2 V2
s z2
d2
L2
X
BC1
d3
V3
L3
BC2
DL2
CSR detector
J. Wu, et al., PAC’05, May 16-20, 2005, Knoxville, TN.
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CSR Relative Bunch Length Monitor
Red curve:
Gaussian
Black curve:
Uniform
Blue curve:
‘Real’
J. Wu, et al., PAC’05, May 16-20, 2005, Knoxville, TN.
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LCLS Feedback Performance (use CSR DP/P)
feedback off
feedback on
DIpk/Ipk0 (%)
DE / E rms  0.09 %
DI / I rms  10.5 %
Dt rms  0.16 ps
J. Wu
(undulator entrance)
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Feedback System Bode Plot at 120 Hz
J. Wu
Define fast-jitter as variations faster than 2 seconds
Slow drift occurs on time-scales > 2 seconds (to 24+ hr)
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Slow Drift Tolerance Limits
(Top 4 rows for De/e < 5%, bottom 4 limited by feedback dynamic range)
Gun-Laser Timing
Bunch Charge
Gun RF Phase
Gun Relative Voltage
L0,1,X,2,3 RF Phase (approx.)
L0,1,X,2,3 RF Voltage (approx.)
2.4* deg-S
3.2
%
2.3 deg-S
0.6
%
5
deg-S
5
%
(Tolerances are peak values, not rms)
* for synchronization, this tolerance might be set to 1 ps (without arrival-time measurement)
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x (deg)
Compensate X-band
Phase Step Error...
x-band phase
LX phase error = 5o
final energy
final peak current
L1 adjustment: phase
+2.1o, voltage -2.1%
final arrival time
J. Wu
23 Nov. 2005
LLRF Meeting
P. Emma
17
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Gun Timing Jitter and Energy Feedback
E
E
Dtf
E > E0
E = E0
Dt0
without energy feedback
E = E0
t
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Dt0
Dtf = Dt0
with energy feedback
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t