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LCLS Longitudinal Feedback and Stability Requirements P. Emma LLRF Review November 23, 2005 PowerPoint Presentation
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LCLS. LCLS Longitudinal Feedback and Stability Requirements P. Emma LLRF Review November 23, 2005. 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 m m (stable to 5%)

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slide1

LCLS

LCLS Longitudinal Feedback and Stability Requirements

P. Emma

LLRF ReviewNovember 23, 2005

critical lcls accelerator parameters
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)

fel power sensitivity to e beam
FEL Power Sensitivity to e- Beam

12% DIpk/Ipk 20% DP/P

0.1% DE/E 0.2% Dlr/lr

electron bunch compression
Electron Bunch Compression

d DE/E

d

d

under-compression

szi

‘chirp’

z

z

z

sz

sdi

Dz = R56d

V = V0sin(kz)

RF Accelerating

Voltage

Path-Length Energy-

Dependent Beamline

compression stability

d

RF phase jitter becomes bunch length jitter…

Compression factor:

Df

Compression Stability

d

z

machine schematic with parameters
Machine Schematic with Parameters

250 MeV

z  0.19 mm

  1.6 %

4.30 GeV

z  0.022 mm

  0.71 %

13.6 GeV

z  0.022 mm

  0.01 %

6 MeV

z  0.83 mm

  0.05 %

135 MeV

z  0.83 mm

  0.10 %

Linac-X

L =0.6 m

rf= -160

rf

gun

Linac-1

L 9 m

rf  -25°

Linac-2

L 330 m

rf  -41°

Linac-3

L 550 m

rf  0°

23-m

Linac-0

L =6 m

undulator

L =130 m

21-1b

21-1d

21-3b

24-6d

25-1a

30-8c

X

...existing linac

BC1

L 6 m

R56 -39 mm

BC2

L 22 m

R56 -25 mm

DL1

L 12 m

R56 0

LTU

L =275 m

R56  0

1 X-klys.

3 klystrons

1 klystron

26 klystrons

45 klystrons

research yard

SLAC linac tunnel

correlated or uncorrelated errors
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 phaseerrors.

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.

longitudinal fast jitter tolerance budget

0.50

X-

X-band

Longitudinal Fast-Jitter Tolerance Budget

tolerances are rms values

laser timing (w.r.t. RF) 

laser energy 

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. 

jitter simulations particle tracking
Jitter Simulations (Particle Tracking)

0.09%

0.004%

Lg

96 fs

Pout

10%

lcls longitudinal beam based feedback

sz1

sz2

V0

d0

gun

BPM

d3

d1

d2

L0

1

2

V1

V2

V3

L2

L3

X

L1

DL2

DL1

BC1

BC2

CSR detector

LCLSLongitudinal Beam-Based Feedback

(stabilizes beam for jitter frequencies < 10 Hz @ 120-Hz rep-rate)

J. Wu, et al., PAC’05, May 16-20, 2005, Knoxville, TN.

csr relative bunch length monitor
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.

slide14

feedback on

DIpk/Ipk0 (%)

LCLS Feedback Performance (use CSR P/P)

feedback off

J. Wu

(undulator entrance)

feedback system bode plot at 120 hz
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)
slow drift tolerance limits
Slow Drift Tolerance Limits

(Top 4 rows for De/e < 5%, bottom 4 limited by feedback dynamic range)

(Tolerances are peak values, not rms)

* for synchronization, this tolerance might be set to 1 ps (without arrival-time measurement)

slide17

Compensate X-band Phase Step Error...

jx(deg)

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

gun timing jitter and energy feedback

E

E

Dtf

E = E0

E = E0

E > E0

Dt0

Dt0

t

t

Gun Timing Jitter and Energy Feedback

Dtf= Dt0

without energy feedback

with energy feedback