LCLS LLRF System

1
LCLS LLRF System

• October 10-13, 2005
• LLRF05
• B. Hong, R. Akre, A. Hill, D. Kotturi, H. Schwarz
• SLAC, Stanford, Menlo Park, CA 94025, USA
• Work supported under DOE Contract
  DE-AC03-76SF00515

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Outline

• Overview of LCLS Injector RF System
• LCLS RF Stability Requirements
• Low Noise RF Distribution System
• RF Control and Monitoring System

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LCLS INJECTOR / LINAC

• The LCLS linac consists of 4 sections. The
  injector L0 (2 S-Band Klystrons) will reside in
  an off axis tunnel at the end of sector 20. L1
  will have 1 S-band and 1 X-band klystron to feed
  their linacs, respectively. L2 (28 klystrons)
  and L3 (48 klystrons) are made up of the current
  main SLAC linac sectors 21 to 30.

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The site layout of the LCLS linac
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LCLS RF Stability Requirements

• Phase and Amplitude requirements by the physics
  design
• LCLS specifications require RF stability of
  0.1rms in amplitude and 100fSrms in a 850nS fill
  time S-Band structure, but the tightest tolerance
  is 125fSrms in a 100nS fill time X-Band
  structure. See Table on next page.
• Bandwidths of the X-band System
• Beam seen due to structure fill time 10MHz
• Structure RF Bandwidth 40MHz
• XL4 Klystron gt 100MHz
• Bandwidths of the S-Band System
• Beam seen due to structure fill time 1.2MHz
• Structure RF Bandwidth 16MHz
• 5045 Klystron 10MHz
• The bandwidth for the system will be determined
  based on noise levels of the as built RF system
  and rise times of interest, somewhere between
  1.2MHz and 10MHz.

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RMS tolerance budget for lt12 rms peak-current
jitter (column 3) or lt0.1 rms final e- energy
jitter (column 4). The tighter tolerance is in
BOLD, underlined text and both criteria,
DI/I0 lt 12 and ?DE/E0? lt 0.1, are satisfied
with the tighter tolerance applied. All
tolerances are rms levels and the voltage and
phase tolerances per klystron for L2 and L3 are
?Nk larger, assuming uncorrelated errors, where
Nk is the number of klystrons per linac.
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Phase Noise of SLAC Master Oscillator
Old Oscillator
New Oscillator
Noise Floor -120dBc/38Hz -136dBc/Hz 120fS rms
Jitter in 5MHz BW
Noise Floor -143dBc/38Hz -158dBc/Hz lt 16fS rms
Jitter in 10MHz BW
New Oscillators Have a noise floor of -157dBc/Hz
_at_ 476MHz
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LINAC Front End RF and Timing System
LCLS must be compatible with the existing linac
operation including PEP timing shifts
Master Oscillator is located 1.3 miles from LCLS
Injector
1.3 Miles to LCLS Injector
PEP PHASE SHIFT ON MAIN DRIVE LINE
MDL RF with TIMING Pulse Sync to DR
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RF Distribution System
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Linac Phase Reference System

• Main Drive Line - 3 1/8 Rigid Coax Anchored to
  Concrete Floor Every Sector
• Must not introduce noise over 2 miles. Diurnal
  Temperature Variations 200fS rms/Sector
• Phase Reference Line - Each Sector Independent
  1/2 Heliax
• 332ft/Sector. Diurnal Temperature Variations
  640fS rms errors

The PAD measures phase noise between the
reference RF and the high power system. The beam
sees 3.5uS of RF from SLED.
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Noise Levels

• RF Reference Noise Floor
• Oscillator -148dBc/Hz SSB at 2856MHz
• RF -138dBc/Hz SSB at 2856MHz
• Integrated Noise
• -138dBc/Hz at 10MHz -65dBc 32fS rms
• SNR 65dB for phase noise
• Added noise from MIXER (LO noise same as RF)
• SNR of 62dB
• ADC noise levels
• SNR of 70dB 14bit ADS5500 at 119MSPS 8.5MHz

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RF Control and Monitor Points Summary

• RF Gun 1 Klystron 3 RF monitors
• Beam Phase Cavity 1 IQ modulator 1
  RF monitor
• L0-A Accelerator 1 Klystron 2 RF
  monitors
• L0-B Accelerator 1 Klystron 2 RF
  monitors
• L0-T Transverse Accelerator 1 Klystron 2 RF
  monitors
• L1-S Station 21-1 B, C, and D accelerators
• 
  1 Klystron 4 RF monitors
• L1-X X-Band accelerator X-Band
• S25-Tcav 1 Klystron 2 RF monitors
• S24-1, 2, 3 Feedback 3 Klystrons
• S29 and S30 Feedback 2 IQ modulators 476MHz
• Totals 2856MHz 10 modulators 16 monitors

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RF Monitor

• LO 2830.5MHz RF 2856MHz
• IF 25.5MHz (8.5MHz x 3 in sync with timing
  fiducial)
• Double-Balanced Mixer
• Mixer IF to Low Pass Filter and Amp
• Amp output to ADC (119MSPS or 102MSPS)

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SLAC Linac RF New Control
The new control system will tie in to the IPA
Chassis with 1kW of drive power available.
Reference will be from the existing phase
reference line or the injector new RF
reference I and Q will be controlled with a
16bit DAC running at 119MHz. Waveforms to the
DAC will be set in an FPGA through a
microcontroller running EPICS on RTEMS.

Existing System
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RF Sub-Systems

• RF Gun
• Beam Phase Cavity
• L0-A Accelerator
• L0-B Accelerator
• L0-T Transverse Accelerator
• L1-S Station 21-1 B, C, and D accelerators
• L1-X X-Band accelerator
• S25-Tcav
• S24-1, 2, 3 Feedback
• S29 and S30 Feedback

• The RF Monitor unit will be capable of measuring
  phase and amplitude for
• 2 RF channels.
• The RF Control unit will be of two versions.
• The fast version will digitize a 1k sample
  of data to drive an IQ modulator.
• The slow version will be capable of setting
  an I and Q value on a pulse to
• pulse basis.

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RF Gun The
RF Gun will be driven from klystron 20-6. The RF
monitors on the gun will be used in feedback
loops to control the operating frequency along
with providing phase and amplitude information to
the longitudinal feedback.
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Beam Phase CavityThe
Beam phase cavity will be used to measure the
bunch position relative to the RF. The
measurement will be done at 120Hz and provide
information to the feedback system. The cavity
is located between L0-A and L0-B accelerator
structures.
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L0-A - First accelerator in the off axis
injectorL0-B has the similar control scheme.
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Injector Transverse Deflector
CavityStation 20-5 and Station 24-8 will be
connected to the new control system and used to
drive the L0 and Sector 25 transverse deflector
cavities, respectively. The cavity is used to
impart on the beam a deflection vs. longitudinal
position correlation. The beam is deflected to a
screen where bunch length can be measured.
20
L1-SThe L1 linac consist of klystron 21-1
powering 3 accelerator structures in the main
linac.
21
X-Band 2nd order correctionAn
X-band accelerator structure will be located at
station 21-2, just before BC1. The beam will be
run on the decelerating crest to remove the
second order curvature in the bunch vs
longitudinal position correlation before
compression in BC1.
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S24-1, 2, and 3 (Three Units)Two
klystrons in sector 24 will be used to correct
the phase and amplitude of the RF as seen by the
beam in LCLS L2 before the beam enters BC2. The
klystron phase and amplitude will be adjusted
based on bunch length and energy measurements. A
third klystron will be capable of use by the
feedback system if one of the other two should
fail.
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S29 and S30 Phase Control (Two Units)The phase
of sectors 29 and 30 will be adjusted in opposite
directions to change the average amplitude of the
RF as seen by the beam as it passes through the
sectors without effecting the average phase as
seen by the beam.