synchronization system for lux john staples, lbnl 26 july 2004
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Synchronization System for LUX
John Staples, LBNL
26 July 2004
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Deflecting cavities 3.9 GHzDipole mode
Accelerating linacs1.3 GHzTESLA-like
Linearizer cavities3.9 GHzLongitudinal mode
Requirements● Synchronization between pump lasers and probe X-rays to better than 50 fsec
● LUX pulse rate of 10 kHz
● 100-150 meter footprint of facility
● Many end-stations, always in flux
● 1.3 and 3.9 GHz supercon cavities, photoinjector
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Pulse Compression
● Start with 20-30 psec from photoinjector, compress to 2-3 psec in arcs
● Final compression to 50 fsec optically– asymmetric crystal on spatially
chirped beam
– 3.9 GHz SC transverse deflecting cavity
● Electron beam timing less critical, deflecting cavity timing very critical
● The end-station lasers must be synchronized to X-ray pulse
Undulator Radiation from head electrons
Radiation from tail electrons
RF deflecting cavity
Electron trajectory
in 2 ps bunch
∆l
Input x-ray pulse >> diffraction limited size and
natural beamsize
Synchronous bunch
Timing jitter results in position/angle jitter of compressed
x-ray pulse
Early bunch
Late bunch
Asymmetrically cut crystal
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Fundamental Approach to Timing● Distribute accurate clock to all accelerator elements and
end-station pump lasers● Measure residual jitter of accelerator components, sum
them up, and transmit to end-station lasers● All elements ride on top of common-mode clock jitter● Differential jitter between X-ray pulse and end-station
lasers is reduced to 50 femtosecond regime● Long term mechanical drifts also very important
– 10 fsec is equivalent to a 3 micron motion
– 1 meter of aluminum or SS grows 3 microns for 0.1 degree C temperature change, invar about 0.1 microns
– transport systems geometric changes must also be monitored
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Why Should This Work?
● Spectral density of timing jitter dominated by low-frequency phenomena– flicker and random walk of frequency and phase
● The coherence time of the significant spectral components is long (audio to sub-audio)
● The jitter components above this frequency range are usually below the noise floor of the monitors
● The integral over frequency space, the time jitter, is dominated by the low-frequency part of the spectrum.
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Available Technology
● Stabilized fiber laser links show promise for transporting timing signals with femtosecond jitter over short distances
● ML lasers have been synchronized to 1 fsec relative jitter using electrical techniques, at 10-14 GHz
● Commodity fiber components are widely available– but the fiber must be actively stabilized
● Don't need a super-stable clock– Crystal oscillators useable, a Poseidon not necessary
– Common-mode jitter of a picosecond acceptable
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NLC Approach: Frisch et al.Transmit 357 MHz timing signal over 15 kmAbout 1 degree X-band over moderate time scales (240 fs)
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Timing jitter 0.58 fs (160 Hz BW)
Timing jitter 1.75 fs (2 MHz BW)
Top of cross-correlation curve
Total time (1 s)Cro
ss-C
orre
latio
n A
mpl
itude
30 fs
0
1(two pulses maximally overlapped)
(two pulses offset by ~ 1/2 pulse width)
Ma et al., Phys. Rev. A 64, 021802(R) (2001)Sheldon et. al. Opt. Lett 27 312 (2002) .
Synchronization electrically of two independent 100 MHz MLlasers, at 100 MHz and 14 GHz.Measured by correlating in non-linear crystal.femtosecond timing attainable electrically. --Reported by David Jones
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Hardware Approach● Start with a good (<1 psec jitter) clock● Distribute clock very accurately (<25 fsec) to all
elements in accelerator and endstations– Use 1550 nm fiber optics components
– Stabilize each fiber distribution link
– Will try to stabilize based on modulation, not optical carrier
– Fiber has much better bandwidth than coax cable
● Local loops stabilize elements within gain/bandwidth limitations, provide residual error signal
● Weighted sum of low-frequency error signal distributed over low-bandwidth digital link to TBCs
● End-station lasers follow low-bandwidth correctors
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Example:Crab Cavity
● 3.9 GHz deflector● include clock,
microphonics● I-Q LLRF system● simulate residual
noise after control loop is closed
● 350 watts klystron output power
● 12 fsec residual
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Timing Distribution
● Require timing distribution system that provides differential-mode jitter of a few femtoseconds to all clients over the entire facility.
● Demonstrate a stabilized fiber network that can satisfy this requirement– fiber has wide bandwidth capability
– use RF techniques to achive jitter stabilization
– if inadequate, revert to interferometric techniques
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Proof-of-Principle Experiment
● The key is low-jitter distribution of a 1 GHz clock● Demonstration is constrained by budget● 1 GHz crystal clock (Wenzel 100 MHz + multipliers)● 1.5 mW 1550 nm DFB single-mode laser
– EDFA increases power up to about 40 mW
– Mach-Zehnder modulator, no chirp
● 100 m single-mode fiber, APC/PC connectors● Low-noise (75K, 1 db NF) RF amplifiers● Low-noise op-amps in LLRF circuits● Piezo stabilization of fiber
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Contributions to Jitter in POP
● LLRF system operational bandwidth rolloff at 1 kHz● 1 dB noise figure of ZRL-1150LN amplifiers following
photodiodes operating at -8 dBm adds about 9 fsec noise at 1 GHz over a 10 kHz bandwidth
● LMH6624 Low-noise op amps following phase detectors will add noise in the fsec range.– Still characterizing ZFM-4 mixers (phase detectors), but contribution
may be significant
● Wenzel clock is measured to have a 1.0 psec jitter
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Example: Noise contribution of op-amp following mixer
Integrate noisecurrent and voltage,including Johnsonnoise over workingbandwidth.
With a 10 kHz 2-pole LPF,the noise integrates to 0.2 uV,for a sub-femtosecond phasejitter.
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Characterizing the Piezo Fiber Optic Phase Modulator
Strong 18 kHz mechanical resonance, Q=139, measured interferometricallyShape and stabilize feedback loop around modulator
Open-loop Bode Plot Nyquist Stability Plot
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Present Status
● Have all the optical components– have measured and verified their specifications
● LLRF system designed, not yet constructed– all items are at hand, including PC boards
● Signal generators, spectrum analyzers, etc. are all acquired
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Future Developments
● Will establish capabilities of a fiber stabilization system at 1 GHz.
● Move on to 10 GHz region● Improve laser itself, if budget permits
– Mode-locked
– A gas-stabilized reference would be nice
● Look at interferometric techniques if necessary