Synchronization System for LUX

John Staples, LBNL

26 July 2004

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

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

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

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.

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

NLC Approach: Frisch et al.Transmit 357 MHz timing signal over 15 kmAbout 1 degree X-band over moderate time scales (240 fs)

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

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

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

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

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

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

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.

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

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

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