Working Group 4
Diagnostics & Synchronization
Mostly by Gwyn Williams and the JLab Team,
Presented by D. Douglas
Where we are and what needs work…
Jefferson Lab, Newport News VA
Charge for working group 4
• Review what works & why
– What good & bad decisions have been made
– What does one need and how much will it cost?
• Beam Instrumentation Requirements
– Multipass BPMs
– Injector Diagnostics – especially phase space tomagraphy
– Exploit synchrotron light!
• Beam Loss & Halo
• Operational Procedures
Synchronization @ LBNL/SLAC
To produce an ultra-stable timing and synchronization system with jitter reduced to the few femtosecond level, we
have developed a laser-based scheme with optical signals distributed over a stabilized optical fiber.
Transmitting precise frequency and timing signals over distances of hundreds of meters, stabilized to a few
femtoseconds (a few parts in 108), is accomplished by measuring the phase delay in an optical fiber and actively
compensating for differences with a piezoelectric modulator. In our scheme, phase differences at optical frequency
are down-converted to 110 MHz. Because phase information is preserved during the heterodyning process, phase
differences at optical frequency can be detected at radio frequencies, using conventional RF electronics. The
radiofrequency reference signal need not be provided with femtosecond accuracy at the far end of the fiber, because
one degree of error at 110 MHz is equivalent to only one degree at the optical frequency, or 0.014 fs.
The system is linear, and signals modulated onto the CW laser carrier at the fiber entrance do not intermodulate with
each other. Moreover, the optical power level is significantly below any nonlinear threshold in the fiber. The laser
frequency itself must be stabilized, so the laser is locked to an absorption line in an acetylene cell.
At present, a 4 km fiber link has been stabilized to the femtosecond level. 2 km of fiber in this link passes under
several roads and through several buildings at LBNL, demonstrating that the fiber stabilization system is robust under
real-world conditions. This technique will soon be used as a backbone to demonstrate synchronization of mode-
locked lasers. Further developments will include integration with controls and low-level RF systems, and high-
resolution diagnostics of photon and electron beams, to provide enhanced feedback control of the integrated
laser/accelerator systems. We are planning to develop and implement similar systems at the LCLS, and
Steve Lidia for the LBNL team
Initial results (LBNL)
• Simplified setup: lasers
coarse 100MHz 1.8
lock electronics 1.3
co-located on optical
• Cross-correlator delay
fiber set to partially overlap
modelocked output modelocked 0.00010000 0.00020000
0.00015000 0.00025000 0.00030000
laser 1 laser 2 pulses
0.8 • Voltage versus time
1570nm 0.6 delay is close to linear
• Error signal sensitivity
-100 0 100 200 is 0.13mV/fs
1MHz bandwidth detector
5.7fs RMS from 1Hz to
0 •Inter-laser link not stabilized
-10 gives short stabilization time
-20 •Currently no acoustic
0 0.5 1 isolation
•Can improve loop gain by
Work in Progress:
for ERLP and 4GLS
STFC Central Laser Facility
• Synchronisation requirements for ERLP are relaxed. The photoinjector laser needs to
be (and has been) locked to the machine RF with jitter <1ps.
• ERLP will act as a testbed for some of the subsystems needed for 4GLS.
• A clock is being developed based on an Er fibre oscillator locked at low frequencies to
a stable RF source.
• A fibre distribution system based on laser pulse propagation will be tested.
• EO timing of electron bunches is planned.
• Synchronisable commercial lasers are being evaluated.
• A phase noise measurement system is operational and is being improved.
• “Local” synchronisation is being developed.
• A conceptual design for 4GLS synchronisation has been produced.
• Electron bunch arrival time appears to be the major outstanding problem. Fast electron
timing sensors will be implemented and the option of feedback control of timing will be
Timing for JLab FEL Amplifier Design
• Since the beam shifts in time due to small RF fluctuations I (S.
Benson) have assumed that we can't hold the electron beam timing to
better than about 1 psec.
• The specification for the laser thus has a pulse length of a few
picoseconds to ensure that there is reasonable overlap despite the
– Most of the amplifier designs assume a picosecond FWHM
micropulse so you don't gain too much by having jitter smaller
than about 100 fsec
– Again is doesn't make much sense to have the laser timing jitter
much better than the electron beam timing jitter.
– If this is done to about 100 fsec it would be fine.
JLab ERL FEL Amplifier Design
E = 120 MeV
135 pC pulses up to 75 MHz
20/120/1 microJ/pulse in UV/IR/THz
250 nm – 14 microns, 0.1 – 5 THz
All sources are simultaneously
produced for pump-probe studies
10 kWatt CW
output for direct
Current plan (hope $$) is to use
the UV components to
demonstrate FEL amplifier
Synchronization Bottom Line
• You get what you pay for!
• State of the Art; LCLS designs
– $1M(+) gets you 10s fsec over kilometers of fiber
• Every System needs a good Master Oscillator
– Expect $50k - $100k
• Small machines (<100 meters) can use temp stabilized
copper for ~$150k
– 1/10 degree, 1.3GHz ~200fsec
– 100fsec target
• Seed laser requirement for JLab FEL Amplifier ~100fsec
Tentative parameters of KEK ERL test facility
Injection energy 5 MeV (10-15 MeV)
Injector beam power 500 kW (1MW)
Beam energy in arcs ~60 MeV (160-200 MeV)
SC cavities for main 9 cells x 4: single module (two modules)
Normalized emittance 1 mm mrad(0.1 mm mrad)
Beam current 10mA – 100mA
RMS bunch length Usual mode: sz=1-2 ps
Short bunch mode: sz ~100 fs
Beam instrumentation for ERL
• Beam profile measurement
– Fluorescence screen for low energy (<10 MeV)
– Optical profile monitor by OTR or SR
– Wire scanner (SEM or Compton scattering)
– High speed gated camera
• Beam position measurement
– BPM (electric)
– BPM (SR or OTR)
• Intensity measurement
– Photocathode, Faraday cup
– SR or OTR based intensity monitor
Beam instrumentation for ERL cont.
• Emittance measurement
– Fluorescence screen with slit
– Quad Scan + OTR
– Wire scanner
• Beam temporal structure
– Streak camera (SR or OTR) (>1 psec bunches)
– Incoherent intensity interferometer (SR or OTR)
– CSR interferometer
– LOLA Streak cavity
• Beam Halo
– Wire scanner or Fork
– Coronagraph (SR or OTR)
• Beam Energy (SR or Dipole +BPM/Viewer at dump)
• Beam loss monitor
Multi-pass BPMs in the FEL LINAC (under
Long term goal is to separate pulses at higher
Micropulse Repetition Rate Frequencies.
1.16 MHz Multi-pass Solution
Coronagraph for halo measurement at KEK
Field lens Relay lens
Baffle plate (Lyot stop)
Baffle plates to reduce
Observation (KEK) Chronograph
in here :
of CCD : 3msec 2.05x10-4
time of CCD : Background
100msec leavel : about
Halo Monitor at JLab FEL
• Forks on 6” stepper motor driven actuators
• Not yet fully exploited
• Able to see nanoamps (or better)
– Use with BLM to enhance performance
Beam Loss (accounting) at KEK
Differential DCCT to measure current
valance (or beam loss) between
accelerated beam and decelerated
Problem is that ~1 microamp is tolerable loss
1 part in 103 for 1 milliamp
1 part in 105 for 100 milliamp
Instrument is not that accurate/stable but if it was the variation in
charge in micropulse to micropulse is no better than 0.1%!
• Set the phase of cavity 1-3
• (this section was updated to show how to use ITV0F06 viewer instead
of IPM0F06 BPM) Note: if you change cavity 1-3 by more than, say,
1/2 degree, you should go back and repeat it all - because it will make
the zero crossing go away. (per Dave Douglas)
• insert ITV0F06
• map ITV0F06 to the monitor 17 (to the WesCam) and make image
• “Beam X–Position (mm)” needs to be (see shift plan for the right
number)?1. If it is, you are done.
• If it is not, open the injector RF slider screen by clicking the
appropriate button on the 4-seater, see Fig. 6. Set the gain on the cavity
1-3 phase slider to 0.1o and adjust the phase to make ITV0F06 “Beam
X–Position (mm)” to be the right number?1
– If the ITV0F06 “Beam X–Position (mm)” more than the right
number, push the phase positive.
– If the ITV0F06 “Beam X–Position (mm)” less than the right
number, push the phase negative.
Repository of available procedures
• Writing procedures saves time during commissioning!
• Too often they are written after the fact
• To help Yuri & ERLP I will post written procedures on open web
• Any donations will be accepted at email@example.com
• This will serve as a starting point ERL commissioning documents
How do we solve the challenges?
• The Beam is NOT Gaussian!
– How can the diagnostics give better information to models
– Novel tomagraphic solutions
• Halo is and will continue to be a problem
– What experiments can be done now?
• Injector requirements
– Proper set-up & monitoring (we still have drifts!)
• Multipass BPMs remain to be a challange
• Synchronization requirements