JSA WG4 summary KJ

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JSA WG4 summary KJ Powered By Docstoc
					          Working Group 4
 Diagnostics & Synchronization
 Mostly by Gwyn Williams and the JLab Team,
           Presented by D. Douglas
Where we are and what needs work…

                   Kevin Jordan
         Jefferson Lab, Newport News VA
 Charge for working group 4

• Review what works & why
   – What good & bad decisions have been made
• Synchronization
   – 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
                             1530nm                                           0.8

                                           f3                 +
                                                                                                                                       •     Cross-correlator delay

           reprate                                                  reprate
            fiber                                                                                                                            set to partially overlap
                                                                                                             120fs FWHM

modelocked  output                                            modelocked      0.00010000                         0.00020000
                                                                                                        0.00015000          0.00025000     0.00030000

laser 1                                                       laser 2                                                                        pulses
                 f2                         f4
      output                                                                                            cross-correlation:
                                                                                                  0.8                 •                      Voltage versus time
                             1570nm                                                               0.6                                        delay is close to linear

                                                                                     voltage, V


                                                 cross-correlator                                  0
                                                                                                                                       •     Error signal sensitivity
                                                                                                    -100             0     100   200         is 0.13mV/fs
                                           1MHz bandwidth detector
                                                                                                                     delay, fs

                                                                              5.7fs RMS from 1Hz to
time, fs

              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:
Synchronisation Activities
   for ERLP and 4GLS

         Graeme Hirst
    STFC Central Laser Facility
                   ERLP/4GLS Summary
• 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
  timing jitter
   – 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
                                                       FEL amplifier
                                                     output for direct
                                                      comparison to
                                                      oscillator FEL

                                     Current plan (hope $$) is to use
                                         the UV components to
                                       demonstrate FEL amplifier
                       Seed Laser
      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

 1st Pass
Pulse <B>
             2nd Pass
            Pulse <A’>

                                Long term goal is to separate pulses at higher
                                  Micropulse Repetition Rate Frequencies.

                         Micropulse Repetition
                           Rate Frequency
                             (9.425 MHz)
1.16 MHz Multi-pass Solution
Coronagraph for halo measurement at KEK

        Anti-reflection disk
                                  Field lens                     Relay lens

                                                    Baffle plate (Lyot stop)
  Objective lens
                                      Opaque disk

               Baffle plates to reduce
        Observation (KEK) Chronograph

Single bunch
65.8mA                           Intensity
                                 in here :
Exposure time
of CCD : 3msec                   2.05x10-4
                                 of peak

 Far tail

 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%!
              Operational Procedures
• 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 jordan@jlab.org
• 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

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