In addition to its normal operation at 250pC, the LCLS has operated with 20pC bunches delivering X-ray beams to users with energies between 800eV and 2 keV and with bunch lengths below 10 fs FWHM. A bunch arrival time monitor and timing transmission system provide users with sub 50 fs synchronization between a laser and the X-rays for pump/probe experiments. We describe the performance and operational experience of the LCLS for short bunch experiments

Bostedt, Christoph

Bozek, John

Brachmann, Axel

Byrd, J. M.

Coffee, Ryan

Decker, Franz-Josef

Ding, Yuantao

Dowell, David

Emma, Paul

Frisch, Josef

Gilevich, Sasha

Haller, Gunther

Hays, Gregory

Hering, Philippe

Hill, Bruce

Huang, G.

Huang, Zhirong

Iverson, Richard

Kanter, Elliot

Kraessig, Bertold

Loos, Henrik

Miahnahri, Alan

Nuhn, Heinz-Dieter

Perazzo, A.

Petree, M.

Ratner, D.

Santra, R.

Smith, T.

Southworth, S.

Turner, J.

Welch, J.

White, W.

Wilcox, R.

Wu, J.

Young, L.

UNT Digital Library

Work supported in part by US Department of Energy contract DE-AC02-76SF00515.FEMTOSECOND OPERATION OF THE LCLS FOR USER EXPERIMENTS*A. Brachmann, C. Bostedt, J. Bozek, R. Coffee,  F.-J. Decker, Y. Ding, D. Dowell, P. Emma, J. Frisch, S. Gilevich, G. Haller, G. Hays, Ph. Hering, B. Hill, Z. Huang, R. Iverson, E. Kanter,  B. Kraessig, H. Loos, A. Miahnahri, H.-D. Nuhn, A. Perazzo, M. Petree, D. Ratner, R. Santra,  T. Smith, S. Southworth,   J. Turner, J. Welch, W. White,, J. Wu, L. Young: SLAC National Laboratory, Stanford CA, USA J. M. Byrd, G. Huang, R. Wilcox: LBNL, Berkley CA, USAAbstractIn addition to its normal operation at 250pC, the LCLS has operated with 20pC bunches delivering X-ray beams to users with energies between 800eV and 2 keV and with bunch lengths below 10 fs FWHM. A bunch arrival time monitor  and  timing  transmission  system  provide  users with sub 50 fs synchronization between a laser and the X-rays  for  pump  /  probe  experiments.  We  describe  the performance and operational experience of the LCLS for short bunch experiments. FEMTOSECOND OPERATIONIn  normal  operation the  LCLS uses  250 pC electron bunches to produce X-ray pulses with lengths between 50 and 500 femtoseconds.  The LCLS can also be operated at 20pC to produce very short  pulses.  The pulse length is below the  resolution  of  the  accelerator  instrumentation but based on indirect  measurements it  is believed to be less than 10fs FWHM [1]. For  short  pulse  mode  the  the  photo-cathode  laser  is apertured to a 0.6mm diameter spot on the cathode with a 3.6  ps  FWHM  pulse.  The  source  laser  is  operated  15 degrees from the Schottky edge produce an electron beam out of the gun with a 700fs RMS bunch length. Typical emittance is 0.14 microns RMS in each plane.The beam is compressed in two bunch compressors to near  maximum peak current to produce a bunch length below  the  20fs  resolution  of  the  transverse  cavity. Simulations using measured beam paramters at 8.3 KeV photon energy give a pulse length around 5 femtoseconds FWHM.Figure 1: Simulated pulse length In  Figure  2 the  FEL pulse  energy  and (uncalibrated) peak electron beam current are plotted as a function of the phase of the L2 compression section.  The FEL does not lase  well  at  full  compression,  probably  due  to  a combination  of   slice  energy  spread  and  CSR induced emittance  growth.  Maximum  compression  is  at  -32.25 degrees, and the FEL lases well at +/- 1 degree from this peak.  The bunch length when operating 1 degree  from maximum compression can be calculated using a simple linear model and gives approximately 5 fs FWHM. Figure 2: FEL power (blue) and Ipk(red)  vs. compressionPUMP-PROBE EXPERIMENTSA variety of experiments can be preformed by pumping the sample with a visible laser and after a variable delay probing with a X-ray pulse. This requires synchronization of  the  FEL and  a  conventional  laser  and  a  system  to measure their relative timing. The LCLS uses S-band RF cavities to measure the pulse to pulse beam time relative to  a  phase  reference  line.  A slow feedback  adjusts  the timing of  a  signal  that  is  transmitted to the experiment pump laser through an interferometrically stabilized fiber link [2]. A block diagram of the timing system is shown in figure 3. The electron beam timing jitter measured at the phase cavities relative to the 476 MHz phase reference line was 130 fs RMS for the 2009 run and 60 fs RMS for the 2010 run. The improvement was due to using a different source for the phase reference as shown in figure 3. The phase SLAC-PUB-14234SLAC National Accelerator Laboratory, Menlo Park, CA 94025cavity system reports the pulse to pulse relative timing of the electron  beam and phase  reference to  allow offline improvement of the experimental data. The RF based beam arrival time measurement sytstm is described here.  The fiber and laser locking systems are described in XXXFigure 3: Timing systemBEAM ARRIVAL TIME MONITORThe  Beam  Arrival  Time  system  uses  single  cell resonant cavities operating at 2805MHz which are excited by  the  bunch  passage.  This  produces  a  decaying exponential tone which is mixed with the 2856 MHz 6 th harmonics of the 476 MHz reference line to produce a 51MHz IF. That signal is digitized with the 119MHz 4 th subharmonic of the 476 MHz.  Figure 4. The input attenuator is used to control the signal level sent to the mixer. The desired level is a trade-off between noise and nonlinearity that would convert bunch charge fluctuations into timing noise. A signal level of -10dBm into  the  mixer  (Marki  Microwave  T3-03)  gives  good performance. The cavity produces signals larger than this for bunch charges above approximately 50 pC. The mixer output  is  followed  by  a  20dB  gain  low  noise,  high linearity  amplifier  (MiniCircuits  ZHL-2010+)  to  match the signal to the digitizer input level. The digitizer is a VME module constructed at SLAC based on a Linear Technology LTC2028 16 bit chip. The external trigger to the digitizer is synchronous with the beam so the waveform is reproducible shot to shot. If the divide-by-4 circuit jumps buckets it is reset by software. Figure 4: Bunch Arrival Time system block diagramPhase Measurement AlgorithmThe phase cavities are constructed from copper and will exhibit a temperature Dependant frequency shift that will result in a change in the average phase of the signal on the order of several picoseconds per degree C, much larger than the tolerance for the system. Fortunately the signal provides a direct measurement of the cavity frequency on each  pulse  which  can  be  used  to  correct  for  the temperature coefficient. The  digitized  signal  is  down-converted  to  provide  a measurement  of  phase  vs.  time.  The  projection  of  this phase back to the beam arrival time provides a signal that is first order independent of  of cavity frequency. Beam Arrival Time Monitor PerformanceComparison of the 2 phase cavity systems provides a measure of their phase drift and noise. Note that while the systems  use  independent  hardware  after  the  reference line,  they are exposed to similar environments so some drifts may not be seen in this comparison. In addition drifts of the X6 multipliers will show up in the cavity to cavity comparison,  but will  not  effect  the operation in the real system: the fiber system is locked to the ouput of the X6 multiplier. Figure  5  shows  the  difference  between  the  times measured by the two systems and the 100 pulse shot-to-shot RMS difference plotted over 4 days of operation at the normal 250pC operating charge. Figure 5: Phase cavity system performance over 4 days showing 13fs RMS noise (blue) and 40fs drift (magenta).When  the  accelerator  is  operating  at  20pC for  short bunches, the phase cavity noise increases due to the small signal  level.  Figure  6  shows  the  performance  over  3 hours.  This is the longest run at  20pC with the present version of the phase cavity system. Figure 6: Operation at 20pC showing 27fs RMS noise and 25fs drift over 3 hours.OVERALL SYSTEM PERFORMANCEThe  noise  and  drift  performance  of  the  fiber  system cannot  be measured  as  installed – there  is  no alternate system for comparison. Based on measurements at LBNL the RMS  noise and drift are believed to be below 10fs. The  experiment  laser  phase  noise  is  25fs  RMS in  a 1KHz  bandwidth,  but  increases  to  120fs  RMS  at broadband  (up  to  100  KHz),  believed  to  be  due  to acoustic noise. There is no direct way to measure the laser timing drift.Preliminary  results  from  laser  pump  /  X-ray  probe experiments  performed in 2009 indicate  a  beam timing jitter of approximately 150 fs between the pump laser and the  X-rays.  This  is  consistent  with  the  electron  beam measurements preformed at the same time. The reference line has been upgraded in 2010 (see figure 3), but new experiment results are not yet available.  The 2009 experiments indicated sub 50-femtosecond pump laser to X-ray timing noise and drift(after off-line correction). This was likely dominated by the noise of  the pump laser relative to the fiber system reference. [3]PERFORMANCE IMPROVEMENTSThe low frequency noise and drift performance of RF components  is  not  generally  specified  by  the manufacturer.  Work is under way to test components to find improved performance substitutes.The algorithm used is non-ideal as it does not make use of the fact  that  the cavity frequency varies slowly with time.  It  also  discards  some  of  the  data  due  to  phase ringing from components in the RF system. Initial tests indicate that a factor of 2 improvement in noise may be possible from algorithm improvements. The  existing  fiber  system  includes  some  RF components that are outside of the stabilization loop and could probably be optimized. The laser locking system is believed to be the largest contributor to timing noise. Work is underway to isolate the  laser  from acoustic  noise  and  to  use  low drift  and temperature  stabilized  cables  and  components  in  the locking system. The  low  (68MHz)  repetition  rate  of  the  laser  mode locked oscillator limits the noise performance if its phase detection  system.  Experiments  are  underway  to  use  an etalon system to multiply the beam rate on the photodiode to improve the measurement phase noise. SYSTEM LIMITATIONSThe X-ray timing may not match the electron timing if not  all  of  the  bunch  lases.  This  system  also  does  not measure the X-ray path length from the undulator to the experimental chamber, or changes in path length from the pump  laser  oscillator  to  the  chamber.  These  effects probably limit the performance of this type of system to a few femtoseconds or some fraction of the electron bunch length. REFERENCES[1] Y. Ding et. al.  “Measurements  and Simulations of Ultra-low Emittance and Ultra-short Electron Beams in the Linac Coherent Light Source”, Phys Rev Lett. 102, 254801, (2009).[2] J.  M.Byrd et.  al.  “Femtosecond Synchronization of Laser Systems for the LCLS”, This conference.[3] J.  M. Glowina et.  al.  “Time Resolved Pump Probe Experiments at the LCLS”, To be published in Optics Express. (2010)

Femtosecond Operation of the LCLS for User Experiments

https://digital.library.unt.edu/ark:/67531/metadc1016063/m2/1/high_res_d/992896.pdf

Femtosecond Operation of the LCLS for User Experiments

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