Proceedings of IBIC2012, Tsukuba, Japan

TUPA41ULTRA-SHORT ELECTRON BUNCH AND X-RAY TEMPORALDIAGNOSTICS WITH AN X-BAND TRANSVERSE DEFLECTINGCAVITY*P. Krejcik†, Y. Ding, J. Frisch, Z. Huang, H. Loos, J. W. Wang, M-H. Wang,SLAC, Menlo Park, CA, USA
C. Behrens, DESY, Hamburg, Germany
P. J. Emma, LBNL, Berkeley, CA, USA
The technique of streaking an electron bunch with a RF
deflecting cavity to measure its bunch length is being
applied in a new way at the Linac Coherent Light Sourcewith the goal of measuring the femtosecond temporalprofile of the FEL photon beam. A powerful X-band
deflecting cavity is being installed downstream of theFEL undulator and the streaked electron beam will beobserved at an energy spectrometer screen at the beam
dump. The single-shot measurements will reveal whichtime slices of the streaked beam have contributed to theFEL process by virtue of their greater energy loss andenergy spread relative to the non-lasing portions of theelectron bunch. Since the diagnostic is locateddownstream of the undulator it can be operated
continuously without interrupting the beam to the users.
The resolution of the new X-band system will becompared to the existing S-band RF deflecting diagnostic
systems at SLAC and consideration is given to therequired RF phase stability tolerances required for
acceptable beam jitter on the monitor. Simulation studies
show that about 1 fs (rms) time resolution is achievable inthe LCLS over a wide range of FEL wavelengths andpulse lengths.

INTRODUCTION
X-ray Free Electron Lasers such as the Linac Coherent
Light Source (LCLS) can produce very short pulses of afew femtoseconds (10-15s) duration [1]. This makes thema powerful tool for observing ultrafast phenomena, suchas molecular dynamics. The challenge for the accelerator
community is to measure this pulse duration at the fslevel. An even greater challenge is to measure both thephoton pulse duration and the electron bunch since theemission process does not result in a one to one
correspondence.
The transverse deflecting cavity (TCAV) is now a wellestablished diagnostic instrument at FEL linacs tomeasure the temporal profile and slice properties of theelectron beam [2]. In this new application the instrument
is installed downstream of the undulator where it streaks
the spent electron beam and is observed on a profile
monitor screen located at the beam dump [3]. This gives aunique opportunity to observe the FEL process in the___________________________________________*This work was supported by Department of Energy Contract No. DEAC0276SF00515† Corresponding author email pkr@slac.stanford.edu

Longitudinal Diagnostics and Synchronization

time-resolved energy profile of the beam without
interrupting beam to the users.

EXISTING TCAV SYSTEM AT SLAC
Two transverse deflecting cavities are currently in use
on the LCLS accelerator and operate at the S-band, 2856
MHz. A short 40 cm section is located in the injector anda longer 2.44 m long structure is located in the main linac,
downstream of the second bunch compressor. Both ofthese structures were actually designed and built in the1960’s [4]. The principle of operation is shown in Figure
1, where a bunch at 0° phase crossing of the rf appears
streaked on a downstream screen.

Figure 1: Principle of operation of the TCAV.
The size of the streaked beam y is related to the bunch
length z by2 k eV     d  s  RF 0 sin  cos   (1) Eswhere y0 is the unstreaked transverse beam size, d ands are the beta functions at the deflector and screen, kRF
the wave number, V0 the rf amplitude, Es the beam energy the betatron phase advance from deflector to screenand  the phase of the rf.
2y2y02z

TCAV Operated with an Energy Spectrometer
The utility of the TCAV diagnostic is further enhanced
if the beam is observed on an energy spectrometer screen
where the energy dispersion is in the plane perpendicular
to the rf deflection. The screen measurement reveals thetime-resolved energy and energy spread. The example
shown in Figure 2 is from the straight-ahead energy
spectrometer screen where the LCLS injector beam isbent in the horizontal plane onto a beam dump. The beamis streaked in the vertical plane so that the curvature of theon-crest rf acceleration of the beam is observed, as well as
the “slice” energy spread of the beam. The latter quantity
is particularly important for determining the performanceof the FEL.
ISBN 978-3-95450-119-9
1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

TUPA41

Proceedings of IBIC2012, Tsukuba, JapanThe X-band structure can also be operated at higher field
gradients allowing us to achieve a deflection voltage
V0=40 MV which is about 2 times higher than on theexisting S-band system. The combined effect is that thenew X-band system will have 8 times better resolution.
With the typical LCLS beam emittances the expected
resolution over the 4-14 GeV beam energy rage isexpected to be 1-2 fs.

XTCAV INSTALLATION AT LCLS

Figure 2: Time-energy correlation measurement.

RESOLUTION OF THE TCAVThe ratio of beam size on the screen to bunch length fromEquation (1) isk  d  s eV0 sin (2)S  y  rfc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○zEewhich leads to a measurement resolution in terms of theratio of the beam emittance N, y to the strength of the kickrf Ee(3) t ,R  y 0  N , yS
 d 2 eV0 sin 
The dependence on wavelength RF indicates that we gaina factor 4 in resolution by choosing X-band (RF=2.62cm)
in place of the existing S-band system ((RF=10.5cm).

Two adjacent 1 m long sections of X-band deflecting
structure have been installed at the end of the undulator
beamline just before the dumpline dipole bends, as seenin Figure 3. They are fed by a single SLAC XL4 klystronhoused together with a modulator in a support building
above the undulator tunnel. This is separate from the main
linac klystron gallery building and requires a new highpower RF installation and control system. RF waveguidemode converters at the klystron and downstairs at thesplitter to the two structures are used to switch between
WR90 rectangular waveguide and a low-loss over-moded
WC293 circular waveguide for the long run between theklystron and the structures. The XL4 klystron delivers 50
MW peak power at 120 Hz and 40 MW is delivered at thestructure input couplers.
Some beamline components have been rearranged toaccommodate the XTCAV and optimize the value of thehorizontal beta function xd at the deflector. The newoptics is shown in Figure 4.

Figure 3: Photo of the XTCAV installed at the downstream end of the undulator beamline.
ISBN 978-3-95450-119-9
2Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, JapanTUPA41

With a vertical dispersion of Dy=0.65 m this gives aresolution of 7 10-6, or 100 keV at 14 GeV.
The camera at this location will be upgraded to give awider field of view to observe the streaked beam. The
image acquisition controls will be upgraded to 120 Hz.

Operational Tolerances

Operation of the upstream TCAV installations isinvasive to user operation of the facility but since theXTCAV is located downstream of the undulator it can be
operated continuously with no impact on the FEL. Some
precautions are taken with the control system to ensurethat accidental mis-phasing of the RF, which results inmissteering of the beam, does not unduly trip the MachineProtection System. The parameters for the operation ofthe XTCAV are summarized in Table 1.
Table 1: Parameters for the XTCAVParameterValueUnit

RF frequency
Structure type11.4242π/3 backwardwave
Horizontal defl.
1118.8211310< 100GHz10mm<168±0.1
48

mrad°F°F
MeV/c120~ 0.1< 0.1140

Hzµs
deg-X%
MW50MW

Structure orientation
Effective structure length
Total flange-flange length
Number of structures
No. of regular cells per structure
Aperture 2aHor. and vert. structure alignment toleranceLongitudinal structure positioning
tolerance
Roll angle tolerance
Operating temperature
Thermal stability
Nominal transverse kick (on crest)@40MW
Maximum Repetition rate
RF pulse duration
RF phase stability (rms) >0.5 Hz
RF relative ampl. stability (rms)Nominal power required atstructure input
Max power output from klystronmcmmmµm

The profile monitor screen where the streaked beam isobserved is located immediately in front of the main beam
dump where the beam has been bent vertically down toseparate it from the photon beam directed straight aheadto the users. The vertical axis of the screen represents theenergy coordinate in the beam image with an energy
resolving power given by  ,R  y N , yDy(4)FEL PHOTON MEASUREMENTS
A characteristic of the SASE process in FEL radiationis that not all parts of the electron bunch radiate
uniformly. Spikes occur in the temporal domain and growrandomly from shot to shot out of the noise spectrum. The
parts of the electron bunch that do contribute to the lasing
process suffer a significantly greater energy loss andenergy spread increase than the neighbouring non-lasing
portions of the bunch. The XTCAV functions as a timeresolved energy spectrometer and reveals which parts ofthe bunch have lased and what their longitudinal profileis.

Figure 5: Beam simulation result at the spectrometer
screen with the FEL on (left) and FEL off (right).
In the example shown in Figure 5 the electron beam
image with the FEL suppressed (at right) is compared tothe beam image with the FEL on (left). Lasing in the FEL
is easily suppressed by introducing a small steering error
in the undulator, taking the electron and photon beam outof coincidence. The portions of the bunch that do laseshow up as spikes with an increased energy spread in thevertical plane and a lower centroid energy in the timeslice.

Reconstruction of the photon temporal profile
The image is divided horizontally into an arbitrary
number of time slices and the centroid energy and energy
spread is calculated for each slice. By subtracting theimage data taken with the FEL off it is possible toISBN 978-3-95450-119-9

Longitudinal Diagnostics and Synchronization

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 4: Optics from the deflector to the spectrometer
screen.

The phase stability requirements of the RF system aredefined by the allowable beam position jitter tolerance atthe spectrometer screen. The 0.1° rms X-band phasetolerance listed in Table 1 corresponds to a horizontal
motion of one sigma of the horizontal beam size. At
larger amplitudes of phase jitter the user will observe that
progressively more shots will fall outside of the screen
viewing angle. The 0.1° phase tolerance puts in turn an 80
ppm voltage stability requirement on the klystron
modulator, which is now achievable with new, state-ofthe-art technology.

TUPA41

Proceedings of IBIC2012, Tsukuba, Japan

quantify the energy shift and energy spread induced by
the FEL process itself.
The horizontal projection of the image with the FEL off
gives the electron beam current profile before theundulator, as shown in Figure 6 at the left. The beam
current profile used in the simulation is the result of startto-end tracking of the beam, as described in reference [3].
The simulation reflects the complex longitudinal profileof a real beam with higher order terms introduced bycompression, CSR and wakefield effects.

The reconstructed profiles shown on the right in Figure
6 are the predicted photon intensities based on the energy
and energy spread inferred from the image. It can be seenthat peaks in the laser intensity do no fully coincide withslices of the beam with the highest peak current, further
underscoring the need for this diagnostic with its ability toreveal both the electron and photon beam profiles.

ACKNOWLEDGMENT
Grateful acknowledgement is given in memory of thelate Ron Akre, who contributed greatly to the many RF
projects at SLAC.

REFERENCES

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○[1] P. Emma et al., Nat. Photon. 4, 641 (2010).
[2] P. Emma, J. Frisch and P. Krejcik, Report No. LCLS-TN00-12, 2000.
[3] Y. Ding et al., Phys. Rev. ST Accel. Beams 14, 120701(2011).
[4] G. A. Loew and O. H. Altenmueller, “Design andapplications of R.F. deflecting structures at SLAC,” SLACPUB-135, Aug. 1965.

Figure 6: Reconstructed beam current profile (left) andphoton intensity (right) for soft x-ray operation (top) andhard x-rays (bottom) from reference [3].

ISBN 978-3-95450-119-9
4Longitudinal Diagnostics and Synchronization