Proceedings of IBIC2012, Tsukuba, Japan

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MEASUREMENT OF SUB-PICOSECOND BUNCH LENGTH WITH THE
INTERFEROMETRY FROM DOUBLE DIFFRACTION RADIATIONTARGET*J.B. Zhang, S.L. Lu, T.M. Yu, H.X. Deng, Shanghai Institute of Applied Physics, China
D.A. Shkitov, M.V. Shevelev, A.P. Potylitsyn#, G.A. Naumenko, Tomsk Polytechnic University,Russia
Reliable and precise methods for non-invasive
diagnostics of sub-picosecond electron bunches arerequired for new accelerator facilities (FEL, LWFA,et al.). Measurements of spectral characteristics ofcoherent radiation generated by such bunches using
interferometer allow to determine a bunch length [1].
Here we present a compare experimental study between
the measurement from the double diffraction radiation
target interferometry and the measurement from theMichelson interferometer.

INTRODUCTION
Non-invasive methods of the sub-picosecond bunch
length diagnostics are very important nowadays for suchaccelerator facilities as free electron lasers with typicalbunch lengths of some hundreds of femtoseconds.
One of the ways to measure the electron bunch lengthis based on coherent radiation that is generated atwavelengths comparable to, or longer than, the bunch
length, when all electrons in the bunch irradiate more orless in phase. The intensity of coherent radiation isproportional to the square of the bunch population. The
spectral distribution of the coherent radiation contains
information about the electron bunch distribution.
Coherent diffraction radiation (CDR) is suggested as themechanisms for coherent radiation generation due to their
non-invasive nature.
DR appears when a charged particle moves rectilinearly
in a vacuum in the vicinity of a medium. If the particle
passes through a slit between two semi planes, it induces
time-varying currents on both semi planes. These currents
generate radiation. Two cones propagate in the directionof specular reflection producing an interference pattern.
The interference pattern obtained by two diffraction
radiation beams from two shifted plates (double DRtarget) may be used instead an interferometer [2].
Recently, the coherent DR interferometry scheme wasestablished [3] on the femtosecond accelerator facility atSINAP [4, 5].

___________________________________________* Work supported by the joint Russian-Chinese grant (RFBR No. 1102-91177 and NSFC No. 11111120065) and partially by the Programof Russian Ministry of Education and Science “Nauka” and Chinese
NSFC grant No. 11175240.
# Corresponding author. Email: pap@interact.phtd.tpu.ru

EXPERIMENTAL SETUP
Our experimental equipments were consisted of a linear
accelerator, a vacuum chamber with targets driven by
motion-control system, a Michelson interferometer anda detector.

Accelerator
The accelerator providing femtosecond electron bunch
facility consists of an S-band thermionic cathode rf-gun,an alpha magnet and a SLAC-type accelerating tube.
The electron beam emerges from an APS type 1.6 cell π/2
mode thermionic rf-gun. After pre-accelerated at the rfgun, the electron beam is transported to the alpha-magnet
via a beamline consisting of the focusing, steering,scraping and beam monitoring components. The alphamagnet is used to compress the electron beam from a few
hundreds of ps to few hundreds of fs in longitudinallength. A SLAC-type accelerating tube is used toaccelerate the electron beam up to high energy tominimize the lengthening force in the drift space.
The design parameters of electron beam are listed inTable 1.
Table 1: Parameters of Electron Beam
Beam energy

20 – 30 MeV

Beam charge

0.068 nC

Normalized emittance~ 10 mm·mrad

Macro-bunch repetition frequency

3.125 – 12.5 Hz

Micro-bunch repetition frequency

2856.2 MHz

Macro-bunch duration2 – 3 μs

Micro-bunch duration (FWHM)

0.3 – 3 ps

Interferometer
To measure the interferograms of the coherent
diffraction radiation from ordinary DR target the typicalMichelson interferometer was employed (see in Fig. 1).
The interferometer consists of a beam splitter, which ismade from a 2-µm-thick nitrocellulose and comes witha metallic coating, providing consistent 30% reflection
and 30% transmission over a very wide bandwidth,a stationary Au-coated hollow reflector mirror, a moving
Au-coated hollow reflector stepped by remote control
system capable of sub-micron steps, and a off-axis
parabolic mirror that focus the re-combined light into thea pyroelectric detector.
ISBN 978-3-95450-119-9

Transverse & Longitudinal Emittance Measurements

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPB65

Proceedings of IBIC2012, Tsukuba, JapanTarget
The DR target was consisted of two plates (with sizes
46×20 mm2) made from the 2 µm aluminum foil coveredon the 0.3 mm polyamide film. Both plates were fixed on
the holder with a possibility to move one plate relative
other one along the beam direction with a step less than
16 µm. During such a movement we measured coherent
DR intensity for each position (interferogram).
In Fig. 3 the side view of doubled DR target is shown
which illustrate the location of the two parallel plates
relative to each other when they are shifted along beam toa distance d, θ0 = 45° is the tilt angle to the beam
trajectory, β is the particles speed.

Figure 1: The experimental setup and the scheme of theinterferometer.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Detector
To detect CDR a room temperature broadband LiTaO3
pyroelectric detector SPI-D-62 THZ (Gentec-EO) wasapplied with the aperture diameter 5 mm which areoptimized for use in our case. Other detector parameters
are presented in Table 2.
Table 2: Detector Parameters
Sensitivity range

0.01 – 3 mm.

Response

150 kV/W

NEP

0.4×10-9 W/(Hz)1/2

Average power (max)

200 mWIn Fig. 2 the curve of detector sensitivity from
specification is shown which was used in simulation.

Figure 2: The curve of detector sensitivity.

Figure 3: The shifted two plates of DR target.

RESULTSFirst we measured the interferogram by applying theMichelson system with changing the position of themovable mirror. This interferogram was measured fromthe coherent diffraction radiation of one plate (ordinaryDR target), when another plate was moved aside. In Fig. 4
the typical time domain interferogram from Michelsoninterferometer is depicted. The base of interferogram isequal to 0.249 μW.

Figure 4: The typical Michelson interferogram.
Then we measured the interferogram using the coherent
diffraction radiation interferometry. In Fig. 5, part of
the whole interferogram measured in experiment isISBN 978-3-95450-119-92Transverse & Longitudinal Emittance Measurements

Proceedings of IBIC2012, Tsukuba, Japan

presented in distance domain. In conducted experiment
we could to get the interferogram only in time domain. In
order to compare the calculated and experimental
distribution and moreover to have a possibility to use theapproach [6] for estimating the bunch length from CDR
interferometry the time domain interferogram wastransformed to distance domain. Knowing the speed one
plate to other 100 μm/s and macropulse repetition rate
6.25 Hz of the accelerator the 16-µm-step value between
two points of experimental interferogram was received.
Also the distribution minimum was shifted to the 0 of thehorizontal axis and cut to the interval (-5; 5) like in thefurther simulation.

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Finally the simulation was carried out for parameters ofexperiment. The parameters are listed in Table 3.
In Fig. 6 below the simulation of the interferogram forvalue of the bunch length equal to 0.2 mm averaged on
the 25 points of detector is shown.
These 25 points on the detector surface are locatedalong and across relative to motion of the beam withthe same step from each other one. Above mentioned
average was done to account for the detector aperture.
The developed model [7] also takes the detector
sensitivity into account in present case.

DISCUSSION

Figure 5: The modified experimental interferogram.
The interferogram in Fig. 5 was measured whenthe movable mirror in the Michelson interferometer wasplaced in the centre of interferogram on Fig. 4.
Table 3: Simulation Parameters
Beam energy

E = 21 MeV

Bunch length

σRMS = 0.2 mm

Wavelength range of simulation

0.1 – 3 mm

Target slit

a = 10 mm

Distance from the target centre tothe centre of detector

L = 260 mm

Figure 7: The spectrum from Michelson interferogramand calculated spectrum.
Moreover we can apply another technique because
the main part of information about the bunch length iscontained in the first oscillation of the interferogram.
Therefore we may try to build a criterion for the bunch
length estimation using this part of interferogram. If wedefine the interferogram function as S ( d ) , thenthe simplest criterion may be represented using
the dimension theory [9] as RMS  f ( x) ,where S (d ) ,x    S ( d )  S (0)  d  d  d maxdmax is the value of variable d in the point of maximal
Figure 6: The simulated interferogram.

 S (d ),  RMS is the bunch length in Gaussiandapproximation of the longitudinal distribution of electrons
ISBN 978-3-95450-119-9
Transverse & Longitudinal Emittance Measurements

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Applying the algorithm for spectra reconstruction
described in [8], we have obtained the spectrum fromMichelson interferogram (see in Fig. 7 red solid curve).
The analysis of this spectrum gives the estimation bunch
length about 0.19 mm that equal to 660 fs. The simulationof such spectrum using the above mentioned model ispresented on the same Fig. 7 (blue dashed curve). This
simple simulation is performed without taking into
account the aperture and the sensitivity of detector withparameters like in Table 3, but with impact-factor 5 mm.

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Proceedings of IBIC2012, Tsukuba, Japan

in bunch, the function f ( x ) is yet undefined. For
definition of this function we calculated the dependencex ( RMS ) from theoretical curves of interferogram in
Gaussian approximation of the longitudinal distribution ofelectrons. Finally with a good accuracy was obtained RMS  x .
Applying the described approach for the experimental
interferogram (Fig. 5) the bunch length was estimatedabout 0.2 mm. For the simulated interferogram (Fig. 6)
was obtained the value about 0.248 mm. The difference of
these values may be explained by a not enough alignmentof the double DR target.
It is clearly seen a good agreement all of results fromtwo techniques which confirms the ability of the proposedtechnique for non-invasive bunch length measurements inthe sub-picosecond range without a complicated scheme
like Michelson interferometer or similar.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○CONCLUSION
The first measurement of the sub-picosecond electron
bunch length was carried out using coherent diffraction
radiation interferometry (non-invasive beam diagnostics
technique).
From the measured interferogram the RMS bunch
length was found to be about 660 femtosecond, which
confirms the ability of the proposed technique for noninvasive bunch length measurements in the subpicosecond range.

The developed technique doesn’t need any complicate
interferometer and additional optics elements.

REFERENCES[1] A. Murokh et al.,”Bunch length measurement ofpicosecond electron beams from a photoinjector using
coherent transition radiation”, NIMA 410 (1998) 452.
[2] A.P.
Potylitsyn,“Coherentdiﬀraction
radiationinterferometry and short bunch length measurements”,
NIMB 227 (2005) 191.
[3] M. Shevelev et al., “Sub-millimeter Bunch Length Noninvasive Diagnostic Based on the Diffraction andCherenkov Radiation”, Journal of Physics: Conference
Series 357 (2012) 012023.
[4] X. Lin, J. Zhang et al., “High Power THz Undulator
Radiation from Linear Accelerator”, Chin. Phys. Lett. Vol.
27 No. 4 (2010) 044101.
[5] J. Zhang, H. Deng et al., “Tunable few-cycle coherent
terahertz radiation with watt-level power from relativistic
femtosecond electron beam”, Nucl. Instr. Meth. A 693(2012) 23.
[6] G.A. Naumenko, A.P. Potylitsyn et al., Izv. Vuzov. Fizika
11/2 (2009) 254.
[7] D.A. Shkitov, A.P. Potylitsyn, “Feasibility of diffraction
radiation from doubled target for beam diagnostics”, Bookof abstracts RREPS’11 (2011) 70.
[8] L. Froehlich, “Bunch Length Measurements Usinga Martin-Puplett Interferometer at the VUV-FEL”, DESYTHESIS 2005-011, FEL-THESIS 2005-02.
[9] L.I. Sedov, Similarity and Dimensional Methods in
Mechanics, (Academic Press, 1959), 363.

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4Transverse & Longitudinal Emittance Measurements