The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

SUB-PICOSECOND COMPRESSION EXPERIMENT BY VELOCITYBUNCHING IN THE S-BAND PHOTO-INJECTOR AND LINAC
N. H. QuyetA), M. UesakaA), L. SerafiniB),
T. UedaA), H. IijimaA), N. KumagaiC), A. MostafaA), F. SakamotoA)
A)

Nuclear Engineering Research Laboratory, The University of Tokyo,2-22 Shirakata-Shirane, Tokai, Naka, Ibaraki 319-1188, JapanB)
Istituto Nazionale di Fisica Nucleare (INFN)-Milan, Via Celoria 16 - 20133 Milan, ItalyC)
Japan Synchrotron Radiation Research Insutitute,
1-1-1 Mikazuki-cho, Sayo-gun, Hyogo, 679-5198, JapanAbstract
The velocity bunching is one of the electron bunch
compression methods, which was proposed by L.Serafini,et al. in 2000. We have carried out the velocity bunchingin the Mg photo-cathode RF injector and the S-band
Linac at the Nuclear Engineering Research Laboratory,
the University of Tokyo. The bunch length is measured
by the femtosecond streak camera. We have achieved theshortest bunch length of 0.5 psec (FWHM), the average
length of 1.3 psec (FWHM) and the fluctuation of 0.4
psec (rms) for 1 nC/ and the injected RF phase of thezero-crossing. The results are consistent with thenumerical analysis, which was done by PARMELA
simulation code.

1. INTRODUCTION
The velocity bunching, which is one of the electron
bunch compression method, was proposed by L. Serafiniin 2000[1]. The compression scheme is based on therectilinear compressor. Since the relevant compression
magnet is not used, several harmful Coherent Synchrotron
Radiation force effects can be avoided.
At Nuclear Engineering Research Laboratory, theUniversity of Tokyo, an S-band linac with an Mg photoinjector (18L) is utilized for experiments of radiation
chemistry as such as the chemical reaction of water. To
realize the high time-resolution in order of subpicosecond, the chicane-type magnetic compressor is
normally used[2,3]. Although the chicane-type
compressor has the ability to produce short bunch enough,we are planning this new compression method.
Normally two accelerating tube are used for thecomplete scheme of velocity bunching as shown in
SPARC project[4], because the energy of compressedbeam in the first tube is extremely low. Therefore, theelectron bunch from the photo-injector is compressed in
the first linac and accelerated in the second linac.
Unfortunately 18L has only one accelerating tube.
However, Helmholtz coils to suppress the emittance
growth due to the space-charge effect are located aroundthe linac. Thus, we have demonstrated the velocity

bunching with concentrating the verification ofcompression scheme using one accelerating tube andHelmholtz coils.

2. THEORY
As mention above, the velocity bunching is based on
the rectilinear compression scheme. The bunch is injectedjust after the zero phase of RF in the accelerating tube.
Consequently, it is compressed due to the velocitydifference between the head and tail of the bunch withslipping backward to the RF crest. Owing to the magnetic
field of the solenoid surrounding the accelerating tube, theemittance growth due to the space-charge effect at thelow-energy region is avoided to realize the Brillouin flow.
Consequently, the electrons in the bunch do not crossover radially.
To calculate the compression factor it must be consider
that the Hamiltonian is an invariant of the motion[1]:H = H0where.

H = γ − β r γ 2 − 1 − α cos ξH 0 = γ 0 − β r γ 02 − 1 − α cos Ψ0Taking the initial conditions of the injection phase andenergy, the compression factor C has been evaluated asfollows:C=δΨ0=δξ ex2δΨ0 sin ξ ex
§ 1 δγ 0 ·¸¸δΨ + ¨¨αγγ© 0 0 ¹240

where δ Ψ0 is the initial bunch width, δξ ex the final
bunch width (at extraction phase), ξ ex the extraction
phase, γ 0 the initial beam energy, δγ 0 / γ 0 the energy
spread. Figure 1 shows the contour plot for 18Lparameters. The vertical axis indicates the beam energy γ.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

4. RESULTS
Figure 4 shows a typical streak image at zero-crossing
injection. The bunch width is measured to be 0.5 psec at
FWHM in the image.

Fig. 1 Contour plot for 18L parameters.

3. EXPERIMENTAL SETUP
Figure 2 shows the schematic view of experimental
setup. The electron beam is generated from the photoinjector which is driven by the third harmonics ofTi:Sapphire laser pulse. The electron bunch with chargeof 1 nC is accelerated up to energy of 4 MeV
approximately. The beam exited will be focused by thesolenoid magnet then injected in the accelerating structure.
It continuously is accelerated by the 2856 MHz frequencyof traveling wave. The aero gel with its index of 1.015
and thickness of 1 cm is used as the Cherenkov radiator.
The energy threshold is 3.0 MeV. The Cherenkov
radiation emitted by the electron bunch moving in theAerogel material, reflected by a flat mirror, converged by
2 lens and finally reflected again by the other mirror intothe femtosecond streak camera through the band passfilter.

Fig. 2 Schematic view of experimental setup.
The amplitude of magnetic field by Helmholtz coils isfixed to be 300 Gauss.

Fig.3 Streak image.
From Fig. 4, it is clearly to observe the efficiency of thevelocity bunching effect. Each point indicates the averageof 30 streak-image shots and the errors corresponds to theroot mean square of them. Here, the error is mainly
caused by the fluctuation of RF-timing. The minimum
bunch width of 0.5 ps for the beam of 1 nC/bunch wasachieved at the injection phase just after the zero-cross.
While the average one is 1.3 ps.
Beyond the zero-phase, the electron bunch tends toincreasing in width - because at that time the bunch will
start to slip ahead and being decompression.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

5. CONCLUSION
From the above results we see that the velocity
bunching is a very useful compression method. The bunch
width of 0.5 ps for the beam of 1nC/bunch can beobtained. Based on the rectilinear compression the effectis very sensitive with the injection, special at the phasenear zero-cross. So that by changing the injection phasewith smaller steps the shorter bunch width could beobserved. Our experiment is somehow difficult to change
the injection phase at every 10 due to the fluctuation of thechanging injection phase device.

REFERENCES

Fig. 4 The bunch width as a function of injected phase.

[1] L. Serafini and M. Ferrario, “Velocity Bunching in
Photo-injectors”, Proc. of ICFA Workshop on thePhysics of, and Science with, the Free-Electron Laser,
Arcidosso, Italy, Sept. 2000, p. 87.
[2] M. Uesaka, et al., Rad. Phys. Chem. 60(2001)303.
[3] Y. Muroya, et al., Nucl. Instrum. & Meth. A,489(2002)554.
[4] M. Boscolo, et al., Proc. of EPAC, Paris, 2002.