Proceedings of the 7th Annual Meeting of Particle Accelerator Society of Japan (August 4-6, 2010, Himeji, Japan)

ELECTRON BUNCH GENERATION FEMTOSECOND PHOTOCATHODE
RF GUN
Koichi Kan , Jinfeng Yang, Takafumi Kondoh, Kimihiro Norizawa, Atsushi Ogata, Takahiro Kozawa, Yoichi Yoshida
The Institute of Scientific and Industrial Research (ISIR) Osaka University Mihogaoka 8-1, Ibaraki, Osaka, 567-0047

Abstract
A femtosecond electron bunch is essential for the observation of ultrafast reactions and phenomena in materials. Inorder to improve the time resolution of pulse radiolysis and ultrafast electron diffraction (UED), that involve the use of
an ultrashort electron bunch and ultrashort light, a femtosecond photocathode RF gun was investigated experimentally.
The bunch length was changed by space-charge effect and RF compression in the RF gun. Emittance of the femtosecond
electron bunch was also varied by space-charge effect. Bunch charge generated in the RF gun was studied theoretically
based on Schottky effect.

フェムト秒フォトカソード RF 電子銃による電子ビーム発生
1.

Introduction

Femtosecond electron bunches on the order of 100 fs orless can be used in accelerator physics applications suchas free electron lasers (FELs) and laser-compton X-ray.
Such electron bunches are also key elements in the studyof ultrafast reactions and phenomena in time-resolved
pump-probe experiments involving the application oftechniques such as ultrafast electron diffraction (UED)and pulse radiolysis. The time resolutions in UED andpulse radiolysis depend on the electron bunch length. InUED, an electron bunch is used as a probe source andultrafast phenomena, such as laser-induced phasetransients, are monitored using electron diffraction
patterns. Pulse radiolysis also involves the use of anelectron bunch and a laser; this technique is a powerfultool that can be used for the observation of ultrafast
radiation-induced phenomena involving the mechanicalmotions of electrons and atomic nuclei in reaction
mechanisms that are studied in physics, chemistry, andbiology. At Osaka University, a photocathode-based
linear accelerator (linac) and a magnetic bunch
compressor were constructed for femtosecond pulse
radiolysis based on a femtosecond electron bunch. A
picosecond electron bunch with a transverse emittance ofapproximately 4 mm-mrad was generated using aphotocathode RF (radio frequency) gun by projecting a
Nd:YLF picosecond laser onto a copper cathode. The
electron bunch was accelerated up to 32 MeV by thebooster linear accelerator with an optimal energy-phasecorrelation in the bunch (the acceleration of the bunchhead was greater than that of the bunch tail) for
compression of the bunch. Finally, the electron bunch was___________________________________________#koichi81@sanken.osaka-u.ac.jp

successfully compressed into femtoseconds, e.g., 98 fs in
rms at 0.2 nC [1]. A femtosecond electron bunch has beenused in pulse radiolysis in order to study the kinetics ofsolvated electrons with time resolution of femtoseconds[2].
However, femtosecond electron bunch of several MeVs
generated by a photocathode RF gun can be also essential
in UED, pulse radiolysis, and FELs in order to observe
ultrafast reactions and phenomena on the order of <100 fs.
In order to generate a femtosecond electron bunch,
femtosecond ultra-violet (UV) light was projected onto
the photocathode RF gun. Bunch length at a linac entrance
was measured with a phase-scan technique [3]. Emittanceat the linac exit was measured with a quadrupole-scantechnique.

2.

Experimental arrangement

Figure 1 shows the experimental arrangement. A 1.6cell S-band (2856 MHz) RF gun with a copper cathodeand a Ti:Sapphire femtosecond laser was used to produce
a femtosecond electron bunch. In the laser system, themode-locked Ti:Sapphire oscillator (Tsunami, producedby Spectra-Physics Co.) was driven with an output of 800mW at 79.3 MHz, the 36th sub-harmonic of the 2856
MHz accelerating RF. The outputs of the oscillator laser
were amplified up to 0.8 mJ/pulse synchronized with a 35
MW klystron in a regenerative amplifier (Spitfire,produced by Spectra-Physics Co.). The regenerative
amplifier was driven at 10 Hz and the laser pulse width is<130 fs in full-width-half-maximum (FWHM), i.e., 56 fs
in root-mean-square (rms). The amplified pulse wasconverted to femtosecond UV light (266 nm) by the THGof nonlinear optics (TPH-Tripler, produced by Minioptictechnology Co.). The maximum power of the UV was 140
μJ/pulse. The femtosecond UV light was injected into the-148-Proceedings of the 7th Annual Meeting of Particle Accelerator Society of Japan (August 4-6, 2010, Himeji, Japan)

RF gun at an incident angle of approximately 2o along theelectron beam direction, where the spot size was reduced
to 2 mm in diameter by an aperture. The maximum of thebunch charge was 100 pC with 15 μJ of UV. The beamenergy at the gun exit was 4.2 MeV.
In bunch charge measurement, a current transformer(CT), which was calibrated with a picoammeter, set at thegun exit was used. The femtosecond electron bunch
produced by the RF gun was accelerated up to 27 MeV by
a 2 m long S-band travelling-wave linac. Transverse
emittance at the linac exit was measured with a standard
quadrupole scan technique, in which the beam size on aYAG screen (YAG1, 100 μm-thick, produced by Ohyo
Koken Kogyo Co.) was varied by a quadrupole magnet(QM). The screen was mounted at 45o with respect to theelectron beam. The electron beam profile on the screen
was acquired by a CCD camera (CCD1) with abackground subtraction process. The emittance wasanalyzed with the dependence of the beam size on theinverse of the quadrupole focal length with least-squares
fitting. Finally, rms bunch length at the linac entrance wasmeasured by a phase-scan technique [3], in whichcorrelated rms energy spread of the electron bunch wasmeasured. A YAG screen (YAG2) was set at 0.7 m
downstream of a bending magnet (BM). The electron
beam profile on the screen was acquired by a CCD
camera (CCD2). The linac can give small/large energy
modulation to short/long electron bunches. The bunch
length was analyzed with the dependence of the correlated
energy spread on the accelerating phase in the linac withleast-squares fitting.
THG

Ti:Sapphire
fs laserSynchronization
KLY
YAG2
YAG1
CCD2
LinacSolenoid
PhotocathodeRF gunQMBM
CCD1Figure 1: Schematic diagram of a femtosecond electron
source.

3.

Introduction

3.1

Bunch charge

the Schottky effect, i. e., field emission due to effective
electric field on the cathode surface, the bunch charge canbe described with accelerating and decelerating field due
to RF and space-charge effect. In the data, QE of the orderon 1.0*10-5 was obtained.

Figure 2: Bunch charge as a function of injection phase of
the femtosecond UV light. In the left figure, the UV spot
size and power were 2 mmφ and ranging from 1 to 15μJ/pulse. In the right figure, the UV spot size and power
were 2 mmφ and ranging from 0.5 to 4 μJ/pulse.
3.2

Beam emittanceIn the measurement of the emittance, a standardquadrupole-scan technique was used with the quadrupole
magnet (QM) and the screen (YAG1). Figure 3 shows theemittance at the linac exit as a function of the magnetic
field of the solenoid set at the gun exit. In the data, thebunch charge was a constant of 50 pC/pulse with 16
μJ/pulse of UV. The laser spot diameter was 2 mm on thecathode surface. The laser injection phase was set to 25 orelative to the zero crossing of the RF field for thereduction of RF emittance. The correlated energy spreadof the electron bunch was minimized at each data bychanging the accelerating phase in the linac. The beamemittance depends on linear and non-linear space-charge
effects, the RF emittance and thermal emittance. In order
to compensate the linear space-charge effect, the magnetic
field of the solenoid was varied. The error bar in the data
represents the error of the least-squares fitting obtained by
the quadrupole scan technique. The best compensation
minimized the emittance to 1.2 πmm-mrad at the solenoid
field of 1.75 kG. Figure 4 shows the dependence of theemittance on space-charge. The emittance also depends on
space-charge effect. The increase in emittance due tospace-charge effect was obtained as a rate of 0.008 mmmrad/pC. The thermal emittance was obtained as 0.8 mmmrad at zero-charge.

Figure 2 shows the bunch charge as a function of theinjection phase of the femtosecond UV light. The laser
energy was varied from 3 to 20 μJ/pulse. The laser spot
diameter was 2 mm at the cathode surface. The solenoid
was fixed at 1.75 kG, which was optimal for emittancecompensation. In this paper, the bunch charge due tospace-charge and Schottky effects was studied
theoretically at once. The bunch charge generated in theRF gun has been expressed with Schottky effect. Based on-149-Figure 3: Emittance at the linac exit as a function ofmagnetic field of the solenoid (left). The laser injection
phase and the bunch charge were 25o and 50 pC/pulse,

Proceedings of the 7th Annual Meeting of Particle Accelerator Society of Japan (August 4-6, 2010, Himeji, Japan)

respectively. Emittance as a function of bunch charge(right).
3.3

diameter on the cathode of 0.5, 1, 2 mm, respectively.

Bunch length measurement

In the measurement of the bunch length, a phase-scantechnique [3] was used with the 45o-bending magnet (BM)and the screen (YAG2), as shown in Fig. 1. The correlated
rms energy spread was decided by the rms beam size on
the screen, which was located at 0.7 m downstream of thebending magnet. According to the reference [3], theenergy spread can be expressed as,
 2
 2

Vl sin(0 )   2 12 
Vl sin(0 )    22
 360
 36022
 dE  11 (1)where σdE is the energy spread, Vl is the accelerating
voltage in the linac. The coefficients of σ11, σ12 and σ22 are
the fitting parameters. The coefficient of σ11 corresponds
to the square of the bunch length.
The bunch length at the gun exit depends on spacecharge effect and the acceleration phase in the RF gun.
Figure 4 shows the rms bunch length as a function of thebunch charge. The bunch charge was varied by the driving
laser power of the regenerative amplifier. The laser spot
diameter on the cathode was ranging from 0.5 to 2 mm. In
the data, the laser injection phase was 25o. RFcompression overwhelming the longitudinal space-charge
effect in the RF gun was observed at low charge, e. g.,
200 fs in rms at 8 pC with the laser spot diameter of 2 mm.
At zero-charge, the bunch length corresponds to the laser
pulse width of 200 fs. At high charge, the bunch length
increased due to space-charge effect, e. g., 400 fs in rms at
70 pC. The bunch length also increased with the smaller
laser spot size due to space-charge effect at the cathodesurface. However, with the low bunch charge can decrease
the bunch length. The bunch length due to space-charge
effect increased linearly with the bunch charge at a rate of
3.4 fs/pC with the laser spot diameter of 2 mm. Figure 5
shows the longtitudinal emittance as a function of thebunch charge. The longitudinal emittance due to spacecharge effect increased linearly with the bunch charge at arate of 0.03 deg-keV/pC.

Figure 5: Longitudinal emittance as a function of bunchcharge. The red, blue and black plots indicate the laser
spot diameter on the cathode of 0.5, 1, 2 mm, respectively.

4.

Conclusion

The femtosecond electron source based on thephotocathode RF gun and the femtosecond laser wasinvestigated experimentally. The bunch length at the linac
entrance was measured with a phase-scan technique. In
the condition of the laser injection phase of 25o, the RFcompression overwhelming the longitudinal space-charge
effect in the RF gun was observed at low charge, e. g.,
200 fs in rms at 8 pC with the laser spot diameter on thecathode of 2 mm. A low longitudinal emittance of 1 degkeV was also obtained. In order to generate an electron
bunch of <100 fs and sub-femtosecond, the femtosecond
electron bunch would be compressed with a magnetic
bunch compressor, which can compensate the secondorder effect. Furthermore, the optimization of the laser
spot size and the bunch charge generate a short-bunch andlow-emittance electron beam which can supress theincrease in compressed bunch length and the aberration in
the compression. The compressed bunch would beinvestigated by the analysis of Coherent Transition
Radiation (CTR) with a Michelson interferometer.

References[1] J. Yang, et al., Nucl. Instr. and Meth. A 556 (2006) 52.
[2] J. Yang et al., "Femtosecond pulse radiolysis andfemtosecond electron diffraction", Nucl. Instr. and Meth. A,
in press.
[3] D. H. Dowell et al., SLAC Report No. SLAC-PUB-9541,
revised 2002.

Figure 4: Rms bunch length as a function of bunch charge.
The red, blue and black plots indicate the laser spot-150-