Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

PHOTOCATHODE FEMTOSECOND RF GUN*
J. Yang#, K. Kan, T. Yamamoto, N. Naruse, Y. Yoshida, K. Tanimura, The Institute of Scientific andIndustrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
J. Urakawa, KEK, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 3050801, Japan.
Abstract

A new photocathode-based femtosecond electron
source is developed in Osaka University to generate alow-emittance femtosecond-bunch electron for revealing
the hidden dynamics of intricate molecular and atomic
processes in materials through time-resolved pulse
radiolysis and time-resolved electron diffraction. A
photocathode rf gun with many improvements wasconstructed
undertheKEK/Osaka
University
collaboration. The transverse and longitudinal dynamics
of femtosecond electron beam in the rf gun were studied
by particle simulation. The growths of the emittance,bunch length and energy spread due to the rf and spacecharge effects were investigated by changing the laserparameters and bunch charge. The simulation indicatesthat a sub-100-fs MeV electron beam with the transverse
emittance of 0.1 mm-mrad and the relative energy spreadof 10-3~10-4 at bunch charge of 0.1-1 pC is achievable in
the rf gun driven by a femtosecond laser light.

INTRODUCTION
High-brightness electron sources, producing short,intense, low-emittance electron bunches, are key elements
not only for new developments in accelerator physics, i.e.
future electron-positron colliders and x-ray free electron
lasers, but also for structure and dynamics research inmaterial science, biology, physics and chemistry.
Femtosecond electron bunches of the order of 100 fs are
essential to reveal the hidden dynamics of intricate
molecular and atomic processes in materials throughexperimentation such as time-resolved pulse radiolysis,
ultrafast electron diffraction (UED) or ultrafast electron
microscopy (UEM). In the pulse radiolysis [1], a shortelectron bunch is used as a pump source. The electroninduced ultrafast reactions are analyzed generally with anultrashort probe light such as femtosecond lasers. Afemtosecond single electron bunch with beam energy
from a few to a few tens MeV is very important to be
utilized in this technique for observing information of themost basic reaction mechanisms on femtosecond timescale, e.g. excitation, ionization, and relaxation of atoms
and molecules.
The UED or UEM provides a unique opportunity for acomplete determination of the transient structures withatomic level detail [2]. A 100-fs-long bunch electron
beam is essential to measure the ultrafast atomic motions
on the fundamental time scale of a single atomic____________________________________________*Work supported by Joint Development Research at KEK, and JSPS
#yang@sanken.osaka-u.ac.jp

vibrational period (100 fs to ~1 ps) for the study ofcomplex phase transient phenomena in solids, the kinetic
pathways of chemical reactions, and the biological
functioning processes.
In order to produce such low-emittance and shortbunch electron beams, a technology of laser-driven
photocathode rf gun [3-6] has been proposed. The rf gun
generates
femtosecond
electronbunches
withfemtosecond laser pulses. The dynamics of thefemtosecond electron beam in the rf gun was studied by
the space-charge tracking code PARMELA.

FEMTOSECOND RF GUN
Figure 1 shows the low-emittance femtosecond-bunchelectron generation system. A new 1.6-cell S-band (2856MHz) rf gun, constructed under the KEK/Osaka
University collaboration, is used. At the level of <1 mmmrad, the normalized emittance can be affected by anumber of small contributions like field asymmetries or
the thermal emittance of the electrons at the cathode,rather than being dominated by a single effect. In order toreduce field asymmetries, the following improvements
were carried out: (1) the laser injection ports wereremoved; (2) a new turner system was designed and (3) a
new insertion function of the photocathode was installed
as shown in Fig. 1. A copper photocathode is used in theexperiment because of the low bunch charge; however,the photocathode is removable. A good-symmetrysolenoid magnet was mounted at the exit of the rf gun tocompensate the transverse emittance growth due to spacecharge effect.
The rf gun is driven by a femtosecond Ti:Sapphire laser.
In the laser system, the Ti:Sapphire oscillator (Tsunami,produced by Spectra-Physics Co.) is mode-locked with afrequency of 79.33 MHz, the 36th sub-harmonic of the2856 MHz accelerating rf. The outputs of the oscillator
laser pulses are captured by a Pockels cell and amplified
up to 1mJ in a regenerative amplifier. The repetition rateof the amplifier is 1 kHz. The amplified pulse isconverted to the ultraviolet (UV) light using two
nonlinear crystals with the maximum energy of 0.15 mJ.
The femtosecond UV light is injected on the cathodesurface at an incident angle of approximately 2o along thedirection of the electron beam using a prism placeddownstream of the gun. The minimum pulse width of theUV light is 170 fs for the 100-fs amplified laser pulse
input. The laser shaping of the UV light in the spatial andtemporal directions is considered to reduce the spacecharge force in the bunch.

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Figure 1: Photocathode femtosecond rf gun and electron diffraction measurement system

FEMTOSECOND BEAM DYNAMICS
Effect of Laser Injection Phase
In the rf gun, the phase compression in the longitudinal
phase space occurs at low laser injection phase (launchphase) because the electrons come out the photocathodeare non-relativistic. This process has been studied for thegeneration of a sub-picosecond electron beam [3]. Figure
2 shows the dependence of the bunch length on the laser
injection phase. The laser pulse width was 100 fs in rms,
the rms spot radius was 1 mm, the peak rf field was 100
MV/m and the bunch charge of 0.1 pC. The simulations
were ended at a 1-m distance point away from the cathodewithout any solenoid fields. The data indicate that thebunch compression due to the rf is occurred at <60o. A
50-fs electron bunch can be generated at 30o with a 100-fs
laser injection. The low energy spread also can beobtained at the low laser injection phase because of thephase compression in the longitudinal phase space. There
are no large changes on the transverse emittance withchanging the injection phase from 20o to 60o.

Figure 2: Bunch length and energy spread versus laser
injection phase. Laser pulse width Tin=100fs, laser spot
radius Rin=1 mm, bunch charge Q=0.1 pC.

However, at the low injection phase in the rf gun, theactual electric field (effective electric field) at the cathodedecreases. The longitudinal self-field of the electron
bunch (i.e. longitudinal space charge force) is dominant in
the rf gun. In order to reduce the longitudinal spacecharge effect, the rf gun should be operated with a highpower rf to increase the electric field, or should beoperated at a low bunch charge.

Effect of Laser Spot Size
Figure 3 shows the bunch length and the transverse
emittance changing with the laser beam spot size. The
space-charge effect causes the increase on the bunch
length at the small laser spot. On the other hand, the rf
effect is dominant at the large laser spot, resulting in theincreases of the bunch length and the transverse emittance.
The minimum bunch length of 50 fs is achieved at thelaser spot radius of 0.7 mm for the laser pulse width of
100 fs at the bunch charge of 0.1 pC. It is depended withthe laser pulse width and the bunch charge. In the case,the thermal emittance of the electrons at the cathode isdominant the beam transverse emittance.

Figure 3: Bunch length and normalized transverse
emittance versus laser spot radius.
In order to reduce the thermal emittance, a copper
cathode was used in the rf gun, because the work functionof the copper is near the UV light energy. The thermal-137-

Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

emittance of the order of 0.1 mm-mrad is achievable byreducing the laser beam size or the effective electric fieldat the cathode.

Effect of Laser Pulse Width
Figure 4 gives the effect of the bunch length, the energy
spread and the transverse emittance on the laser pulsewidth. The bunch length of the electrons increases
linearly with the laser pulse width at 0.1 pC. The bunch iscompressed to be two factors comparing with the input
laser pulse because of the injection of 30o used. A 100-fs
electron bunch can be generated in the rf gun with a 200fs laser pulse injection. The slight increase on thetransverse emittance with the laser pulse width is causedby the rf effect.

Dependence on Acceleration Field Gradient
In UED or UEM, a low-energy electron beam issuitable for the diffraction measurement. The beamenergy generated in the rf gun can be varied by adjusting
the acceleration field gradient. However, the large
growths of the bunch length, the emittance and the energy
spread are occurred with decreasing the field gradientbecause of the space-charge effect. To reduce the growths,
it would be selected a low bunch charge. As given in Fig.
6, a low-emittance electron beam with charge of 0.1 pC,
beam energy of 2 MeV, bunch length of 100 fs and energy
spread of 0.03% is achievable in the rf gun.

Figure 4: Bunch length and normalized transverse
emittance versus laser pulse width.

Figure 6: Normalized transverse emittance, bunch lengthand energy spread versus beam energy.

Dependence on Bunch Charge
If increase the bunch charge from 0 to 2 pC, the bunch
length is only increased from 50 to 70 fs, but thetransverse emittance and the energy spread increase
largely with the bunch charge due to the space-charge
effect, as shown in Fig. 5. In UED or UEM, the ultralowenergy spread beam is required. The bunch charge of 1 pC
is suitable for the generation of a sub-100-fs electron
beam with the relative energy spread of 0.05%, and 0.1
pC for the generation of a femtosecond beam with theenergy spread of 0.01%.

CONCLUSSION
A photocathode-based low-emittance femtosecondbunch electron source was reported. The femtosecond
beam dynamics in the rf gun was studied by particle
simulation. The data indicates that a sub-100-fs MeV
electron source with the normalized emittance of 0.1 mmmrad and the relative energy spread of 10-3~10-4 at bunchcharge of 0.1~1pC is achievable in the photocathode rf
gun driven by a femtosecond laser light.
The produced femtosecond electron beam is able to beused in the time-resolved ultrafast electron diffraction forobserving information of the most basic mechanisms on
the femtosecond time scale. It is a powerful candidate toimprove the time-resolution of UED or UEM into ~100 fs
time region.

REFERENCES

Figure 5: Normalized transverse emittance, bunch lengthand energy spread versus bunch charge.

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[3] J. Yang, et al., Jpn. J. Appl. Phys. 44 (2005) 8703.
[4] M. Babzien, et al., Phys. Rev. E, 57 (1998) 6093.
[5] J. Yang, et al., J. Appl. Phys. 92 (2002) 1608.
[6] X. J. Wang, et al., PAC’97, Canada, 1997, 2793.
[7] J. Yang, et al., Nucl. Instr. Methods A 491 (2002) 15.
[8] J. Yang, et al., Nucl. Instr. Methods A 556 (2006) 52.

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