The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Numerical Investigation of the X-band Femtosecond Linac
Using a Laser Photocathode RF Electron Gun
Hideki HARANO, Kenichi KINOSHITA, Toru VEDA, Fumio SAKAI*, Takahiro WATANABE,
Koji YOSHII and Mitsuru UESAKA
Research Laboratory, University of Tokyo,
Engineering
Nuclear
Tokai, Ibaraki, 319-11 JAPAN* Laser Acceleration Research Group, Japan Atomic Energy Research Institute,
Tokai, Ibaraki, 319-11 JAPANAbstract

As the next stage of the existing subpicosecond
electron linac, the femtosecond ultrafast quantum
phenomena research project is proposed at Nuclear
Engineering Research Laboratory (NERL) of Univ.
of Tokyo. In this study, the feasibility of an X-band
linac with a laser photocathode RF gun is numeri'
cally investigated for the femtosecond linac which isplanned to be installed in the project.
1 Introduction

produce the electron single pulse of 200 fs(FWHM)
and 1 nC[2]. Since the RF gun is triggered by thelaser pulse, it has the intrinsic advantage to reduce
the time jitter between an electron pumping pulse
and a laser probing pulse and to establish precisesynchronization between a linac and a laser in radiation physics and chemistry experiments. In addition, the RF gun has unique features so that it
produces high quality electron beam with low emittance.So the feasibility of the X-band linac systemwith the laser photocathode RF gun is numericallyinvestigated for the femtosecond linac in this study.

The construction plan of" Femtosecond Ultrafast
Quantum Phenomena Research Facility" is progressing at NERL of Univ. of Tokyo as the next planof the existing subpicosecond S-hand (2.856 GHz)
electron linac , which aims at elucidation and application of the beam-material interaction in the femtosecond time domain. The X-band (11.424 GHz)
linac has been scheduled to be introduced in theproject. The X-band RF wavelength is 1/4 shorter
than the S-hand, which can realize the compactification of the system and the excellent bunchingeffect can be expected due to the large gradient of
the acceleration electric field. The optimum designon the system with a 150 kV thermionic gun and

2 Basic System Configuration

Figure.1 shows the basic system configuration ofthe X-band linac proposed in this study. It consistsof the laser photocathode S-hand RF gun, the two
X-band accelerating tubes and a magnetic electron
bunch compressor. The solenoid magnetic field isapplied properly for the transverse beam size suppressiOn.

double subharmonic bunchers (SHB) as an injector
has been carried out using electron orbit calculation
code PARMELA, and the generation of the electron
single pulse of 100 fs(FWHM) and 10 kA has been

Laser

Electron Gun

confirmed[1].
In the laser wake field acceleration experiment
which is carried out as the cooperative research withKEK and JAERI, a laser photocathode S-hand RF
gun has been installed as an injector of the S-hand
18L linac. It has been numerically evaluated to-493-Fig.1

Schematic drawing of the X-band linac
system.

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

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Relative time (ps)Relative time (ps)(a) At the exit of the injector
section.

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29. 5 r-----t------t-----t--- -----1
2 1. o_s.-2----_+1-----10~--~1!------!2
Relative time (ps)(b) At the exit of !he second accelerating tube.

(c) At the exit of the chicane.

Fig.2 Electron bunch structures in the longitudinal phase space.
The RF gun has a solenoid magnetic field forwhich is far shorter than the X-band RF's one pe-leading to the parallel beam while suppressing theemittance growth.

riod of 87.5 ps. Therefore, the single bunch pulse
generation is possible without SHB, which makes

tion. The same calculation parameters as ref.[2] arethe system simple as shown in Fig.l. This electron
bunch has an extremely small transverse emittanceHere, the range(z :S 60cm) inwhich the field exists is defined as an injector sec-used for this section. The periodic inner structureof the X-band accelerating tube is the same as theof about 1.9 1r mm · mrad in normalized rms which

second tube in ref.[1], which is designed to operate

The beam diameter just after the injector section

the 11.424 GHz constant impedance, travelling wave

is 13.4 mm(tail-to-tail) and larger than the ¢8.4 mm
aperture of the X-band tube. Therefore, the transverse beam size was suppressed by the solenoid mag-and 2/3 1r mode. Each tube is composed of 78 cells.
The second tube is for the energy modulation of theelectron bunch for the magnetic pulse compression
and its phase is adjusted by the mechanical phaseshifter. The RF power is supplied by the X-band
klystron. 30 MW is supplied to the first tube and 10
MW to the second. The chicane type is used for themagnetic electron bunch compressor. The dimension is fixed equal to ref.[2], and only the magnetic
field strength is adjusted to the energy distributionof the electron bunch.

is advantageous for the magnetic pulse compression.

netic field as shown in Fig.3. The rms normalized
transverse emittance C:x,rms and the beam size Xmax
defined as f3c:x,rms are also plotted in Fig.3 where(3 is the beta fuction. The electron beam becomesparallel with the smaller diameter after overfocused,which indicates that this field has the similar effectto the combination of two magnetic lenses. Figure.3

J

also indicates that the emittance increase near thefocus due to the space charge is less than 1.5 times.
The emittance grows inside the accelerating tube

3 Results

due to the RF -produced lateral force especially for

Figure.2(a) shows the the electron pulse right aJ-

the low energy electron far from the axis. Therefore

ter the injector section, calculated by PARMELA.

the first tube is placed so that its entrance is near

The electric charge of the electron pulse is assumed

the focus.

to be 1 nC. The pulse width is 14 ps(tail-to-tail)-494-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Z=60

injector sectiol!lZ=140

drift space

XACC1z=208.2 z=255.2'drift space' XACC24 Conclusion

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i:

i

:

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Fig.3

Distribution of the Solenoid magnetic
field Bz and evolutions of the rms normalized transverse emittance cx,rms and beani.
size Xmax·

For carrying out the pulse compression by thechicane, the energy modulation is necessary so thata rear electron in the bunch has a higher energy,and it is desirable that the energy distribution isrectilinear[2]. The electron energy must be around
30 MeV from the viewpoint of electron beam applications. The phase of the accelerating tube wasadjusted to satisfy these requirements as well as possible. The electron pulse at the exit of the second
tube is shown in Fig.2(b). The electrons are accelerated to about 37.5 MeV. The pulse width is slightly
compressed since the electrons are accelerated on thestable phase in the first tube.
The electron pulse at the chicane exit is shownin Fig.2( c). The pulse width is compressed to about
200 fs(FWHM). The initial 1 nC electrons all ptmetrate. The emittance is about 3.5 1r mm · mrad, lessthan twice of the initial value. The maximum magnetic field strength of the chicane is 1.24kG.

By the combination of the laser photocathode Shand RF gun, the X-band accelerating tubes andthe chicane type pulse compressor' it was confirmed
numerically that the electron single bunch pulse canbe generated with the pulse width of 200 fs, the peakcurrent of 2 kA and the ultra 'low emittance. The
condition of the injector ·section was fixed in thisstudy, however, the system is expected to be more
optimized by adjusting this section to match to theX-band accelerating tube. For example, the system
will be compacted if the solenoid magnets attachedwith the RF gun !s p1odified in order to generate
the parallel beam with a proper size to the tube
aperture. If the pulse width at th~ injector exit. is..
shotened by some contrivance such as shortening thelaser pulse width, the linearity of the electron energy
distribution is improved at the magnetic compressor's entrance, which will cause the pulse compression efficiency higher.

.

References[1] A.Takeshita et al., "Optimization of design of the100 fs 10 kA X-band linac", Proc. of the 21th LinearAccelerator Meeting (1996) pp.299-301.
[2] K.Kinoshita et al., "Improvement of NERL Shand linac aiming at femtosecond! electron pulse",
Proc. of the 22th, Linear Accelerator Meeting (1997).

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