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

Generation of Femtosecond Electron Single Pulse at the S-hand LinacKenichi KINOSHITA, Mitsuru UESAKA, Taka,hiro WATANABE, Toru UEDA, Koji YOSHII, Hideki HARANO,
Fumio SAKAI*; Hideyuki KOTAKI*, Kazuhisa NAKAJIMA**, Hiroshi NAKANISHI**, Atsushi OGATA
Nuclear Engineering Research Laboratory
2-22 Shirakata-shirane,Tokai-mura,Naka-gun,Ibaraki 319-ll,Japan
Japan Atomic Energy Research Institute
2-4 Shirakata-shirane,Tokai-mura,Ibaraki 319-ll,Japan*High Energy Accelerator Organization
1-1 Oho,Tsukuba,Ibara,lii,Japan 305
1 Introduction

At the LINAC in Nuclear Engineering ResearchLaboratory Linac(NERL LINAC), femtosecond quantum phenomena researches are promoted at present,
and the generation of the femtosecond single electronpulse is one of the inportant subjects there. Though
the 700fs(FWHM) electron single pulse using the achromatic arc-type magnetic pulse has achieved in NERL
LINAC, the shorter pulse for several applications isexpected. In NERL LINAC the laser wakefield acceleration experiment and backward Thomson scattering X-ray generation experiment have been carried out
in cooperation with High Energy Accelerator Organizaion(KEK) and Japan Atomic Energy Research Institution(JAERI). Here, the 200fs(FWHM) electron pulse
is required. Energy-phase relation of the electron beam
must be controlled precisely and the energy dispersion
on the identical phase must be decreased in order toestablish precise magnetic pulse compression. In NERL
LINAC the thermionic electron gun has been used for anelectron source. A laser photocathode RF electron gun,however, can produce smaller emittance electron beam,
and i~ is more advantageous for the magnetic pulse compressiOn.
2 Modified 18L Linac system

Femtosecond electron pulse generator using thechicane-type magnetic pulse compressor is composed of the laser photocathode RF electron gun,the accelerating tube and the chicane type magThe configuranetic pulse compressor( Chicane).
tion of the 18L Linac is shown in fig.
1.
Laser

2.2 Constant gradient travelling-wave-type accelerating
tube
constant gradient travelling-wave-type S-hand accelerating tube whose frequency is 2856MHz is used forthe accelerating tube. The electromagnetic-field distribution in the accelerating tube was calculated withthe electromagnetic field analysis code SUPERFISH,
and it was used for the electron orbit calculation by
PARMELA. Though two accelerating tubes for the acceleration and for energy modulation are basically necessary for the magnetic pulse compression, there is orily
one accelerator. However, the method that the accelerating tube is used for acceleration and energy modulation simultaneously is examined considering the fact thelaser photo cathod RF electron gun can produce electrons that have almost relativistic energy.

2.3 Chicane-type magnetic pulse compressor
The chicane consists of four rectangular bendingmagnets. In the chicane, a larger energy electron takes
a shorter path. Therefore, the pulse compression is possible by giving the energy distribution in which· a lower
energy electron is put in the front part of a bunch. Whenthe z -axis is taken in the propagating direction of thebeam and the energy distribution of electrons in a bunchis written using coordinate ( = 2{ ( z0 - z) (A is the RF
wavelength) as E(() = C0 + C 1 ( + C 2( 2 , the position ofan electron after the chicane, written by (', becomes,c2c;)c1 ).,,., = ( 1 + b1 Co( + ( b1 Co + b2 C6 (2 .

(1)where b1 and b2 is the coefficients that are determinedby shape and configuration of the chicane like,b1 = 4Ro arcsin

S-band Accelerating Tube(!!.._)- y'~4RoD
Ro-D2
2RoD 2 L

Photo Cathod RF
Electroo ~(~ _Fig. 1 : Schematic configuration of the modified 18L
linac updraft sectionD2)3/2'b _4RoD 2
3R0 D 4 L
2- (~ _ D2)3/2 + (R~ _ D2)5/2

3RoD2 L

2.1 Laser photocathode RF electron gun(2)(3)
In the laser ,photocathode RF electron gun, photoelectrons that rise from the cathode surface by injection Ro, D,and L are an the radius of curvature of the elecof the YLF laser are accelerated by the RF of the reso- · tron of nominal energy, the width of the magnet, andnance cavity. The frequency of the RF is 2856MHz. The the gap between the magnets (shown in Fig. 2). The
pulse length of the electron pulse which rises in this elec- magnetic field is given so that the first order coefficienttron gun is several picoseconds, and the energy is about of (in eq. (1) becomes zero in the magnetic pulse compression. Thus, the pulse length after the compression
4.6MeV.

+ (~ _ D2)3/2 ·-479-

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

depends on the secondary term, when the first order coefficient of (is made to be zero in equation (??zeta-p)).
The value of the secondary term varies when the gap
L of the magnets in the chicane changes. The relationship between L and the value of the secondary term forthe electron pulse accelerated by 181 linac is shown inFig. 3. It can be seen that the pulse can be shortenedwhen L is extended. In the design L was determinedso as to obtain the shortest pulse by the calculation ofPARMELA.

Dipolesr+---,~/~~,...,--.....,

Vacuum chamber

LD

3 P ARMEL A simulation
The electron orbit calculation code PARMELA wasused for the calculation. The pulse length of the laser
in the electron gun was assumed to be 5ps(FWHM), theacceleration electric field amplitude of the electron gun
be lOOMV /m, and the electric field amplitude of the accelerating tube be about 8.5MV /m. Figure 4 is the longitudinal phase-space electron distribution at the exit of
the photocathode RF electron gun. Here, the electriccharge of the electron pulse is assumed to be lnC. The
pulse length is about lOps and energy is about 5.2MeV
in Fig. 4, and this pulse is sufficiently short and havesufficiently high energy for the acceleration in the accelerating tube. The normalized rms tranverse emittanceof the electron pulse is about 271' mm·mrad, and it is far
lower than that of our thermionic electron gun, which
is about 1007rmm·mrad. Figure 5 is the electron distribution and pulse shape at the exit of the .accelerating
tube. Electrons of lower and higher energy are put inthe front and rear parts, respectively so that the pulse
is- compressible by the chicane. Figure 6 is the results at
the exit of the chicane. The pulse length is compressed
to about 200fs(FWHM).

Fig. 2 : Chicane

4 Conclusion
The numerical analysis for the generation of the femtosecond electron pulse in the system that consists of
the laser photocathode RF electron gun and the S-handlinac was carried out using the electron orbit calculation
code PARMELA, and the chicane type magnetic pulse
compressor was designed. By the simulation it was confirmed that the electron pulse with the pulse length of200fs(FWHM) can be generated in that system.

2.00

1.90
~"'-3;

1.80

<'I

J-.1'

1.70

1.60

References

1. 50 L..LLL-I....L..L-L-'--1-L..L......__,L..L.JL...L...JL...J....JLJ....I-L..J-'--l-L..J....L..i--'-'...L..J....I
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35

L (m)

Fig. 3 : Magnet interval and pulse lengthi

2828

108212181i

147

q_8

85.28

5.23. •· . A·~; b..;:._''C!>~~~.

~

l·'~'l•"<i:fP;\·:,·

2
5.18

5.13

40204820.3

20.3

18.8

18.8

i
~

17.3

Fig. 4 : The electron distribution in
the logitudinal phase-space at theexit of the electron gun

14. ~88Fig. 5 : The electron distribution in
the logitudinal phase-space at theexit of the accelerating tube

-480-~~
012..-··--·1--- .

.

17.3

15.8-404
relative time (ps)1·~ <>

relative time (ps)15.8

relative time (ps)==-j------3-272"'·854

relative time

5.33~

~

1470
relative time~[1] M.Uesaka et al, Phys. Rev. E, Vol.50 (1994),
pp.3068-3076[2] K.N akajima et al., Proc. of the 17th Particle Accelerator Conf., Vancouver, 1997 (in press).

14. '! 2•r···.
02

relative time (ps)Fig. 6 : The electron distribution inthe logitudinal phase-space at theexit of the chicane