. JAERI-Conf 95-021

3a1-1

Calculation of Electron Orbital for the Design ofthe X-band LINAC
Akira TAKESIDTA, Takahiro KOZAWA, Mitsuru UESAKA, Toshiaki KOBAYASHI,Toru UEDA and Kenzo MIYA

Nuclear Engineering Research Laboratory, University of Tokyo, 2-22 Shirakara-Shirane Tokaimura Naka-gun lbaraki, 319-11, JapanIn Older to generate a 100fs single pulse, an electron

Abstract

beam from the electron gun must be compressed till apulse length is shorter than one period of the X-band

The X-baod linear accelerator, which can generate sucha short pulse as lOOfs, is under design at the Nuclear
Engineering Research Laboratory, the University ofTokyo. The design study of the X-band linac is carriedout by using PARMELA and SUPERFISH. Then it isfound that 600 ps (tail-to-tail) emission from athermionic gun can be bunched to about lps (FWHM)
at the end of the :?nd accelerating tube and 80 % of theinitial charge is transmitted.

RF (87.5 ps).
Taking that point into account, transport simulationof an electron beam has been carried out by using
PARMELA and the feasibility of a generation of afemtosecond single pulse has been confinned.

2. Design
The layout of the X-baod Iinac designed here is shownin Fig. 1 It consists of a thermionic gun, asubharmonie buncher (SHB), two accelerating tubes
and achromatic magnetic pulse compression system.
And there are solenoid coils along the linac to generate
axial magnetic field for controlling the transverse beamsize. The final energy of the beam is 35MeV.
Parameters of the components and calculated results aredescribed in the following.
Thermionic Gun The present thermionic gun
Y796 has achieved 500 ps (tail-to-tail) at the voltage of90 kV so far[4]. In this design, Gun voltage isincreased up to 290 kV. This is because both anincreaSe of emission current and a decrease of the spacecharge effect are expected. Other parameters are
determined as follows. Rms normalized emittance is 50:n;mm mrad, the pulse length (tail-to-tail) is 600 ps,
peak current i~ 10 A. and the space distribution of anelectron beam is assumed to be K-V distribution.

1. Introduction
Femtosecond technology is going to become a key
technology in radiation physics, chemistry and materialscience to investigate ultrafast and fundamental
quantum phenomena. Up to now, 700 ps electron
single bunch has been successfully generated at the Sbaod linear accelerator of the University of Tokyo bymagnetic pulse compression[!]. However the X-band
Jinac whose main RF frequency is 11.424 GHz isthought to be more effective for generation of shorter
electron single pulse than the S-band linac. The X-band
linac for a linear collider is already under design at
Stanford[2] and KEK[3]. The X-band linac is discussed
as the femtosecond accelerator in this paper. As to thedesign of the X-band Iinac, the most difficult problem
is an increase of the space charge effect, which isaccompanied with shortening of an electron bunch.

140mm

r-960mm';~

100mm~650mmRegular!
::...

600mm

Regular

200kV
Thermionic Gun
476MHzAmp.

2856MHz KlystronFig. 1 Layout of the X-bandlinac-404-

11.424GHz
KlystronJAERI-Conf 95-021

current There are 52 coils. Radius of a coil changes
from 50 mm to 150 mm and supplied CUirent to thecoil changes from 200 to 6000 A tum. The
longitudinal magnetic field distn"bution by these coils
is shown Fig. 2. The magnetic field at the entmoce ofthe fonner accelerating tube is about 1000 Gauss.
Variation of both pulse length and beam size as afunction of the distance from the anode is shown in
Fig. 3. It shows that the required conditions aresatisfied at the entmoce of accelerator.
Accelerating tube There are two acceleratingtubes in this lioac. The fonner consists of a buocher
section and a regular section and the latter consists of
only regular section. This is because magnetic pulse
compression is scheduled and the 2nd accelerating tube
is used for energy modulation. It is designed to carrythe 11.424GHz constant-impedance travelling wave
with the 2/3'1t mode. Input power for each acceleratingtube from the X-band Klystron is about 15MW and thefield gradient at the entmoce of the accelerating tube isjust ·below 40MV/m. In the :tiiSt accelerating tube, the:tiiSt 6 cavities form the buncher section and the rest 69
cells form a regular section. The second acceleratingtube consists of 69 regular cells. Travelling wave
inside the accelerating tubes is numerically evaluatedby SUPERFISH as an input to PARMELA. Theresults are explained as follows .. Table 1 shows the5 H8 Even though the pulse from the gun can bebunched to 87.5 ps by only one.476 MHz SHB, maxenergy gain must be more than 0.09 MV, which
reduces engineering design mergin. Therefore double
SHB system is adopted ·here. In this system, thevoltage required for each SHB is reduced down to0.05MV. Frequencies of SHB are determined 476 MHz
and 2856 MHz, respectively, as the subharmonic
numbeJ:s of 11.424 GHz.
Solenoid Coil In the non-relativistic region
along the· low energy part, the space charge effect has
an large influence on a growth of emittance in thetransverse direction. As the aperture of the acceleratingtube is 8.4mm in diameter, the beam must becontrolled in the transveiSe direction by an externallongitudinal magnetic field. In the calculation withPARMELA, the external magnetic field is supplied byvarious kinds of circular coils with different radius and1600
1400
1200
'Ui'!as 1000

· Table 1 Parameters in the buncher section.s 800NIllcell No. Length of a cell600

Energy of the beam Velocity

L {em){MeV)

40010.64

0.304

0.78c

200

20.71

0.505

0.86c

30.75

0.728

0.91c

40.78

0.832

0.92c

50.81

1.085

0.9Sc

60.83

1.343

0.96coo

100

50150

250

200

Accelerating TubeInjection
Z(cm)Fig.2 Manetic field distribution along the accelerator

x,y ( c m) [FWHM]
300 ~--~---r----r---,..----.------, 0.5
Beam size
250 ~0.4 ·+ ••y2000.3
150
0.2
100
0.1
50

Time (ps) [FWHM]

QSHB

204060Z (em)

80

100SHB

Fig.3 Beam size and pulse length along the injector-405-0120

JAERI-Conf 95-021

variation of the length of each cell, the energy andvelocity of an electron beam through the buncher
section. Parameters of the regular section determined
by SUPERFISH is shown in Table 2 Table 3 shows
the final beam parameters calculated by PARMELA. In
this calculation, 80 % of the initial electric charge istransmitted and the resultant pulse length is 1.04 ps(FWHM). At the present S-band linac, the pulse length
at the end of the 2nd accelerating tube is about lOps
and it is compressed 1/10 by magnetic pulse
compression. If we can achieve the same compression
ratio, a 100 ps single pulse can be expected by this Xband linac.

Table 2 Parameters in the regular section

Q value6663

Shunt Impedance(ro)78.034

Attenuation
Constant (a)0.473N/m

Group

Velocity[1] M. Uesaka et. al., Physical Review E Vol. 4(1994) pp. 3068-3076[2] A D. Yaremian et. al., SLAC-PUB-6610 August(1994)[3] H. Sakai et. al., Proceedings of the 18th Linear
Accelerator Meeting July (1993)[4] T. Ueda et. al., Proceedings of The 20th Linear
Accelerator Meeting September (1995)MQ/m

3.79%

Table 3 Final beam profile[FWHM]

1.04(mm)

X= 0.55
y =0.58

Momentum(mrad)

Px =7.8
Py =7.5

Beam size

Transport simulation of an electron beam is carriedout by using PARMELA and SUPERFISH associatedwith the design of the X-band linac for generating afemtosecond single pulse. The layout of the accelerator
is arranged as follows. After the thermionic gun, there ·are the double SHB, two accelerating tubes andachromatic magnetic pulse compression system.
According to the results by PARMELA, 80% of theinitial electric charge is transmitted and pulse length at
the end of the 2nd accelerating tube is 1.04 ps(FWHM). Therefore, the feasibility of generating the100 femtosecond single pulse is confirmed.

4. Reference(Vg/c)

Pulse length(ps)3. Conclusion-406-