The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Design of CW Microtrons using a 500 MHz RF Cavity for Industrial Application
Hirofumi TANAKA
Mitsubishi Electric Corporation, Electromechanical Systems Dept., Advanced Technology R & D Center,
8-1-1, Tsukaguchi-honmachi,Amagasaki City, HYOGO 661-8661, JapanAbstract
The CW microtrons with the 500 MHz RF cavity areproposed for industrial application: The acceleration
beam energy and power are from 5 MeV to 16 MeV, andseveral tens of kW, respectively. A beam tracking study
shows that the CW microtrons are practicable. Slowpositrons of about 109 slow4/sec can be produced using
the 16 MeV CW microtron, according to an estimation
based on previous measurement values.

The electron beam at the first turn cannot pass byoutside the RF cavity after the first bending magnet.
Therefore, an inverse-bending magnet (BM-) is situatednear the bending magnet. The electron beam crosses theRF cavity for the second time in the inverse direction of
the first crossing. A transverse beam focussing isobtained with only edge effects of the bending magnets
and a QM near the RF cavity. Basic parameters of theCW microtrons are shown in Tab. 1.

1 Introduction
High power electron beams are necessary for industrial
application: electron beam and X-rays irradiation.
Maximum beam energy for the application is around 10
MeV Application to slow positron production is also
necessary for high power electron beams. The beamenergy is from 15 MeV to several hundreds of MeV.
Electrostatic accelerators and linear accelerators havebeen used for the application. However, development ofaccelerators operated in a continuous wave (CW) mode isof considerable practical interest for the industrialapplication because the accelerators require compactness.
A CW accelerator RHODOTRON [1] has been developedby IBA, and the electron energy and the beam power arelOMeV and 200 kW, respectively. However, the energy
would be too low for slow positron production.
A CW microtron with a 500 MHz normalconducting RF
cavity is proposed to apply for these fields and afeasibility study has been made. The essential interests
are compactness and low electrical efficiency. This paper
describes results of the beam dynamics design. Attention
will be paid mainly to effects of RF fields and spacecharge. An estimation result of slow positron productionis also described.

1.5

0.5

0.0

-0.5

-I. 0.__._....._...l.J._....._...J....o......................._........._,_...._,_....._.....__._.......J...................J....o_.........._,

-2.5

-2.0

-1.5

-1.0

Figure 1 shows a schematic drawing of a 5 MeV CW
microtron. An electron beam is injected with a chicane
magnet from an injection line. In order to match anaccelerating RF frequency, the electron gun is pulsed at
the RF frequency. There are two solenoid magnets at theinjection line for a transverse beam focussing. The RFcavity is a conventional multi cell type, which isfrequently used for an electron storage ring. The
frequency is selected around 500 MHz that is determinedby a capable input power of the RF cavity and the total
size of the microtron.

0.0

0.5

1.0

1.5

Fig. 1 A schematic drawing of a 5 MeV CW microtron.

Tab. 1 Basic parameters of CW microtrons.
Energy (MeV)
510
Application field
X-raysElectron

16Slow
positron6259455.6
48

Wall loss of an RF cavity (k.W)

56~

no~

no~

Footprint (mx m)

67
2.9 X
1.6133
3.6 X
1.6133
4.1X
2.1

Average Beam Current(mA)

The number ofRF cells
The number of turns

2 CW microtrons-0.5

3. Beam simulations of the CW microtron
Beam simulations were mainly doneparameters of the 5 MeV CW microtron.

-107-with theThe 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

negligible when an average beam current is aroundlOrnA, and electron beam energy is 70 keV.

3.1 Injection energy

Injection beam energy is from 70 keV to 90 keV. A low
RF field strength makes a low energy gain and large
energy dispersion of an electron beam, when the beamcrosses an RF cavity. The reason is that velocity of thelow energy beam is much slower than velocity of light,
and the phase slip is large. A beam tracking with 3
dimensional magnetic and electric fields was made.
Figure 2(a) shows electron beam energy along a reference
orbit, when the beam crosses a 2-cells RF cavity for the
first time. 'Thn electrons were simulated whose initialacceleration phases are different every 5 degrees. Theinjection energy is 70 keV. Figure 2(b) shows horizontal
beam orbits when the electrons are injected the RF cavity
with r = 0.01 m and r' = 0 radian. The initial phaseconditions are the same as the Fig. 2(a). The initial phasedifference produces a difference of an energy gain andtransverse beam focussing. Figure 3 shows energy
dispersion as a function of the injection energy. A phasewidth of the beam is 30 degrees. Injection energy of a 2cells type and a 4-cells type is needed to be more than 70
keV and 90 keV, respectively, in order to suppress theenergy dispersion within 2 %.

c:0'§
~

<Jl

3.

'0,.,

~
Ql
c:

u.J

Injection Energy [keV]

Fig. 3 Energy dispersion as a function of injectionenergy. An initial accelerating phase width is30degrees.
0. 03
0.02 .
0.01

:0
~

~
a;

a;
E

E

(a)

1.or--~~~~~..,( b )-~(b) 0.020 r--..,...........,..,~~"T"""'.....,

L

-0.01

c.

-0.01

XX

-0.02

0.8
~

~

0.6x(m)x(m)::'<w 0.4

0.0 .__.......___...._......._._..._--.J~00.2

0.4

0.6

~8s(m)

1~0.000 ...._...._......,__..._......._--...J
0.0

0.2

0.4

0.6

0.8

1.0s(m)Fig.4 Horizontal phase spaces at the entrance of theRF cavity: Beam energy is 70 keV and average
beam currents are 0 mA with (a) and 200 mA
with (b).

Fig.2 Energy and orbits in the RF cavity: Injection
beam energy is 70 keV and initial acceleration
phases are from -180 degrees to -135 degrees.
(a) Electron beam energy along a reference
orbit; (b) horizontal beam orbits.

~150E!
i'l£100-~3.2 Space charge effect50

Injection energy is several tens of keV, and space charge
effect is thought to be large when a high intensity beamis accelerated. The effect was simulated with a beam
tracking. The tracking was done from an exit of theelectron gun to an entrance of the RF cavity. Figure 4
shows horizontal phase spaces without and with spacecharge effect at the entrance of the RF cavity. Initial
beam conditions are sx = f y = 37mm.mrad ,fJx = p y = 0.023 , and ax= ay = 0 . The emittances areunnormalized sigma values.
Figure 5 shows horizontal beam emittances at the RFcavity as a function of an average beam current. Theemittances were calculated from phase space
distributions of 2000 particles. The space charge effect isAverage beam current [rnA]

Fig. 5 Horizontal beam emittances at the RF cavity as afunction of an average beam current.
3.3 Beam properties

Figure 6 shows roughly approximated beamenvelopes of the CW microtron. Effects of RF electricand magnetic fields were not considered. Beamemittances of the horizontal and vertical coordinates
are considered to ones after crossing the RF cavity forthe first time. Energy dispersion is 2 %.

-108-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

A beam tracking from the exit of the electron gun tothe exit of the CW microtron was done. The following
effects were approximately taken into account: (1) RFelectric and magnetic :fields, (2) space charge, (3) and,static magnetic :fields. Figures 7(a)(b) show phasespaces at the exit of the CW microtron. Initialacceleration phase width is 30 degrees. Initialacceleration phases of the Figs. 7(a) and (b) are -160
degrees and -175 degrees, respectively. Themaximum and minimum acceleration energy can beobtained using the paramete rs at the exit of the CW
microtron. The electron beam can be accelerated withpracticable beam sizes.
Horizontal beam envelope [m] versus distance [m]accelerator with a different acceleration method can beproposed at the energy of around 5 MeV, which is morecompact than the CW microtron. The feasibility study ismprogress.
Acknowledgement
The author would like to thank Dr. T. Nakanishi, Dr. C.
Tsukishima, Dr. Y. H. Pu, Mr. M. Kimura, and, Mr. T.
Hirano of our company for helpful discussions.
References[1] Y. Jongen, et al., ''RHODOTRON Accelerators for
Industrial Electron-Beam Processing: a Progress Report",
in a catalogue ofiBA, (1995).
[2] S. Okada, " Positron Laboratories in the World",
RADIOISOTOPES, 42, (1993), 423. (In Japanese.)[3] H. Tanaka, T. Nakanishi ," Slow Positron Production
using an 18 MeV Electron Linac'', NIM B 62, (1991), 259.
(a)~___,--c~~~~--""j' - " '"fr; fl•-;E_Ifl~~-0.020000

jr_~_. _ _,_.__ l_.,_ _. . . . . l.l. . . . . . , . . . . ~..J.---~i-0. 01-D. 01

Q,qgQg_QQ_ ___ --~----~-~-----'------ ~-------~'-Betatron envelope__-0.005

00. 005

x (m)

(b)

Momentum envelope

0. 01

..

-0.005

J. .. .I. ••• J. ...

0L.......,,,,__;,_J._j

0. 005

0. 01

Y (m)

o.o1 ,... .,. . ....,......... r ..,...,....,...,...T..,....,....,...,....,....,...,....,.........

Note: Linear optics

iEJJ "~ ~~~~--1Fig. 6 Roughly approximated beam envelopes of the CWmicrotron. Energy dispersion is 2 %.
4. Production of slow positrons
Slow positron production using the CW microtron
produces the following advantages: a linear storage
section is not required, and, the amount of shielding issignificantly reduced. It is difficult to estimate yields ofpositrons, that depend on shapes of targets, moderators,
and transfer lines. Measurement values [2][3] which
various laboratories have been done show that theproduction efficiency is roughly described the following
equation between 15 MeV and 20 MeV:
N slow- positron IN electron

L . . _. . . ~:~·..1.---~ .l.. . . .

-0. OJ-0.01-0.01['___,-0.065-0.06, __ J .. _, __ ,,_,., ,___ ;-0.055-0. 05>-D-Oll---'---•---' _,___!_ ---•---•----•----' ,__ ,___ , ___ ,___ ;__., ___ ,___ c._-0.045-0.01-0.00500.005

Fig. 7 Phase spaces at the exit of the CW microtron:
the initial RF acceleration phases are -160
degrees with (a) and -175 degrees with (b).
~ 10 1012u"'= 2-+ 20 x 10-12 E3(MeV) .

The coefficients (2 and 20) are minimum and maximum
values, respectively. The number of slow positrons andfootprint areas of the CW microtron are estimated andshown in Fig. 8. The electron beam power is 90 kW. Slow
positrons of 1o'l slow-e+/s can be obtained with thecompact accelerator. Heat removal of the targets is themost difficult problem and rotate targets are required.
5. Summaey and perspective
The CW microtron with the 500 MHz RF cavity is
proposed and is found practicable. A recirculating-109-l0.01

Y (m)

x (m)

...

>

CD

10 "'"'0c..

V)

Areal~'!

'iArea10 8!
'1~5~~16~~1~7~~1~8~~19~~2~0~~21 8Beam energy (MeV)

Fig. 8 Positron production amount (maximum andminimum estimation values) and footprint areasof the accelerators as a function of electron beamenergy The electron beam power is 90 kW