The 13th Symposium on Accelerator Science ·and Technology, Suita, Osaka, Japan, October 2001

BEAM DYNAMICS IN A CW MICROTRON WITH A 500 MHz RF CAVITY
FOR INDUSTRIAL APPLICATIONS
H. Tanaka, Y. Pu, T. H. Kim, Y. Makita, C. Tsukishima, T. Nakanishi, and S. Nakamura
Mitsubishi Electric Corporation, Electromechanical Systems Dept., Advanced Technology R & D Center, 8-1-1
Tsukaguchi-Honmachi, Amagasaki-City, 661-8661 Hyogo, JapanAbstract
A CW microtron with a 500 MHz rf cavity is
proposed for industrial applications. The machine is ofa racetrack type. The acceleration energy and the beam
power of a prototype machine are 5 MeV and 30 kW,respectively. The rf cavity is a conventional normalconducting 2-cell cavity with coupling slots: The
injection energy is 80 keV, and a 500 MHz CW gun isused. The bending magnets are divided into two
subsections to adjust the acceleration phases of every
tum. Beam tracking studies show that the transverse
beam stability can be obtained with fringe fields of thebending magnets and two QMs near the rf cavity. Thestudies also show that the CW electron gun is practicable,
and an 80 ke V - 30 rr mm-mrad electron beam can beobtained.

magnets and a QM magnet at the injection line for atransverse beam focusing. The chicane magnet iscomposed of three bending magnets. The rf cavity is aconventional 2-cell cavity with nose cones andinductive coupling slots, which is frequently used for
an electron storage ring. The frequency is selectedaround 500 MHz that is determined by an input powerof the rf cavity and the total size of the microtron.
The electron beam after passing the rf cavity for thefirst time cannot pass by outside the rf cavity .after the
first bending magnet. Therefore, an inverse-bending
magnet (BM3) is situated near the bending magnet.
The electron beam crosses the rf cavity for the second
time in the inverse direction of the first crossing. A
transverse beam focusing is obtained with only edge
effects of the bending magnets and two OMs near therf cavity.

1 INTRODUCTION
High power electron beams are necessary forindustrial applications: X-rays irradiation and electronirradiation. Development of accelerators operated in acontinuous wave (CW) mode is of considerablepractical interest for the industrial applications [1] [2].
The CW microtron with a 500 MHz normalconducting rf cavity is proposed to apply for thesefields [3]. The essential interests are compactness andhigh electrical efficiency. A key issue of designing themicrotron is that the injection energy is 80 keY( f3 = 0.5 ), and the velocity of an electron beamchanges with every tum. Hence the energy gain fromthe rf cavity in each passage changes. A bending
magnet with a new structure is proposed to adjust
beam orbit length of each turn, and acceleration phases
can be adjusted appropriately. This paper describes
results of the beam dynamics design of the CW
microtron. The main subjects are a design of a CW
electron gun for the CW microtron, and transv~rse beamstability of the CW microtron.

Figure 1: Schematic drawing of the CW microtron.

Table 1: Basic parameters of the CW microtron.
EnergyNumber of turns
Average beam current
Injection energy
rf frequency
Number of rf cells
Wall loss of the rf cavity

2 CW MICROTRON
Figure 1 shows a schematic drawing of the 5 MeV
CW microtron. An 80 keY electron beam is injectedwith a chicane magnet from an injection line. In orderto match an accelerating rf frequency, the electron gun
is pulsed at the rf frequency. There are two solenoid

5MeV
66mA
80keV
500MHz
240kW

A fixed relation among magnetic fields of bending
magnets, an rf frequency, and energy gain at an rf
cavity is needed for synchronous acceleration in aconventional microtron [4]. As for the proposed- 86-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

microtron, the electron velocity, however, changes
with every tum, and the energy gain from the rf cavity
in each passage changes. Therefore, the rf acceleration
phase slip becomes large, when a conventional
bending magnet is used. We propose a bending magnet
with a new structure to adjust beam orbit length ofeach tum. The bending magnet is divided into two
subsections described as BM1 and BM2 in Fig. 1. The
two subsections have different magnetic fields, andbending angles of the two subsections is adjusted sothat beam orbit length of each tum is appropriate forthe acceleration. The parameters are optimized with acomputer optimization program so that an electron
beam with the widest acceleration phase can beaccelerated. Basic parameters of the CW microtron are
shown in Tab. 1.

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Figure 2: Beam current of the pulse electron gun asfunctions of grid voltage and acceleration voltage.

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3 BEAM SIMULATION STUDIES

vvv""""""

3.1 CW electron gun/'~"The 80 keV electron gun is a conventional triode
type with a 0.5 cm2 dispenser cathode. A 500 MHz
CW emission is made by varying a grid voltage in .thetriode. A signal source of the grid voltage is a pick-up
signal of the rf cavity, and the grid voltage is biased toa DC level. Two sections were made to optimize ashape of the triode: 1) a section from a cathode to thegrid, 2) a section from the grid to an exit of theelectron gun, because the former section is very finestructure as compared with the latter section, The
optimization of the triode was done with a beam
tracking in combination with the two sections in
consideration of space charge effect.
The simulation method was confirmed by comparing
calculation results with measured results of an electrongun. The gun is a conventional pulse electron gun of atriode type, which is used for a linear accelerator.
Figure 2 shows a beam current as functions of gridvoltage (Vg) and acceleration voltage. The figure
shows that the calculation results agree with themeasured results.
Figures 3 and 4 show a beam current and anormalized · beam emittance · of the CW gun as afunction of grid voltage (Vg), respectively. Amaximum beam current needed is estimated to be 100
rnA on account of attaining an average beam current of
6 rnA. Figure 3 shows that the grid voltage needed is
below +2 V. Figure 4 shows that a normalized beamemittance is calculated to be 30 7C mm-mrad. A phasewidth of the CW emission is estimated to be about 70
degrees, when an rf power of 60 W is supplied.

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I':-55Vpj.V)

0101520Figure 3: Beam current of the CW electron gun as afunction of grid voltage.

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51015Vg(VjFigure 4: Normalized beam emittances (100 %) of theCW electron gun as a function of grid voltage.
3.2 Transverse beam stability

Figure 5 shows electric field pattern for a TM010 modein the rf cavity. Figure 6 shows electromagnetic field
patterns for a TM010 mode along the beam axis for .
various radii. The rf fields have much effect on thetransverse beam focusing at the low energy. Figure 7
shows horizontal beam envelopes on different initialacceleration phases, when the beam passes through the rf
cavity for the first time. Figure 8 shows horizontal phasespaces at the exit of the rf cavity. The initial acceleration- 87-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

phases are as the same as those in Fig. 7, and twiss
parameters at the entrance of the rf cavity are a =0, andf3 =2 in all cases. The initial acceleration phase difference
produces a difference of transverse beam focusing
significantly.
A beam tracking from the CW electron gun to the exitof the microtron was done. Figure 9 shows energy
dispersion of the electron beam at the exit of the CW
microtron. The energy spread is calculated to be ± 2.2 %.
The study also shows that an electron beam can be
accelerated up to 5 MeV with practicable beam sizes:± 20 mm, ± 4 mm in the horizontal and verticalcoordinates, respectively, and a capable acceleration
phase width is more than 20 degrees.

Figure 6: Electromagnetic field patterns for a TMOlO
mode along the beam axis for various radii: z=O is thecenter of the rf cavity.

ACKNOWLEDGEMENT S
The Authors would like to thank Messrs. T. Hirano, K.
Hanakawa, M. Yamada, Dr. N. Nakamura, and manymembers of our company for helpful discussions.

REFERENCES

0.0 0.2 0.4 0.6 0.8s(m)0.0 0.2 0.4 0.0 G.d 1.J

l.C

0.0 0.2 0.4s(m)0.8 1.0

Figure 7: Beam envelopes at the rf cavity (first turn).
Initial acceleration rf phases are -290 o (right), -275 o(center), and -260° (left).

[1] J. Pottier, "A new type of rf electron accelerator: therhodotron", NIM B 40/41, 943, 1989.
[2]H. Kwon, et al., "Conceptual Design of 10 MeV,
100 kW CW Electron Accelerator for IndustrialApplication", PAC '1999, 2558, 1999.
[3] H. Tanaka, "Design of CW Microtrons using a500 MHz rf Cavity for Industrial Application",
Proceedings of the 12th Symposium on
Accelerator Science and Technology, 107, 1999.
[4] Per Lidbjork, "Microtrons", CERN 94-01, Vol. 2,971,199420"0f'E¥0·~~-20-40-10-50

510x(mm)Figure 8: Horizontal phase spaces at the exit of the rfcavity. Initial acceleration rf phases are -290 °, -275 °, and-260°.

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4.8

5.0

5.2

Energy (MeV)

Figure 5: Electric field pattern for a TMOlO mode in the rfcavity.

Figure 9:Energy spectrum at the exit of the CW microtron.

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