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

DESIGN OF AN LEBT OF A CW MICROTRON
T. H. Kim, and H. Tanaka
Mitsubishi Electric Corporation, Electromechanical Systems Dept., Advanced Technology R&D Center,
8-1-1, Tsukaguchi-honmachi, Amagasaki City, HYOGO 661-8661, Japan

Abstract
A low energy beam transport (LEBT) has been designed
for a CW microtron. The LEBT mainly consists of twosolenoid magnets, a bending magnet, a quadrupole magnet,
and a chicane magnet. Fringe fields of the bendingmagnets are optimized by adjusting gaps of auxiliary polepieces (clamps) of the bending magnets. Calculation andmeasurement results of magnetic fields show that a fieldintegral (PINT) related to the fringe field is proportional
to the gaps of the clamps. The studies also show twodimensional calculation results and three dimensional
calculation result of the PINT are within accuracies of10% and 0.5 %, respectively.

Electron Guns
0600

800

1000
Unit:mmTable 1: Beam parameters at an exit of an electron gun.
Beam energy
80 keV
Normalized emittance (1 a)
20 7rmm · mrad
TWISS parameters a-3-0
B0-1

Table 2: Design beam parameters at an exit of the LEBT.
TWISS Parameters
a-1 - 1B0-2'E5"a.
0Qj>

r:"

..

E"

Ill

25
20
15
1050-5-1002400

Figure 1: Schematic drawing of the LEBT of the CW
microtron.

1 INTRODUCTION
High power electron beams are necessary for industrial
applications: electron beam and X-rays irradiation.
Electrostatic accelerators and linear accelerators havebeen used for the applications. However, accelerators
operated in a continuous wave (CW) mode are moresuitable for these applications. We have proposed a CW
microtron with a 500 MHz RF cavity for theapplications [1 ]. In the CW microtron system, an LEBT
should be compact and have large acceptance for a highpower electron beam. Therefore, a bending radius of abending magnet of the LEBT is small, which has mucheffect on transverse beam focusing. Careful considerationof a fringe field of the bending magnets is necessary. This
paper describes a lattice design of the LEBT taken into
account a fringe field effect and a design of the bendingmagnet.

2000.20.4

LATTICE DESGIN

0.60.8

1.2

Distance (m)Figure 1 shows a schematic drawing of the LEBT of theCW microtron. The LEBT mainly . consists of twosolenoid magnets (SOU, SOL2), a bending magnet(BM1 ), a quadrupole magnet (QM) and a chicane magnet(BM2, BM3, BM4). An electron beam is injected to theCW microtron with the chicane magnet.
Electron beam parameters at an exit of an electron gun
are studied with beam simulation and shown inTable 1 [2]. Table 2 shows design parameters at an exitof the LEBT. The LEBT is designed using MAD [3] andTRANSPORT [4]. In the LEBT, the length of the chicane
magnet is determined by the layout of the CW(a) a==-1, /3:::0.03'E5252015"a. 100Qj>

r:5..E" -50
Ill"
-10
0

0.2

0.4

0.6

0.8

1.2

Distance (m)(b) a==O, /1==0.03
Figure 2: Two typical examples of beam envelopes of theLEBT. Emittances are of 2 a.

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

microtron. The bending radius of the BM2 is small, which
has much effect on transverse beam focusing. Edge anglesand fringe fields of the bending magnets should beadjusted accurately. The fringe field effect is introduced
into MAD and TRANSPORT as a field integral (FINT).
Calculation results show that the FINT of BMl, BM2,and BM3 are 0.39, 0.33, and 0.33, respectively. Figure 2
shows two typical examples of beam envelopes of theLEBT. Instruments of the LEBT were designed and madebased on these results.

30.40.20 ........l!f"":__L. _ _ _ _ _ l _ _ _ _.:L"']IItll--..1..

BENDING MAGNET-100Studies of the fringe fields of the bending magnets weredone with a magnetic field calculation and measurement.
The FINT is calculated with POISSON and TOSCA. TheFINT is given by-50

100500S (mm)(a) BM2
1.2 , - - - - - - - - · - - - - - - - - - - - - - - ,PINT =foo By(s)(Bo -By(s)) ds .
'g. Bo 2-oo•

Measurement-POISSON
0.8

where By(s) is the magnitude of the fringing field on themagnetic mid-plane, B0 is the asymptotic value of By(s)well inside the magnet entrance, and g is the full gap ofthe magnet [5]. The FINT of the bending magnets are
smaller than that of a typical magnet (0.45) [4]. Figure 3
shows a schematic drawing of the BM2. The BM2 has
auxiliary pole pieces (clamps). Figure 4 shows calculationand measurement results of the magnetic fielddistributions. The magnetic field calculations were donewith POISSON. The calculation result of the BM3 is inagreement with the measurement results. The calculation
result of the BM2 is different from the measurement
results significantly. Figure 5 shows calculation andmeasurement results of the FINT as a function of the gapsof the clamps. These results show that the FINT isproportional to the gaps of the clamps. The calculation
results of the FINT of the BM2 and the BM3 areaccuracies of 10% and 3%, respectively. A magnetic fieldcalculation of a three dimensional model of the BM2 wasdone with TOSCA. Figure 6 shows a magnetic field
distribution of the BM2. The FINT calculated with0~ 0.6ID0.40.2-50500100

S (mm)(b) BM3
Figure 4: Calculation and measurement results of themagnetic field distributions.
0.45............. .....

0.4~l1-E o.35

e·Measurement--POISSONTOSCA
b.

0.3
0.25
2035

3025

Half gap of clamps (mm)(a) BM2
0.45~0.35'""'

LL

0.3_.......

-

0.4

Coil

[Measur~ment-...
-lit- POISSON----··--

0.25
20302535

Half gap of clamps (mm)(b) BM3
Figure 5: Calculation and measurement results of theFINT as a function of the gap of the clamps.

Figure 3: Schematic drawing of the BM2.

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ACKNOWLEDGEMENTS
0.04

The authors would like to thank Mr. K. Ogata and Mr.
N. Matsumoto of To kin Machinery Corporation formeasurements of the magnetic field of the magnets. They
also wish to thank Dr. T. Nakanishi, Dr. Y. H. Pu, Mr. Y.
Makita and Mr T. Hirano of our company for helpful
discussions.

B(T)

0.035
0.03
0.025
0.02
0.015

REFERENCES

0.01
0.005
400)

Figure 6: Magnetic field distribution of BM2
TOSCA is in agreement with the measured FINT, as
shown in Figure 5. Difference between the accuracies of
the calculations with POISSON is due to three
dimensional effects of shapes of the bending magnets.
Because the BM3 is an approximately rectangularbending magnet, the accuracy of the calculation results of
the BM3 is better than that of the BM2. A bending
magnet which is short along a beam orbit should bedesigned with a three dimensional magnetic field analysis
program.

4[1] H. Tanaka, "Design of CW Microtrons using a 500
MHz RF Cavity for Industrial Application",
Proceedings of the 12th Symposium on Accelerator
Science and Technology, Saitama, Japan, 107, 1999.
[2] H. Tanaka, Y. Pu, T. H. Kim, Y. Makita, C.
Tsukishima, T. Nakanishi, and S. Nkamura, "BeamDynamics in a CW microtron with a 500 MHz RFcavity for Industrial Applications", Proceedings ofthe 13th Symposium on Accelerator Science andTechnology, Osaka, Japan, 2001.
[3] H. Grote, and F. C. Iselin, "The MAD Program
Ver.8.19 User's Reference Manual", CERN/SL/9013, Geneva, Switzerland, 1996.
[4] D. C. Carey, K. L. Brown, and F. Rothacker, "ThirdOrder TRANSPORT with MAD input A Computer
Program for Designing Charged Particle BeamTransport Systems", SLAC-R-530, USA, 1998.
[5] K. L. Brown, "A First- and Second-Order Matrix
Theory for the Design of Beam Transport Systems
and Charged Particle Spectrometers", SLAC-R-75,
USA, 1982.

SUMMARY

The LEBT of the CW microtron is designed and isproved to be practicable. The instruments of the LEBT
have been made and the beam test will be done.

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