Proceedings of IBIC2012, Tsukuba, Japan

MOPA02270° ELECTRON BEAM BENDING SYSTEM USING TWO SECTORMAGNETS FOR THERAPY APPLICATION

Abstract
The 270 degree doubly achromatic beam bending magnet system using two sector magnets has been designed
mainly for treating cancer and skin diseases. The main requirements of the design of two magnet system is to focus
an electron beam having a spot size less than 3 mm × 3mm, energy spread within 3% and divergence angle ≤ 3mrad at the target. To achieve these parameters the simulation was carried out using Lorentz-3EM software. The
beam spot, divergence angle and energy spread were observed with respect to the variation in angles of sector magnets and drift distance. From the simulated results, it has
been optimized that the first and second magnet has an angle 195 degree and 75 degree and the drift distance 64 mm.
It is also observed that at the 1396, 2878 and 4677 A-turn,the optimized design produces 3324, 6221 and 9317 Gaussof magnetic field at median plane require to bend 6, 12 and18 MeV electron beam respectively. The output parametersof the optimized design are energy spread 3 %, divergenceangle ∼ 2.8 mrad and spot size 2.6 mm.

INTRODUCTIONRadiotherapy using electron and photon represent a mostdiffused technique to treat tumor diseases. With the advent of high energy linear and circular accelerators, electron / photon have become a viable option in treating superficial tumors up to the depth of about 5-10 cm. In suchcase, the dose of radiation absorbed correlates directly withthe energy of the beam and its deposition of energy in tissues, which results in damage to DNA strands and diminishes the cell’s ability to replicate indefinitely. For the last
several years, electron accelerators are extensively being
used in the medical field with special applications of photon and electron beam for cancer therapy and various skindiseases [1].
High energy medical electron linacs are usually mounted
horizontally because of larger length of linac tube. The
emergent electron beam from the accelerating tube is deflected magnetically through 90° or 270° into a verticalplane to hit an X-ray target or electron scatterer [2]. A
small, stable, and axially symmetric beam spot on the target is needed. For fully rotational medical electron linacs,∗ sanjay@physics.unipune.ac.in

it is necessary both to minimize physical height of the machine and to maintain 100 cm distance between the x-ray
target and rotational centre. In addition, it is also a basic
requirement that the bending magnet is doubly achromatici.e. the position and angle of the output beam is independent of input beam energy.
The 90° deflection system can bend a mono-energetic
beam on axis to a point at the X-ray target, but the spreadof energies of the actual beam results in a spread of such
focal point at the X-ray target. Also, the radial displacement and angular divergence of the entrant beam resultsin spread of exit beam. To overcome the above problems
a 270° bending magnet system is used. This system isdoubly achromatic and can be made using one, two andthree magnets [3]. Based on the orbit dimension and size,
two dipole bending magnet system is advantageous. Therefore, an objective of the present paper is to provide a 270°
beam bending using two dipole magnet system in which
first bend is with 195° and other bend is with 75° to minimize the height of the orbit above the accelerator beamline.

MATERIALS AND METHOD
The linear electron accelerator available at Society ofApplied Microwave Electronics and Engineering Research,
Mumbai having energies 6,9,12, 15 and 18 MeV [4], average current 80 µA, pulsed current 130 mA, pulsed width
6 µs, repetition rate 150 PPS is going to be used for thepresent study. The linear accelerator produces beam of 6 to20 MeV with energy spread of 7%.
The Lorentz-3EM software has been used for studying
the trajectory of charged particles through electric and/or
magnetic fields in three dimensional geometry. The magnetic field between the poles and trajectory for various electron energies have been calculated with the help of Lorentz3EM software for better accuracy.

MAGNET DESIGNIn order to take advantage of the low height of an elementary single dipole and at the same time take advantage of achromatic system, a double focusing doubly achromatic magnet system was designed. It consists of twodipole magnets which are used to bend the 6 to 20 MeV

ISBN 978-3-95450-119-9
Accelerator Technology1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Shahzad Akhter, V.N. Bhoraskar, S.D. Dhole∗ ,Department of Physics, University of Pune, Ganeshkhind, Pune, India
B.J. Patil, Department of Physics, Abasaheb Garware College, Karve Road, Pune, india
S.T. Chavan, S.N. Pethe, R. Krishnan, SAMEER, IIT Powai Campus, Mumbai, India

MOPA02

Proceedings of IBIC2012, Tsukuba, Japanenergy electron beam by 270° onto the X-ray target within
the acceptable geometry. The main requirements of the design of magnet system is to focus the electron beam, having
a spot size less than 3 mm diameter, beam energy spreadwithin ± 3% and divergence angle ≤ 3 mrad on the X-raytarget.
The magnet system consist of two magnets, out of which
one magnet deflects the electron beam by an angle greaterthan 180° and other magnet by less than 90°. If a finite energy spread beam is injected into a magnet with more than
180° deflection (first magnet), the output beam will be convergent. If the same beam is injected vertically upwards
into a less than 90° deflection magnet (second magnet),
the output beam would be diverging beam. The amountof convergence and divergence of beam is depending upon
the bending angle of each magnet and the pole face angle atthe exit edge of first magnet and entrant of the second magnet. [5] Therefore, simulation has been carried out using
Lorentz-3EM software for compensating the convergenceangle with divergence angle and choosing the appropriate
drift distance between both the magnet.

current passing through the coil gives different values ofmagnetic field. To compensate and to bend the desired energy of electron beam, a current was accordingly adjusted
for the measurement of output parameters. A cross sectional view of the optimized two magnet system is shownin Fig. 2 for 270° beam bending.
Drift distance

Second magnet
Firstmagnet

Output electron beam

Incident electron beamc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○SIMULATION
For this, at first design the two sector magnets of angle
180° and 90° having radius of curvature 65 mm and thedrift space between both the magnets was taken 65 mm.
The schematic for simulation is shown in Fig. 1. Then, anelectron beam of diameter 3 mm, energy spread of ± 7%,
Gaussian profile in intensity was made incident on the poleface of first magnet. The incident electron beam parameters
were adjusted such that it should match the actual electron
beam from Linac tube.
The output beam coming out from the second magnet
was detected at the bremsstrahlung target position which iskept at 70 mm ± 3 mm from the exit pole face of secondmagnet. The energy spread, beam divergence, and spot sizeof the beam were observed at bremsstrahlung target. For
both the magnets, a common yoke of H-shape has been
designed. The Steel 12L14 material was used for yoke andmagnets. Both sector magnets have common magnetizing
coils which has internal water cooled system. The pole gap
between dipole magnet was kept 10 ± 0.1 mm.
The requirements of the design are (a) the central electron beam should remain perpendicular to the target surface
at the distance of 70 mm from the exit edge of the second
magnet. (b)target beam spot should be less than 3 mm. To
achieve these requirements, the magnet angle and drift distance were varied in Lorentz-3EM. From the variations, it
has been observed that as the magnet angle increases thebeam angle with respect to target also increases. Moreover,as the drift distance increase the beam spot size decreases
up to ∼ 2.8 mm at a drift distance of 64 mm and further
with increase in the drift distance the spot size increases.
From these results, it has been optimized that the first magnet of an angle ∼195°, second magnet of an angle ∼75°
and the drift distance ∼64 mm ± 1 mm. The variation inBremsstrahlung target

Figure 1: Schematic of simulation setup for two magnetsystem (Not to the scale)Figure 2: A view of proposed two magnet system
The width and height of the optimized magnet polepieces are 37 mm and 70 mm respectively, taking into theconsideration of the fringing field effect on the electronbeam. The contour of magnetic field distribution in themagnet at median plane is shown in Fig. 3. Contour shows
the uniform contribution of magnetic field over the polepieces.

ISBN 978-3-95450-119-92

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Proceedings of IBIC2012, Tsukuba, Japan

MOPA02

added to the second magnet. Very small amount of current
require to be passed through these coils. The magnetic fieldchange produced due to trim coil has been estimated and it
is shown in Table 1. From the table it is observed that ∼3%field change is obtained with 135 Amp-turns/coil of trimcoil.
Firstmagnet

Figure 3: Contour of magnetic field distribution in the median plane of the first and second magnet.

Table 1: Effect of Trim Coil on Second Magnet
I(A/coil)NI(/coil)

B(G)

∆B
(G)

% change(%)

01.50
2.65
4.00
5.25
6.75

030
53
80
105
1356605
6695
6717
6743
6770
6805

090
112
138
165
2000
1.36
1.69
2.09
2.50
3.03

Moreover, to reduce the energy spread at the target, anenergy defining slit was introduced in the magnet system.
Therefore, an aluminum slit was introduced at the exit poleface of the first magnet, where large dispersion of beam
was observed. The aluminum slit was specifically used tominimize the bremsstrahlung production.
Overall, the variation in magnetic filed as a function ofdistance along the radius of curvature in the median plane
for the optimized system is shown in Fig. 5. It is observedthat at the 1396, 2878 and 4677 A-turn, the optimized design produces 3324, 6221 and 9317 Gauss of magnetic field
to bend the electron beam of energy 6, 12, and 18 MeV respectively.

Figure 4: Variation in magnetic filed with ampere-turn withand without triangular shim at second magnet.

RESULTS AND DISCUSSION
The optimized design of two magnet system does not
fulfill the requirement of actual medical Linac for therapy.
So the variation of magnetic field and other output parameters due to incorporation of triangular shim, trim coil andenergy defining slit in the magnet system have been studied.
The initial magnetic field versus current measurements
observed that the second magnet saturated more at higher
current than the first magnet. Therefore, a triangular shim
was introduced at the base of second magnet. The variation in magnetic field with amp-turns for with and without
triangular shim at the base of second magnet is shown inFig. 4. It is observed from the figure that with an introduction of triangular shim the magnetic field increases andfurther gets saturated with increase in the total amp-turns.
To make slight corrections in the angle and /or the position of the beam on the target, a set of trim coils have been

Figure 5: Magnetic field Vs distance along the radius ofcurvature on the median-plane for first and second magnet.
For a typical case study, a beam of energy 12 MeV, beam
diameter 3 mm and energy spread of ± 7 % has been passed
through the optimized design of the two magnet system.
The trajectory of an electron beam is shown in Fig. 6. It isobserved that the convergence angle due to first magnet has

ISBN 978-3-95450-119-9
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Copyright ○Secondmagnet

MOPA02

Proceedings of IBIC2012, Tsukuba, JapanCONCLUSION
The optimized design of two magnet system gives uniform distribution of magnetic field and extracts 6 to 18
MeV electron beam energy spread 3 %, divergence angle∼ 2.8 mrad and spot size 2.6 mm ± 0.05 mm. This system
is useful for the radiation therapy.

REFERENCES[1] C.J. Karzmark, Med. Phys. 11(2) (1984) 105.
[2] W. Houwen, et al., Med. Phys. 21(7) (1994) 1065.
[3] J.J. Livingood, The optics of dipole magnets, (AcademicPress, New York, 1969).
[4] R. Krishnan, et al., Proceedings of PAC09, Vancouver, BC,Canada (2007).

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 6: Trajectory of 12 MeV electron beam in two magnet system.

[5] R.M. Hutcheon, E.A. Heighway, Nucl. Instr. Meth., 187(1981) 81-87.

been compensated by the divergence angle produce due tothe second magnet.
Therefore, the output parameters of the optimized designof the two magnet beam bending system for medical Linac
are energy spread ±3 %, divergence angle ∼ 2.8 mrad andspot size 2.6 ± 0.05 mm. The optimized parameter for twomagnet system is given in Table 2:
Table 2: Optimized Parameter of the Two Magnet System
Beam Particles
Operating Beam Energy
Energy Variation
Beam Current (Average)
Input Beam Diameter
Output beam spot size
Output beam divergence
Main Chamber
Magnet Type
Magnet material
Pole gap
Radius of curvatureElectrons
6 to 18 MeV±3%∼ 80 µAmp∼ 3 mm, ≤ 7 mrad
∼ 2.6 ± 0.05 mm∼ 2.8 mrad
30.4 × 35 × 25 cm3
Two Pole (C-shaped)
Steel 12L14
10 ± 0.1 mm
64 mm ± 1 mm

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