The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

A Magnet System with Reduced Gap Size for Beam ScanningTetsuro NORIMINE, Hiroshi AKIYAMA, Koji MATSUDA and Kazuo HIRAMOTO
Hitachi Research Laboratory, Hitachi Ltd
2-1, Omika-cho 7-chome, Hitachi-shi Ibarakiken, 319-12 Japan

Abstract
A magnet system with reduced gap size for beam scanning
is presented. In the beam delivery system, the displacement of
the beam center position at the second scanning magnet isredlced by setting the first one where betatron oscillation
phase difference is 180 regrees between them. In thissystem , the gap size of the second scanning magnet is reduced
from 45 mm to 30 mm when its length is 300 rom anddeflecting angle is about 60 mrad to irradiate a 200 rom X 200
mm region.

irradiate2-dimensional regions.
In these beam delivery systems, there are two typicalirradiation systems. One of them is the wobbler scatterer
system[2] which uses the beam spread out by the scatterer.
And the other is the spot beam scanning system[3] which uses
the small sized beam( ~ ~ 10 mm) .
At the second scanning magnet of these beam scanningsystems, it needs wire gapsize in orrer to pass the beam
deflected by the frrst one. And electric power consumed at thesecond scanning magnet becomes large.
Then, we present a new magnet systems using beamscanning system to reduce the gap size of the second scanningmagnet.

1 Introduction
In recent years, proton therapy , which has goodcharacteristics of rose localization, has been successfully
applied to cancer treatments . Usually, treatments requirelarge uniform radiation fields, as large as 200 mm X 200 mm.
Since the size of the beam which is extracted from anaccelerator is small, it is required to spread the beam.
To obtain such large uniform radiation fields, various
beam delivery systems have been developed and are categorizedby passive and active beam delivery systems[!].
Passive beam relivery systems employ single or rouble
scatterers to spread the beam and flatten the beam intensity.
And the lateral part of the spread beam is cut off by thecollimator along the shape of tumor.
In active beam delivery systems, two scanning magnets areplaced in tanrem with their magnetic field directions
orthogonal to one another and to the beam direction in order to
Quadrupole Magnet
Ql-Q6

2 Scheme of beam scanning to reduce the gap size
The scheme of beam scanning using the scanning magnets
with redlced gap size is as follows . (i)The first scanningmagnetS 1 and the second one S2 are installed where the phasedifference of the betatron oscillations in the reflected plane
between them is 180 regrees. (ii) The first scanning magnet
S 1 is placed at the position upstream to the last bending
magnet BM3 of the gantry. The beam is deflected in deflecting
plane of bending magnets of the gantry to keep the gap size ofBM3 small. (iii) The second scanning magnet S2 is placed at
the position downstream to BM3 and deflects the beam in thedirection vertical to the deflecting plane of bending magnets ofthe gantry.
If these two scanning magnets are set as (i), the position ofS2 becomes the node of the deflection by the scanning magnet
S 1. And then, the beam deflected by S 1 where X=O returns toBending Magnet
BM2p =l.Sm ~----.:::_::::~:-1-Tt-t-f-:::.:::First Scanning XMagnet~5Sl.'/··-

Bending Magnet
BMlp ==l.Sm

Rotation
Radius 4.2m

Second _ScanningMagnet
S22.7mExtractedBeamIIIIsocenter----- ---------------------------------- ----------- ---~- -~---------~
Rotation Axis

I~----------===::.::..:=~----.l (b) In(a) In the Deflected Planeof the Bending Magnet ( X -S Plane)

7.2m

the Gap Directionof the Bending Magnet( Y direction)

Fig. 1 An Example of the Present Scanning Magnet System-317-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

X=O at S2.
Figure 1 shows an example of the magnet system to which
the present scheme is applied. In this system, the coordinates
are as follows. s is the length along the beam resign
trajectory. X is the position in the magnetic midplane of thegantry bending magnet. Y is orthogonal to S and X
coordinates.
If the length of the second scanning magnet is L, and themaximum deflected angle by the first scanning magnet in theSeCond one is 8 , the maximum beam displacement is
L 8 I 2 in the second one. If two scanning magnets are placedin tandem, the maximum beam displacement is larger than L
8 . Thus, this scheme redlces the gap size of the second
scanning magnet and its electronic power.
Furthermore, because there is only one scanning magnet
between the last bending magnet and isocenter, rotational
radius can be shorter than the gantry in which the two scanningmagnets are put in tanrem cbwnstream of the last bending
magnet.

plane of the bending magnet. The first scanning magnet S 1 isinstalled at the position between cpaciupole magnets Q4 andQ5. It is found that the difference of the phases of the betatronoscillations is 180 regrees. To create an uniform radiation
field as large as 200 mm X 200 mm , the scanning width is set
to be ± 110 mm at the isocenter. It needs 57 mraddeflection
on the first scanning magnet. If the scanning magnet length is
300 mm, it needs 0.46 T to deflect the beam.
At the second scanning magnet, the maximum scanning
width by the first scanning magnet is ± 9 mm and half beamsize is ± 4 mm . Taking the clearance into consideration, the~8~~s~3 Application of the present scheme/3x15·:::~,', /;',,,10'''/3y''''

5An optics of the gantry has been designed on condition thatthe extracted proton beam from a synchrotron has
characteristics as shown in Table 1. The beam extracted fromsynchrotron has small emmitance[4]. The beam energy is70-250 MeV. The size of the radiation field is 200 mm X 200mm. The needed deflection gradient is about 60 mrad.

246s(m)8101214Fig. 2 Betatron functions of the Gantry

Table 1. Beam parameters
Unnormalized Emittance(Horizontal,Vertical)
Beam Energy
Momentum Spread
Magnetic Rigidity.Size of Radiation Field-3rrmmmrad

7JX

570- 250MeV
below+0.2%
1.23 - 2.43 Tm
200 mmX 200 mm

The extracted beam from the accelerator is transported tothe gantry. The gantry has three bending and six cpadupole
magnets. Bending magnets are 50 regrees upward andcbwnward ones and a 90 regrees cbwnward one. Radius ofbending magnets is 1.5 m, rotating radius is 4.2m. The
maximum magnetic field of the bending magnets is 1.6 T.
Optics conditions are set in order to obtain approximately
the same sizes in X andY directions when the gantry is rotated
Optics resign conditions are (i} {3 x.=/3 y- 4-5 m and a xa y = 0 at the isocenter to make a spot beam, (ii) dispersion
functions and their gradients "' x = 'fJ x ' = 0, 'fJ y = 'fJ y' = 0 at
the entrance of the gantry, and (iii) 'fJ x = 'fJ x'= 0 at the exitofBM3.
Figure 2 shows betatron functions of the gantry to whichthe present scheme is applied. Both {3 xand {3 yare small in
the gantry beam transport system. Since the half beam size of
the Y direction of the gantry is below 6· mm, the 20 mm gap
size is sufficient to pass the beam in the gantry bending
magnet.
Figure 3 shows dispersion functions of this gantry. The
gantry optics is cbubly achromatic at entrance of the gantry
and theexitofthe bendingmagnetBM3, then beamsizesofXand Y direction at the isocenter become nearly equal.
Figure 4 shows betatron oscillation phase in the deflected-318-0-5 ...................._._.__._.._._........................._._._....._..._._..................._._.._._...__.
0246s(m)8101214Fig. 3 Dispersion functions of the Gantry

100Ji=5-100-200-300

L.....,...._,_..,L._,.._..L.-'c...L....!..............~_._L.J_..._._...L.o...........-'-'-'--'-'

0246s(m)8121014~D~ ~~~~ D~D~

QI BMI Q2 Q3Q4SIQ5BM2 Q6BM3S2Fig. 4 Betatron Oscilation Phase in the Deflected Plane

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

gap size of the second scanning magnet becomes about
30mm.If we set the two scanning magnets aqjacent at theposition downstream to the last bending magnet, the gap sizeof the second scanning magnet is needed about 45 mm.
This system is also effective to reduce the rotating radius of
the gantry because two scanning magnets are placed separately
upstream and oownstream of the last gantry bending magnet.
Then, this magnet system can save the space between the last
bending magnet and isocenter.

4 Summary
We presented a new beam scanning magnet system. Thissystem is useful to reduce the gap size of the second scanningmagnet and reduce the electric power consumed by it.
Applying this system to a rotating gantry, it is possible toreduce its rotating radius because two scanning magnets could
be placed separately upstream and oownstream of the last
bending magnet of the gantry . And this scanning system
would save the space to install the gantry in the treatment
room.

References[1] W.T.Chu et al. ''Instrumentation for treatment of cancer
using proton and light-ion beams", Rev. Sci. Instrum.,Vol.64 (8), pp. 2055-2122,1993[2]H. Ohara, et al. "STUDIES ON PHYSICS AND
BIOLOGY OF CARBON BEAMS OF RII<EN AND NIRS",
Proceedings of the NIRS International Workshop of Heavy
Charged Particle Therapy and Related Subjects, pp. 13-21,
1991[3] John E. Milburn et al. " Raster Scanning Magnets for
Relativistic Heavy Ions", Proceedings of the1987 IEEE
Particle Accelerator Conference Vol. 3, pp. 2000-2002,1987[4] K.Noda et al. " Slow beam extraction by a transverse RF
field with AM and FM", Nuclear Instruments andMethods inPhysics Research A374 pp. 269-277, 1996-319-