The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

DEVELOPMENT OF HIGH FIELD DIPOLE MAGNET
AND POWER SUPPLY FOR PROTON THERAPY
K. Endo and K. Egawa, KEK, Tsukuba, Japan
Z. Fang, NIRS & KEK, Chiba, Japan
M.Mizobata and I. Uchiki, Mitsubishi Electric Corp., Kobe, Japan
Abstract
A small dipole magnet of 3 T and its pulsed power
supply feeding 200 kA at maximum are being developed
for the table-top proton synchrotron for the protontherapy. The experimental field distribution is consistent
with the 3D dynamic field simulation. The dipole field
was measured at the time interval of 2 µsec with 15 tiny
search coils aligned accurately to the radial direction at 5mm pitch. The dipole is excited by the discharge currentof the capacitor bank of 200 kJ with the rise time of about
5 msec. Performances of the dipole and power supply aredescribed with the numerical simulations.

INTRODUCTION
The dedicated medical proton accelerators are designed
to have a maximum energy of 250 MeV or slightly more,
however, in most cases about 80% of patients could betreated with the proton energy less than 200 MeV
guessing from the maximum tumor depth distribution
treated so far [1]. The radiotherapy using accelerated
protons and/or heavy ions is recognized as an effective
method to cure the malignant tumor offering the QOL(quality of life). It is now in an aged society especially
important to promote this advanced method countrywide
and/or worldwide. Several dedicated proton accelerators(synchrotron and cyclotron) are built and used at the
medical centers. However, they are not enough to treat all
patients who really want the advanced radiotherapy. So
far the accelerator itself is very expensive and requires thespecial facility to install. To promote this radiotherapy ina reasonable cost, the smaller and cheaper accelerator but
limited in the maximum energy is desired to develop. The
compact accelerators serve as complements to the large
facility and will enhance the advanced cancer treatment at
the residential districts.
Synchrotron is convenient to change the beam energy
from pulse to pulse if the treatment depth to the patient isrequired to control locally to avoid the dose to the normal
tissue. To make the proton synchrotron of 200 MeV forthe radiation therapy small enough for installation andconvenient for the daily clinical treatment in the hospitalenvironment, the development of the high field compact
dipole magnet with performance of the accelerator grade
is indispensable [2, 3, 4]. Another requirement to thecompact ring is to reduce the overall longitudinal
dimension of the RF cavity with the average accelerating
gradient of ~40 kV/m [5]. Its RF frequency range is wellwide and the accelerating voltage is very large toaccomplish the acceleration within the duration while theohmic heat dissipation can allow the temperature rise of
the dipole coil. The RF issues are treated by the paper ofthis conference [6].
According to the size of the horizontal beam aperture,
the cross-sectional dimensions of the dipole become large.
If they are limited by the ring size, the iron core saturation
becomes large when attaining a high magnetic field.
There is a room for a trade off between the maximum
field strength and the beam aperture. The present optics
design has a preference to the beam aperture so as toobtain the sufficient beam intensity extracted for themedical treatment. The peak field of the dipole issuppressed to 3 T at 200 MeV, however, the dipole core
saturates considerably.
The pulsed power supply was also manufactured. It has
a base on the charge/discharge of the energy storage
capacitor. The discharge current steps up through thepulse transformer to attain the peak current of 200 kA
corresponding to the dipole field of 3 T. The power
supply and the dipole magnet compose a resonant circuitof which rise time is used for the acceleration [7].

PARAMETERS OF 2 LATTICE VERSIONSMain machine parameters are given in Table 1 with therevised parameters for the missing quadrupole version
and their layouts are shown in Fig.1. The joint design
study by BINP and Frascati is also given for reference.
Each design has the different dispersion and thehorizontal/vertical tunes depending on the cell structure.
Present lattices are FODOFB with small dispersion at thedipole magnet and DOB with the reduced circumference.
Either lattice has the same dipole magnet parameters.
Table 1: Parameters for 200 MeV proton synchrotron.
Max. energy
Inj. energy
Circumference
Av. diameter
Bending radiusMax. dipole field
PeriodTune, Q x/Q y
Max. dispersion
Cell structure
Frequency range
Acc. voltage
Acc. time

BINP/Frascati
KEK
200.0
200.0
200.0
12.0
2.0
2.0
6.4
11.9
9.5
2.0
3.6
3.0
0.54
0.72
0.72
4.0
3.0
3.0
4
44
1.42/0.54 2.25/1.25 1.6/0.6
0.63
0.5
0.8
BODO
FODOFB
DOB
7.4~26.5
1.9~16.2
2-18
12.4
13.0
13.0
3.5
5.0
5.0unit
MeV
MeVmmmTmMHzkVmsec

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

QF

QD

DOB Proton ringBB
0.1m
50.15m
7.5 ρ=0.72m

0.625mQF

FODOFB Proton ring
R=1.8m

QD

R=1. 5m

300.6m2m.7
ρ=00.1m
0.15m
0.375mQD

The experimental data reproduce fairly well thenumerically obtained distributions. The effective magnet
length along the central orbit is given in Fig.7. At low
field the effective length is longer by about 20 mm than
the simulation. The time base differs between thecalculation and measurement because the real magnet
excitation depends on the resonant condition of the circuit.
So the experimental time base is shortened by adjusting
the measured current to that of calculation.

Figure 1: Layouts of the compact proton synchrotron(FODOFB and DOB versions).

DIPOLE MAGNET AND PERFORMANCEThe ring consists of 4 dipole magnets which are seriallyconnected with the pulsed power supply of 200 kA atpeak. A prototype dipole magnet whose cross-section isshown in Fig.2 was designed by using the time-dependent
3D simulation code. The pole end profile was also
simulated approximating the continuous curvature(Rogowsky curve) or the slant plane by the horizontal
steps as shown in Fig.3. According to simulations bothends of the pole extending 5 cm from the core edges aretapered to attain a uniform effective dipole length asshown in Fig.4 and the completed dipole magnet is givenin Fig.5.

Figure 3: Pole end profile for the field calculation.
(a) Plan view of the half dipole and cut plane, (b)the pole end profile and the field distribution by
the color codes on the cut plane.

Effective length (m)

1.15

1.145

1.14

r=70cm (m)r=72cm (m)r=74cm (m)1.135

1.13012345t (msec)

Figure 4: Numerically obtained effective length
as a function of time. The squares correspond tothe central orbit. The others are those at ±2 cm.

Figure 2: Cross-sectional dimensions of the dipole (top),
and field distribution at max. excitation (bottom).
The field distribution is measured by using 15 searchcoils precisely aligned to the radial direction in the probe
holder which are moved manually at every 0.9 or 0.45
deg. step on the field measuring table on which the many
precise pin holes are drilled to align the probe at the exact
positions by inserting a pin. Induced 15 voltages are
saved at every 2 µsec and integrated to convert them tothe field data with a personal computer. The field
distributions normalized at the center of the dipole are
given in Fig.6 with the results of the field calculation.

Figure 5: Completed dipole magnet connection to
the pulse transformer with a bus-bar.
The effective magnet length is obtained by integrating
the measured field map numerically on the gap medium
plane. As the search coil moves on the field measuring

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Normalized By at dipole center

table to form a circular arc both inside and outside of thedipole. However, at the outside of the magnet theintegration should be taken along the tangential line on
which the magnetic field was interpolated by the methodof an iso-parametric transformation in conjunction withthe generalized matrix inversion.
1.02
10.98
Bmeas(1.5ms)
Bmeas(2.0ms)
Bmeas(3.0ms)
Bmeas(3.5ms)
Bmeas(3.7ms)
Bcal(1.5ms)

0.96
0.94

Bcal(2.0ms)
Bcal(3.0ms)
Bcal(3.5ms)
Bcal(4.0ms)
Bcal(5.0ms)

The power supply shown in Fig.8 has the capacitor
bank (10 mF, 6.5 kV) to excite 4 dipoles.
Before manufacturing the pulsed power supply, thecharge, discharge and residual energy recovery circuits
are simulated using PSpice code for all dipoles seriallyconnected. The current pulse width is adjusted to 10 msec
(50 Hz equivalent) by changing the circuit parameterssuch as the pulse transformer winding ratio, capacitor,charging voltage and etc. The measured current pulse andthe central dipole field are plotted in Fig.9 when theenergy recovery circuit is working.
Authors greatly acknowledge a favor of Prof. Y. Hiraoand Dr. S. Yamada of NIRS to perform this work.

0.92
-40

-20

02040x (mm)Figure 6: Measured and calculated field distributions.
1.17

3.5

B0(T) for 50Hz

32.5

1.14

2

1.13

1.5

Case81
measured

1.12

1

1.11

0.5

1.10

Figure 8: Completed pulse power supply fordipole magnets. From left to right; charging block,control block, 200 kJ capacitor bank, discharge
block and pulse transformer (shown in Fig.1).
30123

45Time (msec)

B (T )

To generate 3 T in the gap of 5 cm in height with asingle turn coil, the excitation current is as large as 200kA. The coil dimensions are small because of the compact
construction of the dipole magnet. The cooling capacityof the heat dissipated during the excitation determines thecurrent pulse width. Assuming the sinusoidal excitation
with a peak current I p and a pulse rate η repetitions/sec,
the effective current is I eff = 0.5I p η / f for the rise andfall time equivalent to f Hz. As this case is equivalent to50 Hz, I eff = 14.1 η [kA] . In practice, the current differsfrom the sinusoidal pattern due to the saturation of themagnet and the repetition rate is 5 Hz or less dependingon the coil temperature. To avoid the skin effect the coil
is made of the strand Cu cable impregnated with theepoxy resin of which thermal conductivity is considerably
so low to arrow only 1 Hz repetition rate. To improve thissituation, it will be considered to replace the coil with thatmade of the OFC hollow conductor after the study.
As the discharge current of the capacitor bank isutilized to excite the dipole, the tracking control is madeby the precise current control of the quad after the dipole
current which serves as a reference signal to the quad
power supply. This kind of control is relatively easy
because the quad current is about one tenth of the dipole
current or less for the DOB lattice version.

250

I p e a k = 1 9 8 .6 k A
1 .5

150

1100500 .5

PULSED POWER SUPPLY3002002B (T)

Figure 7: Measured and calculated (Case81)
effective magnet length.

B p e a k = 3 .0 3 1 T

2 .5Current (kA)

1.15

B0(T) for 50Hz halfsinusoidal excitation

Effective length (m)1.16

C u r r e n t (k A )new B andC urrT0000 .0 0 50 .0 10 .0 1 50 .0 2t (s e c )Figure 9: Measured time-dependent excitation curve.

REFERENCES[1] Y. Hirao, “Results from HIMAC and Other Therapy
Facilities in Japan,” Proc. Cyclotrons 2001, pp.8-12.
[2] K. Endo et al, “Compact Proton and Carbon Ion
Synchrotron for Radiation Therapy,” Proc.
EPAC2002, Paris, pp.2733-5.
[3] K. Endo et al, “Development of High Field Dipoleand High Current Pulse Power Supply for Compact
Proton Synchrotron,” PAC’03, Portland, to bepublished.
[4] K. Endo et al, “Development of Compact Proton
Synchrotron for Radiation Therapy,” ARTA2003,
Tokyo, to be published.
[5] Z. Fang et al, “RF Cavities and Power Amplifier for
the Compact Proton Synchrotron,” PAC’03, Portland,
to be published.
[6] Z. Fang et al, “RF Cavity for Table-Top Proton
Synchrotron,” this conference.
[7] K. Endo et al, “Resonant Pulse Power Supply for
Compact Proton and/or Heavy Ion Synchrotron,”
Proc. APAC2001, Beijing, pp.636-8.