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

STABILIZATION METHODS OF THE RCNP CYCLOTRON
T. Saito, S. Ninomiya, H. Tamura and K. Sato,
Research Center for Nuclear Physics, Osaka University, 10-1 Mihogaoka, lbaraki, Osaka 567-0047,
JAPAN

is significantly improved. Finally ultra-precise beams
have been accelerated successfully for a long time.

Abstract
Stable operation of a cyclotron is the basis foracceleration of high quality beams. A high stability of anaccelerator is needed not only for narrow energy spreadbeams but also high time resolution beams or high power
beams. Persistent ultra-precise proton beams with dElE"=;:1.5xl04 have been successfully acc.elerated up to 392
MeV. For stable operation of the cyclotron, among
others stability of magnetic fields is essential. To
stabilise magnetic fields, the temperature of the coolingwater for coils is controlled and the temperature changeof the magnet cores is able to be kept less thanO.Oldeg,/day and the magnetic field drift is suppressed
less than 10"6/day.

1 INTRODUCTION
The RCNP cyclotron complex consists of a six sector
ring cyclotron (K=400) and an injector A VF cyclotron(K=140). The cyclotron complex has been operated toserve various kinds of beams for the nuclear physics
experiments and high-quality stable beams in narrow
momentum spread have been strongly required. In orderto accelerate beams stably, high stability of the magnetic
fields and the rf acceleration fields is needed.
Since the first beam, various efforts for improving thebeam quality have been done. Three variable frequency
single gap acceleration cavities and a single gap flattopping cavity are used to accelerate high quality
beams.[l] Extremely high stability is required for
relative phase between the acceleration and flat-topping
voltage. Typical phase excursion of the cavity voltage isless than O.ldeg./lOOhrs. Voltage variations are less than
0.01 and 0.05% for the acceleration voltage and the flattopping voltage, respectively.
With respect to themagnetic fields, the current stability of the main coils forthe injector cyclotron and the ring cyclotron are better
than 4xl0·6 , and those of the trim coils are better thanlxl0· 5 •Despite the stable operation of the electric devices, along-term stable operation was still a remaining problem.
In summer, 1996 it was found a correlation between
temperature of the iron cores of the magnets and themagnetic field. Suppressing temperature changes of theiron core with adjusting the temperature of the coolingwater for the magnets, the stability of the magnetic field

2 THERMAL EFFECTS ON MAGNETIC
FIELDS
In general, magnetic flux density B of anelectromagnet is given by:
B"=;:U 1-Lr/g (1)Where U: magnetomotive force/.L 0 : permeability of vacuum
g: magnet gapAs long as considering only small thermal expansion of amagnet, flux density depends only a magnet gap. Morethan 90% of magnetomotive force is spent to generatemagnetic field at the magnet gap, thermal effect onmagnetization of iron core is negligible.
The action of magnetic field upon beams ions depends
not only flux density but also a size of a magnet pole.
The size is also influenced by thermal expansion as wellas the magnet gap. This effect is important to a sector
magnet. This implies that a feedback control to keep afield strength based on a measurement of a magnetic field
by a NMR method does not function effectively. But incase of compact cyclotron, of which most beam orbits
are within a magnet pole, the field correction by a NMR
monitor is effective under small and uniform variation ofmagnetic fields.[2],[3]A correlation between temperature and magnetic field
is complicated. An un-uniformed temperature variation
brings complicated deformation of an iron core. In caseof temperature change of the cooling water for the ring
trim coil, temperature change of + 1"C causes change in
gradient of field drift of -1.6x10-5/day. Under suchsmall temperature change, thermal deformation isuniform. On the other hand, large temperature change
generates complicated deformation of pole-tip andposition dependent field variation is observed. Little
data of temperature distributions and degree of freedom
to control temperature, it is practically impossible tocontrol field drift by adjusting temperature of an iron core.
Only practical way is keeping of same distribution of ironcore temperature.

3 CAUSES OF TEMPERATURE CHANGE
There are various factors in temperature change of
cyclotron magnet core.

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

The trim coils made of ceramic coated copper plates
are fixed on the pole surface of sector magnets directly
through 0.125 mm polyimid film without heat insulators.
According to good thermal· contacts, the thermal time
constant of about 6 hours is observed.
Heat conductivity between main coils and iron cores
are relatively small, but temperature control of coolingwater for the main coils is important. As powerconsumption of main coils varies with a change ofaccelerated particles, temperature changes of the coils arelarge without temperature control of the cooling water.
The thermal time constant of about 100 hours creates
long-term drift of the magnetic field. The cyclotron
complex is required to accelerate high quality stablebeams lastingly for more than a week. The time constantis harmful characteristic for stable operation of thecyclotrons.
The sectors No.1, No.2 and No.3 contain magnetic
channels in the gaps. The temperature rises of thechannels are large and thermal effects are not negligible.
To reduce thermal effects, a water cooling system for themagnetic channels was made. The water temperature iscontrolled 20±0.2.
Every temperatures of cooling water for main coils,
trim coils and magnetic channels are independentlysettled.
The energized rf cavities are also heat sources andsome effects are observed. The effects are negligible.
Thermal contacts between cavities and iron cores aresmall. In usual operation, the temperature rise of thecavities is small and almost constant. The temperature ofcooling water for the cavities is controlled within ±0.1'C.
Thermal effect of room temperature is small as long as
room temperature is kept constant within ± 1'C or so.
Temperature change of 1'C causes temperature drift of.about 0.1 'C/week to iron cores. This drift is tolerable.
The stability of room temperature for the AVF cyclotron
is not well. Some counter-plan may be needed.

4 TEMPERATURE CONTROL OF THE
MAIN COILSThe coils of the main magnets are consisted of pancake
made of hollow conductor. The structural reason of thepancake, there is a thermal contact between going andreturning waterway of cooling water. · Owing to thethermal contact, temperature of the midpoint of theconductor is higher than the outlet of cooling water. The
midpoint is closest to the iron core, the temperature of the
midpoint should be kept constant. By simple analysis
shows the temperature distribution along waterway isgiven by:y=(tout-tin){ -qx2/2+(q+ 112)x }+tin (2)wheretin: temperature at inlet
tout: temperature at outlet

q: heat conductivity

In the ring cyclotron surface temperature of the main
coil near the magnet pol~ is directly measured together
with the cooling water temperatures. Figure 1 shows
temperature distribution calculated using observed data.
41.5
41
40.5
40
39.5
39
38.5
38
37.5
37
36.5

_./

/,
/

0//

"""

/"'"

/

0.5

1.5

2

Figure 1. Typical temperature distribution of hollow
conductor of a pancake along waterway.
The temperature at inlet and outlet are 37.l 'C and 39.4'C,
respectively. 0 corresponds to the inlet of cooling waterand 2 corresponds to the outlet. Maximum temperaturenear midpoint is 41.1 'C . The data are observed atacceleration of 300MeV proton beams (coil curreri.t425A). Maximum temperature difference between inlet
and outlet is 5.4 'C at acceleration of 400MeV alphaparticles (coil current 628A). To maintain a constanttemperature at midpoint, dynamic range of 8'C is needed
for the cooling system according to various acceleration
condition (coil current 253-628A). To keep constanttemperature at midpoint of coils at a shift of acceleration
particle, the temperature change of iron core minimized
and rapid start-up of magnetic field is realized .

5 START UP PROPERTY OF MAGNETIC
FIELD
Figure 2 shows trend data of temperature of ring sector
magnet after 2 months interruption for maintenance. Themagnets are warmed up with high temperature coolingwater before excitation of the magnet for a week. Heatgenerated by a water pump itself can heat up coolingwater up to 40'C. It needs about 90 hours to arrive at
stable situation of temperature and magnetic field. Driftof the magnetic field was less than 10-5/day. Thetemperatures of cooling water of main and trim coils wereadjusted on occasion to stabilize magnet temperature.
Figure 3 shows the magnetic field deviation of thering cyclotron after cycling for change acceleration
particle. Keeping constant iron temperature with proper
setting of coolingwater temperature, field drift
converged less than 10-(j within 100 minutes after start ofcycling. Proper cycling procedure is also needed for
rapid convergence of eddy current effects.

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

44
water temperature(main coil)

42
40
38

water temperature(trim coil)

36
1...

34

pole temperature

32~300+102030406050

Start of peration

709080100hours

Figure 2. Temperature of the ring sector magnet. Start up trend after 2 months interruption for maintenance.

O.E+OO-5.E-06 .__ _ _......-_ _ _ ___.._ _ _ ___,__ _ _ __.__ _ __,..._.__ _ _ _....__ _ ___.
60708090100110120130

Elapsed time from start of cycling (minutes)

Figure 3 Magnetic field deviation of the ring cyclotron after cycling.

6 COLNCLUSIONS
Stable operation of the cyclotron is essential toaccelerate an ultra-precise beams in a long term.
With proper setting of cooling water temperature,stability and start up property of magnetic field issignificantly improved. Control of thermal deformationof large structure such as magnets of an accelerator ispractically impossible. The key technic is to keep or toreach quickly stationary state of thermal equilibrium.

REFERENCES
[1]

[2]

[3]

- 70-T. Saito, M. Uraki and I. Miura, Proc.of the
14th Conf. on Cyclotrons and their Applications,
Cape Town, South Africa, 1995, pp 169-172
S. Ninomiya, T. Saito, H. Tamura and K. Sato,
Proc. of the 16th Conf. on Cyclotrons and theirApplications, Michigan USA, to be published
W. Brautigalm, R. Brings, Proc.of the 14thConf. on Cyclotrons and their Applications,
Cape Town, South Africa, 1995, pp280-283