HIGHER HARMONIC MODE OPERATION IN BABY CYCLOTRON
Yasumasa Kaneda, Hyogo Yoshida and Takashi Karasawa*
The Japan Steel ~vorks, Ltd. Muroran-shi, Hokkaido*Institute of Physical and Chemical Research, .Saitama
The main items of J.S.W Baby Cyclotron are to produce-the short lived
Radio Isotope, carbon-11, nitrogen-13, oxygen-15, and fluorine-18.
The features are compactsize and reliable operation. High magnetic fieldused to make.the cyclotron small and higher harmonic mode accelerations areadopted to accelerate both proton and deuteron with same magnetic .field andshorten the resonator. The performances are listed on table l. · Plane viewof Baby Cyclotron is shown Fig.l.
Both 2nd and 4th·aqceleraticn modes are used for proton and deuteronrespectively to the prototype and the first commercial cyclotron. Two
45-degree dees in push-push excitation are used and their geometricalconfiguration is symmetrical.
Prior to acceleration test, orbits were traced by a computer for bothmodes in the magnetic field distribution measured on the electro magnets ofcyclotron. In the course of computer studies, we found that there exista large displacement of orbit center at 4th harmonic mode acceleration.
It is well known that an orbit center of an ion is displaced by the firstharmonic in the magnetic field. At the higher harmonic mode acceleration,
orbit center displacement is caused by occurrence of asymmetries of both themagnetic field and the acceleration field. The later is generated by phasedifference of the ion arriving at the acceleration gaps.!) \ve express thedisplacement of the center by the poller coordinates, (f,~), and variationof f and <p per revolution byL! p andL] rp, respectively.z.l. For the displacement
caused by the asymmetry of the electric acceleration field in addition themagnetic field, the following equations hold for the 4th harmonic mode
acceleration,
A P= 2l2V/E p sinQsin2'f +)trc, sin(\0-\v.)
IJ'f= -4V/E sinQ(l-l//2cos29') +7tr/pC,cos('f- 'If,) + 2 (Vr-1)
Where V is half of the peak to peak- R.F voltage, E is the energy of an ion at
the radius r,FQ is the phase of an ion from the peak of P.F frequency.C 1 and¥• are the amplitude and phase of the first term in a Fourier expansion of theazimuthal variation in the magnetic field. Vr is the radial betatronoscillation frequency.
As the result of computation of the orbit center displacement fordifferent positions of the ion sourse, a small change of the ion source
position is responsible for a large displacement of the orbit center.
It is known from the equation that the smaller the value of sinQ is, thesmaller the displacement is. The reduction of the value of sinG can beachieved by the high relative magnetic field near the center of the ~agnet.
In order to reduce the displacement of the center of the ion orbit further, weinstalled two pairs of harmonic coils at proper positions on the pole faces ofthe magnet. The harmonic coils generate the first harmonics of the magnetic
field at arbitrary azimuth. The first harmonics generated by the coils could
reduce the displacement caused by the asymmetries of both the electric
acceleration field and the magnetic field.
On the prototype adDpte.d the methods Tientioned above, they are very
important for reducing the displacement of the orbit center ~fter searchingfor an optimum accelerating condition using shadow technique?> Specially, thegood efficiency of the harmonic coils became clear not only for the orbit
center problem but also for the beam extraction.
So the first commercial cyclotron was designed by detailed numericalcalculations and experiTiental study, we could make a displaceTient of the orbit
center small in the initial stage. Now, we can get stabilized extracted beam-291-

and sufficient amolli~t of Radio Isotope by Baby cyclotron.
Performances of Baby Cyclotron

Table l.

~1. tern

Energy(HeV)

Extraction radius(em)'Prototype
protonproton4..7102456
2deuteron
730

1.85

(:r<IHz)

Harmonic mode

deuteron

9.4

Average magnetic field(T)RF frequency

lst commercial type

I

1.52

1.8

46.5

55

244'I-1~RGZT

30X

DOOR,--1

VA CU'c11 ?Ul1PACCELERATION B0X

Fig.l

Plane view of Baby CyclotronRe£'erencesl) T.Ka::-asawa and Y.Kaneda: Reports of I.P.C.R., (in Japa!1ese), .22, 35(1977).
2) H.L.Eagedoorn and N.F .. Verster: l·;.I.I¥1. 18-19, P.217 (l962).
3) J.R.Richardson: Progress in Nucl. Techniques
Instrunentation (I), p.66,
1965 North-Holland Publ. Amsterdarr..

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