STATUS REPORT ON THE INS SF CYCLOTRON

XI-5

K. Sato, T. Tanabe, M. Fujita, T. Yamazaki, T. Honma,
N. Yamazaki, Y. Sakurada, Y. Ohshiro, T. Yamada~ H. Ogawa~M. Sekiguchi andY. Hirao
Institute for Nuclear Study, University of Tokyo*National Institute of Radiolagical Science
Abstract
The INS 176 ern Sector Focusing Cyclotron is a type of thevariable energy and multi-particle acceleration. The firstextracted beam was obtained in April, 1974. Eight targetstations are now available for experiments.
This paper describes the status, operation experiences,performances and improvements.
1.

Introduction
The INS SF cyclotron has three spiral sectors and a selfoscillator RF system.
The beam transport system consists oflow, medium and high resolution courses. Fig~.l and 2 show aplan view of the cyclotron and the layout of the beamtransport system, respectively. Design study, mechanicalstructure and measurement of the magnetic field have beenreported in the previous papers 1,2).
After the initial construction of the cyclotron was completed, various kinds of ions have been accelerated to variousenergies. In these operations, we had some troubles and wenoticed that improvements on each element were required for
obtaining a more stable operation and a better beam quality.
Considerable efforts were devoted to improve the various partsof the cyclotron and develope new beams.

Fig. 1Planview of the INS SF cyclotron

265

38-2

3B-3

Fig.2

Layout of the beam transport
Abbreviations are; Vl=vacuum valve #l;STlH=steerer #1,horizontal;S=slit and stopper box;E=emittance measurement
device;QT=quadrupole triplet;VP=vacuum pump;SW=switchingmagnet;P=profile monitor;QDl-l=quadrupol e doublet #1
operating on the power supply #l;SS=shield shutter;BA=beam analyzer;SX=sextupole magnet;2B-l=beam course #1 in
the cave 2B.

Status and Goal of Performance
Variable energy and multi-particle acceleration
Table 1 shows the design values of the maximum energy andthe achieved values of the energy and intensity for various kindsof ions.
A goal of the maximum proton energy is 48 MeV, while theachieved maximum energy is 35 MeV, which is restricted mainly by
the limited frequency range of the self-oscillator RF system(see section 3.2.).
2.
2.1.

Beam transport system
The high resolution courses can be operated either in adispersive or in a non-dispersive mode. Typical performances inthe both modes to the 3B-l course are as follows; about 2% of theextracted beam from the cyclotron is transmitted to the target in
the dispersive mode with a beam spot size of 1.5 mm(W)x 2.5 mm(H),and about 25% in the non-dispersive mode with a beam spot size of
2.6 mm(W)x 5.2 wm(H).
The energy resolution was measured in the dispersive mode.
The narrow resonance at 14,233 MeV in the excitation function ofl2c(p,p)l2c was observed. Horizontal widths of the object and imageslits(S6 and 88 in Fig.2) were set to be 1 mm. The measured 6E ofabout 2 keV was somewhat larger than the calculated resolution of11500, which is corrected up to the second order aberration byusing two sextupole magnets(SXl and SX2 in Fig.2).
2.2

266

Table 1Ionpp (poL)

Goal and status of the beam performance

Goal
Designed Energy(MeV)

XI-5Status(March, 1978)
EnergyExternal Beam Currents(MeV)

7 7 -483548308llA

d3 He++

13 -

3434

815 nAllA

19 -90908llA

4He++

25

68

688llA

12c4+

8888305

llA

14N4+

76

40 *SuOllA

14N5+

- 118

115

4o3llA

1605+

- 103

104

4o5llA

1606+

- 153

129

Oo23 llA

20Ne5+

83

60 *OolllA

20Ne6+

- 119

115

L5llA

20Ne7+- 162

150

Oo02 llA* 3rd harmonic acceleration
Cyclotron
3.
3.1. Electromagnet
The cyclotron has 11 pairs of circular trim coils, of which
8 pairs are usually used, since we have only 8 power supplie~.
More power supplies are necessary for proton beam of a higher
energy than the achieved.
3.2. RF system
The resonator is a coaxial type of a 1/4 A mode with amovable short. The resonant frequency of the resonator ranges
from 7.5 MHZ to 22.5 MHz. The RF system consists of a booster,a self-oscillator, a dee voltage stabilizer and a frequencystabilizer. The power is supplied to the resonator through an
RF coupler, which is a movable capacitor on the top of a coaxial
The RF system is described in refs. 1 and 3.
feeder.
In December of 1976, a serious damage to the coaxial feeder
happened. The inner conductor was isolated with alumina insuIn the first design,lators at the both ends of the feeder.
the space between the inner and outer conductors was in thevacuum. A high dee voltage operation of about 90 kV caused adischarge inside the feeder. After this damage, such a discharge took place even in a lower dee voltage. An alumina
In this design, theinsulator of a new design has been made.
space inside the feeder becomes to be in the atomosphericpressure and the va~uum is sealed at the top of the feeder byusing "o" rings. After this improvement, no serious damage tothe feeder has occured.
The frequency range of the RF system has been limited toThe upper limit may be caused by the follow7.5 MEz-17.5 MHz.
ing characteristics of the resonator including the RF coupler.
At a higher frequency, the input impedance is too samll tomatch the internal resistance of the power tube. The impedance267

increases as the capacitor of the RF coupler approaches to thedee. Unfortunately however, it is impossible, because undesirable discharges occur at the gap between the dee and the coupler.
At a lower frequency, the load impedance is large enough tomatch the internal resistance of the power tube. Such a highload impedance results in high grid current and high griddissipation. Actually, the grid current exceeds the maximum
rating. Moreover, the RF oscillation does not start easily.
In order to overcome this situation, a resistance of the gridleak has been raised from 1 k~ to 1.6 k~ and a grounded capacitor(200pF) has been attached to the plate of the power tube in thecase of the lower frequency.
3.3. Ion sources
A standard ion source of a hot filament type is used forlight ions such as p,d, 3He++and a, and is shown in Fig.3.
The filament material has been recently changed from tantalum towolfram, because a wolfram rod of 2 mm in diameter is available
on the market. In heavier arc operation, for example, with 200v and 1 A in arc voltage and current, a rather short life (10hours) of the filament is observed and is a problem on the ion
source still to be solved.
A heavy ion source of a cold cathode PIG type has beendeveloped.
It is shown in Fig.4. Table 1 shows the heavy ion
beams obtained by this source. Typical working point of thepulsed arc discharge in a 33% duty cycle operation is, forexample, 150 V and 7 A in average arc voltage and current forthe production of 20Ne6+ ions. The cathode life is 4 - 14 hours.
Unfortunately, the base insulater of alumina was destroyed in a
recent run. This damage was supposedly caused bymechanicallyweak points of the alumina.

1, ANODE (COPPER}
2, CATHOOE(TANTAlUM)

3, INSULATOR (BORON NITRIDE l4, INSULATOR(ALUMIN.A)
5, SLIT(TANTALUI<I)

6, CAP (TANTALUM)
7, WATER
8, GAS

0Fig. 3268

Scm

Sectioned views of theion source of a hot
filament typeFig. 4

Sections through theheavy ion source of acold cathode PIG type

XI-5

I
I

I

I:Jc~~/..e~-1_t----Ji ""'t-,---'

I:JI~

rti trI

I
I

Fig.5 Layout of the central region for an internal ion source
3.4. Central Region
The initial structure of the central region has beenreported in the paper4). A gap between the dee and the dummy
dee was 42 mm even at the central region and very wide. Sucha structure was recongnized to be inadequate, especially in the
3rd harmonic acceleration of heavy ions, from the fact that theobtained beam intensity of 14N4+ ions in the 3rd harmonic mode
was less than that of l4N5+ions in the fundamental mode. Thewider gap is considered to affect severely the ion path in the
3rd harmonic acceleration.
A new central region has been designed and installed. Fig.
5 shows the improved central region. The dee-and dummy deeinserts make a gap narrower and shield
the initial ion pathfrom the RF field. After the improvement, a reasonable beam
intensity has been obtained in the 3rd harmonic acceleration ofheavy ions. An improvement on the puller system has beencontinued; the base of the nuller slit was cooled with a forced
water and an interceptor of the unwanted beam was set on thebase.
3.5. Beam extraction
The beam extraction system consists of two electrostatic
deflectors. Parallel plates and quadrupole type electrodes
were chosen for the first and second deflectors, respectively.
In our design, the septum electrodes are isolated and we canmeasure septum current.
The extraction efficiency through two deflectors is 4060%.
In August of 1976, a damage of the septum of the firstdeflector occured. Entrance of the septum has a V-slot
structure of 0.2 mm thickness and it melted. Several holes
were also opened on the septum. At that time, the septum
current was not limited in the cyclotron operation. Since then269

we have operated in the restricted maximum septum current of
4 ~A· Recently, we examined
the septum and a slight damage
was noticed at the entrance of the septum. So the limited valueof 4 ~A is considered to be reasonable.
Considering the extraction efficiency of the beam deflector
system and the restriction on the septum current, intensity ofthe extracted beam will not be increased so much. Now, adeflector system of a new design has been designed and isunder construction.
3.6. Beam diagnostic elements
Three beam probes can be used at the same time; dee probe,deflector probe and main probe.
The dee and deflector probes
are of a differential type.
The main prob6 is used as one of
the fourtypes of differential, three fingers, phase and beamstopper.
In a standard service of the cyclotron, the main probe
is operated as the beam stopper.
Beam emittance of the extracted beam is measured by using
the emittance measuring deviceS), which is placed at the initialstage of the beam transport system.
Recently, the device isconnected to the computer TOSBAC-40C for quick measurement andanalysis.
3.7. Operation
The ion source and the puller are set at the calculated
positions.
The position of the ion source is adjusted byrotating its own axis so that the beam becomes maximum. Currentsof the trim coils are, in principle, set to the calculatedvalue. But currents of the most inside and outside trim coils
are adjusted by operators.
In order to obtain the maximum beam
current , operators adjust the following other parameters;
position of the center plug, currents of the valley andharmonic coils, positions of the septum and deflector, and thedeflector voltage. Dee voltage is fixed to a calculated value,
which depends upon the energy, the kind of the particle and theturn number.
4Beam Transport System
4.1. Achromatic beam transport
Beam analyzing system of the high resolution courses
consists of two quadrupole and two dipole magnets. These arearranged in order of QDDQ.
This system cannot be operated ina doubly achromatic mode, while it can work in a singly (image)achromatic mode.
In recent experience of the achromatic
operation for the heavy ion beam, it was found that the beamenergy resolution became worse when slits of the beam transport system were made narrower. This effect is supposedly due
to a slit scattering.
REFERENCES
l. Y. Hirao et al., Seventh International Conference on
Cyclotrons and their Applications (Zurich, 1975) p.l03
2. Y. Hirao et al., ibid., p.312
3. M. Fujita et al., Proc. of this Conference, 24Ca7
4. INS Annual Report 1974, p.S
5. T. Honma et al., Proc. of this Conference, 23Ap2270