Proceedings of IBIC2012, Tsukuba, Japan

MOPA07

DEVELOPMENT OF PHASE PROBE FOR THE NIRS SMALLCYCLOTRON HM-18
Satoru Hojo#, Ken Katagiri, Akira Goto, NIRS, 4-9-1 Anagawa, Inage, Chiba, Japan
Yuichi Takahashi, Toshihiro Honma, AEC, 3-8-5 Konakadai, Inage, Chiba, JapanThe small cyclotron HM-18 of the National Institute ofRadiological Sciences (NIRS) allows us to accelerate Hand D- ion at fixed energies of 18 and 9 MeV,
respectively. It has four trim coils for generation of theisochronous fields. Until recently, currents of the fourtrim coils had been adjusted only by monitoring theoutput beam intensity. In order to exactly produce theisochronous fields, a phase probe has been newly installedin the HM-18. The phase probe has a simple structure inwhich four copper electrode plates of 55~76 mm x 68 mm
in area are glued to a copper base plate with a polyimide
insulator sheet sandwiched between them. The
thicknesses of the copper plates and the polyimide are 0.1
mm. This structure has an advantage that it can be easily
installed in the cyclotron; only one part of a pair of upperand lower electrodes, which is usually adopted incyclotrons, is simply attached on the surface of the(lower) sector pole. The development of the phase probe
and some results of a preliminary beam test using it arereported.

INTRODUCTION
The small cyclotron HM-18 of the NIRS has been
operated for use in RI production since 1994[1]. The HM18 cyclotron is a negative-ion accelerator that waspurchased from Sumitomo Heavy Industry, Ltd. A layoutof the HM-18 is shown in Figure 1. Two carbon-foil
strippers are located at a radius of 435 mm for beam
extraction; they are located in the opposite sides to eachother. In one side beams are delivered to a beam transport
line, which is jointed to that of the NIRS-930 cyclotron.
In the other side four target ports are attached directly tothe beam chamber for production of short-lived radio
isotopes such as 11C and 18F. The HM-18 has an internal
cold-cathode ion source that produces H- and D- ions,which are accelerated up to 18 and 9 MeV, respectively,
with the acceleration frequency of 45 MHz. The
acceleration harmonics are 2 and 4 for H- and D- ions,respectively. Four trim coils are used for generation of theisochronous fields. Until recently, currents of the four
trim coils had been adjusted only by monitoring theoutput beam intensity; the isochronous field wasindefinite because the beam phase was not able to bemeasured. Therefore, a phase probe has been newly
installed in order to exactly produce the isochronous
fields in the HM-18.
___________________________________________# s_hojo@nirs.go.jp

Figure1: Schematic layout of the HM-18 cyclotron.

STRUCTURE OF PHASE PROBE
Photograph and cross-sectional view of the new phaseprobe is shown in Figure 2. This phase probe has a simple
structure. Usually, a phase probe is composed of a pair ofupper and lower electrodes, but the new phase probe for
the HM-18 is composed only of lower part. It consists offour copper plates of 55~76 mm (azimuthal width) x 68mm (radial length) in area glued to a copper base platewith a polyimide insulator sheet sandwiched between
them. Adhesive double-coated tapes are used for theattachment of them. The thicknesses of the copper plates
and the polyimide sheet are 0.1 mm. Semi-rigid coaxial
cables are used for transport of beam signals in the beamchamber. The outer conductor of the cable is made ofcopper tube with 1.2 mm in diameter. The ends of theinner and outer conductor are soldered to the fins of thepickup electrodes and the grounded copper plate,respectively, as shown in Figure 2.
This structure has an advantage that it can be easily
installed in the cyclotron. Photograph of the phase probe
installed in the HM-18 is shown in Figure 3. The phaseprobe and signal cables are simply attached on the surfaceof the lower sector pole with adhesive aluminium tapes.

ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

MOPA07

Proceedings of IBIC2012, Tsukuba, JapanThe layout of the new phase probe and four trim coils(C1~C4) in the HM-18 is shown in Figure 4. The inneredge of the first electrode (PP1) is set at the radial
position of 150 mm from the cyclotron center, and theouter edge of the fourth electrode (PP4) is set at 425 mm.

Figure 4: Layout of the new phase probe and four trimcoils (C1~C4).

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○MEASUREMENT OF BEAM PHASE AND
ADJUSTMENT OF TRIM COIL
CURRENTS
Beam pickup signals from the phase probe wereobserved with an oscilloscope. An example of waveform
for a 18 MeV H- beam is shown in Figure 5. In this casethe beam intensities were 22 A at the radial position of150 mm and 20 A at the extraction of the cyclotron.
Cross sectional view at A-A
Figure 2: Photograph and cross-sectional view of thephase probe.

Figure 3: Photograph of the phase probe installed in theHM-18.

Figure 5: Waveform of a beam pickup signal from thePP1 for a 18 MeV H- beam along with the rf signal. Theyellow line indicates the pickup signal and the red line therf signal (45 MHz). The pickup signal was obtained bytaking the difference between the two signals when thebeam is on and off. The beam intensity was 20 A.

ISBN 978-3-95450-119-9
2Beam Charge and Current Monitors

Proceedings of IBIC2012, Tsukuba, Japan

MOPA07

At first, the phase excursion of the beam was deduced
from the pickup signals thus obtained of the fourelectrodes for the parameter set of main and trim coilcurrents that had been used in routine operation. The
result is shown in Figure 6 by blue diamonds. The
deviation of the beam phase was 17o at the maximum.
Then, the coil currents, mainly those of the main and C2
coils, were adjusted in order to make the phase excursion
as small as possible. The obtained result is shown by red
squares in Figure 6. The phase excursion could be
reduced to 9o at the maximum. The beam intensity was
not improved by this adjustment.
Figure 7: Phase excursion of a 9 MeV D- beam. The blue
diamonds indicate the excursion for the parameter set ofmain and trim coil currents that had been used in routine
operation. The red squares indicate the one obtained after
adjustment mainly of the main current. The beam
intensity decreased from 14 A to 11 A in thisadjustment.

Figure 6: Phase excursion of a 18 MeV H- beam. The blue
diamonds indicate the excursion for the parameter set ofmain and trim coil currents that had been used in routine
operation. The red squares indicate the one obtained after
adjustment mainly of the main and C2 coil currents.
The measurement of the phase excursion and theadjustment of the coil currents were also performed for a
9 MeV D- beam. The results are shown in Figure 7. The
blue diamonds indicate the excursion for the parameterset of main and trim coil currents that had been used inroutine operation. The phase increased almost
monotonously with the radius up to 60o. This phase
excursion was improved by adjustment mainly of themain coil current; however, the beam intensity decreased
from 14 A to 11 A. This means that for theoptimization it is necessary to adjust not only themagnetic field distribution in the acceleration region but
also the one in the center region, which affects the beam
intensity significantly.

In order to help measure the phases of 18 MeV H- and 9
MeV D- beams, which are accelerated in the existing HM18 cyclotron, a phase probe has been newly developed. It
has a simple structure consisting of four copper electrodeplates, a polyimide insulator sheet and a copper base platethat are sandwiched and glued with adhesive doublecoated tapes. The thicknesses of the copper plates and thepolyimide sheet are 0.1 mm. The phase probe is simply
attached on the surface of the lower sector pole withadhesive aluminium tapes.
It was confirmed that this phase probe could provide
signals sensible with an oscilloscope for the phasemeasurement. It was found that the magnetic field
distributions of the HM-18 were not necessarily
isochronous ones: in particular for a 9 MeV D- beam. Thephase probe is expected to allow us to obtain the optimummagnetic field distributions for the HM-18.
The flexible structure of this phase probe can beapplicable in other places such as the inner surface of apipe in a beam transport line.

REFERENCES[1] S.Hojo et al., “Present status of cyclotrons (NIRS-930, HM-18) at NIRS”, Proceedings of thePASJ2012, WEPS008, (2012).

ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○CONCLUSION