RECENT PROGRESS IN HIGH RESOLUTION BEAM OPERATION OFTHE RCNP CYCLOTRON SYSTEMYoji Inata*, Yoichi Ohe*, Hajime Saito**, Shiro Ninomiya, Takane Saito and Kenji Sato
RCNP , Osaka University, Mihogaoka 10-1 Ibaraki –shi Osaka 567-0047 ,Japan* RCNP SAS Operator Group, ** SHI Accelerator Service Ltd.(SAS), RCNP

Abstract
The RCNP accelerator system consists of a K400,6
sector ring cyclotron and a K140 AVF cyclotron. Many
improvements and efforts have been dedicated especiallyfor the high resolution beam operation. It enables us todeliver a 420MeV He 3 beam of less than 15keV
resolution in the dispersion matching mode and 90keV
resolution in the achromatic transportation mode at atarget point.
Optimization of flat topping in the ring cyclotron, finetuning of the AVF cyclotron beam with a phase slit
system and the highly stabilized magnet systemmonitored with NMR probes are the main contributors.
Figure 1. Energy width of Ring Cyclotron Beam

1 INTRODUCTION
The accelerator system of Research Center for Nuclear
Physics (RCNP), Osaka University consists of a K400, 6
sector ring cyclotron and a K 140 AVF cyclotron.
Usually the AVF cyclotron is operated as an injector forthe ring cyclotron. The system runs for about eightmonths in a year. Remaining four months, about twomonths each in summer and winter, are dedicated formaintenance and improvement work. SHI Accelerator
Service Ltd. (SAS) is entrusted for the operation under
the supervision of the RCNP stuff. Currently there arenine resident operators working on the operation twentyfour hours a day, seven days a week with two people ineach team (working twelve hours a day). The operators
are also involved in the maintenance and improvement
work.
As has been well known, high resolution nuclear
reaction experiments are being conducted in the RCNP.
Many progresses have been observed in the last threeyears in delivering high resolution beams. Figure 1 shows
evolution of the beam energy width, measured by amagnetic spectrometer Grand Raiden through scatteringexperiment with an achromatic beam transport mode.
It is seen in the figure that noticeable improvements
were made around the year 2000. The SAS operators
contributed in many ways for these improvements, which
are summarized as follows;
A. Optimizing flat topping condition in the ringcyclotron.
B. Fine tuning the AVF cyclotron in injection andextraction,
C. Stabilizing the magnetic field, monitored by
improved NMR system of eight digits .

2 OPTIMIZING OF THE FLAT TOPPING
CONDITION
An RF cavity of the third harmonic frequency isequipped in the ring cyclotron for flat topping (FT) of theacceleration voltage wave form.
It was difficult to tune the FT system in phase andvoltage, because;a. There is no way to observe the synthetic voltage waveform,b. It is difficult to measure the FT voltage accurately.
c. Voltage distribution is different between the main
cavity and the FT cavity. Local voltage matching ismeaningless.
A voltage correction factor of 1.3 had been used for along time in the setting of the FT voltage.
One day in 2000, there was an opportunity to operate thesystem just after recovery of an FT trouble. A highresolution beam was obtained unexpectedly.
An
inspection revealed the correction factor was accidentally
set to be 1.75. The new correction factor has also madephase adjustment much easier.

Figure 2. FT voltage and beam energy width (Proton 300 MeV)

Then fine tuning of the FT voltage after the phaseadjustment has been recognized to be very effective to thefinal ultimate resolution. Figure 2 shows an example ofthe voltage tuning.

3 FINE TUNING IN THE AVF
CYCLOTRON
Figure 3 shows a beam pattern in the outer region of theAVF cyclotron measured by a deflector entrance probe.
Accelerated particle is He3 to inject to the ring cyclotron
for the final 450MeV beam. It is seen in the figure that theturns are almost separated especially in the outermostregion.

There are two functions of the DEE side phase slit. One
is to shape the beam in radial direction, the other is todeflect the beam orbit by the electric field penetratingfrom an opening of the electrode 2. The effect of theopening is shown in the figure 4. The line b indicates abeam orbit at a timing of the peak accelerating voltage,line a indicates it at ten degrees earlier than the peak andline c indicates ten degrees later. The orbit of the line ais shifted over the line b. It enables us to cut line acomponent by the ordinary phase slit, and to select thebest part of the injecting beam.
With this adjustment, beam time length is shortened andthen turns are separated.

4 NMR FIELD MONITOR IN THE AVF
CYCLOTRON (TO THE EIGHT DIGITS)

Figure 3. Beam pattern in the outer region measured by deflector
entrance probe(thick line: differential probe thin line: integral probe)

Main points in tuning for separated turns are as follows;a. Optimizing DEE side phase slit position andbuncher condition. Cutting peripheral beam by an
ordinary phase slit.
b. Precise setting of the AVF cyclotron magnetic fieldunder isochronous field established .
c. Fine tuning of valley coils in the outermost region.

Recently it became clear that the beam quality improvesat highly stabilized magnetic field through stabilizing thetemperature of the cyclotron magnets. It is required tomonitor the magnetic field stability with the accuracy of1ppm.
Although an NMR instrument can measure field in theprecision, it was difficult to apply to the AVF cyclotron.
Figure 5 shows the magnetic field distribution, where flatpart of the magnetic field is very little. To perform therequired measurement of the accuracy, it must be ensured
that the NMR probed head is small enough, field gradientover the probe is properly corrected2) and a controller has
8 digit capability.

The DEE side phase slit1) is a supplemental phase slit
equipped inside the DEE electrode of the AVF cyclotron.
While the slit was installed in 1997, it was not usedeffectively for a long time.
Recently it has been realized that the slit can be very
effective, based on the progress of beam turn monitoring
and beam injecting condition with a buncher. Using theDEE side phase slit and the ordinary phase silt
simultaneously, beam length is shortened very effectively.
Conceptual structure is shown in the Figure 4. Electrode3 is the DEE side phase slit. Number 4
Figure 5. Magnetic field distribution in the AVF cyclotron.

Figure 6. show the improved probe head and acorrecting coil set. Increasing turn number of themodulation coil, the modulation amplitude is increased
to10mT from conventional 1-2mT.

Figure 4. DEE side phase slit and incidence beam orbit

The achievements described above are the result of
every day work of the following members taking part insustaining operation of the RCNP cyclotron system.
RCNP STAFF
Mituru Kibayashi, Hitoshi Tamura, Shiro Ninomiya,Takane Saito, Kenji SatoSAS Operator Group
Sigeyuki Mine , Zennosuke Taisei , Yoji Inata , Hideki
Gotoh, Hiroyuki Yana , Yoichi Ohe ,Masaaki Hikake , Yasuteru Kotaka, Kinya Masuda
Figure 6. NMR probe head and correcting coil set

5 REFERENCES
Figure 7 shows the correcting coils. The correcting coils
are thin and do not disturb the isochronous field.
Conventional NMR controller was of 7 digits. The 8digits capability is necessary to monitor the field in the0.1ppm accuracy and the controller has been developed.
By detecting a very little field change, it has become
possible to recover the beam fluctuation.

Figure 7. Correcting coils[1] Y. Sasaki, RCNP Annual Report 1996, 178-181[2] M. Kibayashi, Proc. of the 12th Symposium onAccelerator Science and Technology, Wako(1999) 319321