The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Vacuum Design ofRF-resonator for the RIKEN Intermediate-stage Ring Cyclotron
Toshikazu HIASA, Yukio KUMATA, Kouichi UNO, Toshinori MITSUMOTO*, Naruhiko SAKAMOTO*,
Kumio IKEGAMI*, Kazuo SUGII*, Yoshitoshi MIYAZAWA*, andAkira GOTO*
Sumitomo Heavy Industries, Ltd. (SHI)
5-2, Soubiraki-cho, Niihama-shi, Ehime 792-8588, Japan*RIKEN (The Institute ofPhysical and Chemical Research)
2-1, Hirosawa, Wako-shi, Saitama 351-0198, JapanAbstract
A four-sector ring cyclotron (K=980) has been designed
as an accelerator in the RIKEN RI beam factory project.
Since heavy and highly-charged ions are mainly accelerated
in this cyclotron, the chambers such as an RF-resonatormust be kept ultra-high vacuum. From this point of view,
the structure of the main resonator has been studied so as toreduce the out-gassing into the beam space. A structure of
duplicated walls has been designed; this structure reduces
the out-gassing into the beam space less than half of thetotal out-gassing into the resonator. The displacement of theresonator by the atmospheric pressure has also been
confirmed to be tolerable.

1. Introduction
A four-sector ring cyclotron has been designed as an
Intermediate-stage Ring Cyclotron (IRC: K=980) [1] whichis installed between a four-sector ring cyclotron (RRC:K=540) and a six-sector superconducting ring cyclotron(SRC: K=2500). Heavy ions are accelerated through thesethree cyclotrons up to high energy (some hundreds MeV/u),and are utilized to produce radioactive ion beams (2]. The
IRC is under manufacturing at Niihama works, SumitomoHeavy Industries, Ltd. (SHI).
Figure 1 shows a plan view of the IRC. The vacuum
chambers of the IRC consist of four sector magnet
chambers, two main resonator chambers and two valley
chambers including a flattop resonator. Ions are accelerated
on the median plane in these chambers.
The pressure must be kept ultra-high vacuum in order toreduce the beam loss due to the scattering by residual gas
molecules. The ultra-high vacuum allows us to achieve highbeam transmission efficiency, and as a result, reduce
activation of the cyclotron components.
The pressure required in their path is about 1x10-6 Pa.
Actual pressure of the RRC, the size of which is similar tothat of the IRC, is about 2x10- 6 Pa [3], twice the requiredpressure for the IRC. In case of the RRC, the out-gassing of
the main resonator is approximately 60 % of total outgassing. Therefore we made it our main design issue toreduce the out-gassing of the main resonators.

Fig. 1 Plan view of the RIKEN IRC.

2. Structure of Main Resonator
Figure 2 shows a schematic drawing of the IRC main
resonator designed. The main resonator is of variablefrequency (18-38.2 MHz) single gap type, similar to that of
Osaka University RCNP ring cyclotron [4]. The resonant
frequency is varied by rotating a pair of flapping panels.
The panels are electrically connected to a copper wall withhinges and flexible copper panels.
We have decided to adopt a structure of duplicatedwalls for the resonator chamber as shown in Fig. 3, in orderto save a total cost for manufacturing compared with thecase of the RRC main resonator [5], which uses a copperclad stainless steel wall. Outer stainless steel wall of 45mm
in thickness is a structural material for partition between thevacuum and the atmosphere, so that the outer wall hasenough rigidity against the atmospheric pressure. Innercopper wall of 4mm in thickness is used as cavity conductor.
Seamless copper tubes are soldered on the outer surface ofthe copper wall for cooling water flow. In order to achieve
an ultra-high vacuum, we have designed the inner volumeof the resonator to be separated into two parts with thecopper wall from the viewpoint of vacuum. The 25mm gap~293~

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

between the stainless steel and copper walls is called a "subvacuum part", which has large out-gassing elements likerubber o-rings and is evacuated by two turbo-molecular
pumps of 0.5 m3/s. The inside of the copper wall is called an"ultra-high vacuum part", which is evacuated by threecryogenic pumps of 10 m3/s. A turbo-molecular pump of 5
m 3/s is installed on the "ultra-high vacuum part" as primaryevacuation. The total pumping speed is 30 m 3/s, the same as
the case of the RRC. The "ultra-high vacuum part" and the"sub vacuum part" are sealed each other by only metal ·touching. Therefore a small amount of gas flows from the"sub vacuum part" to the "ultra-high vacuum part".

Trimmer

Acceleration
Electrodesurface between magnet chambers and resonators are sealedby pneumatic expansion seal [6], so that the resonators areable to be withdrawn easily along rails for the maintenance.
The main resonator of Osaka University RCNP ring
cyclotron also has duplicated walls. However, it has manyholes on the copper walls to get sufficient conductance
between the separated parts. In this case, the two separated
parts are considered as one vacuum chamber.

3. Out-gassing
As mentioned above, the vacuum chamber of the IRC
main resonator can be considered as separated two parts.
The two parts are evacuated with a "differential pumping
method", the "ultra-high vacuum part" is evacuated by atotal pumping speed of 30 m3/s, while the "sub vacuum
part" is evacuated by a total pumping speed of 1 m 3/s. Thereis a small conductance between two parts. Figure 4 shows aschematic diagram for the calculation of the differential
pumping method.

Cryogenic Pump

Chamber 1

Chamber 1~~--

Turbo-molecular Pump
(for Sub Vacuum)

Connecting

Fig. 2 Schematic drawing of the IRC main resonator.

~~
Chamber 2

Chamber 2Fig. 4 Schematic diagram for the calculation of thedifferential pumping method. P 10 , P 1 and P 20 , P 2 are thepressure (Pa) in each chamber before and after connecting
respectively. Q 10 , Q1 and Q20 , Q2 are the out-gassing(Pa·m 3/s) in each chamber before and after connecting
respectively. C is the conductance (m 3/s) between chamber
1 and 2.

Soldered Copper Tube

Ultra-high Vacuum(lxiQ·6 Pa)
Copper WallAt the equilibrium condition after connecting chamber
1 with 2, the out-gassing in each chamber are expressed bythe equation

Stainless Steel
4Fig. 3 Schematic drawing of the structure of duplicatedwalls.
A mechanical safety valve and an electrical safety valve
are installed in duplication between the "ultra-high vacuum
part" and the "sub vacuum part", so as to avoid mechanicaldisplacement by the pressure difference on the copper wall.
The magnetic fluid sealed bearings are adopted as therotating bearings for the rod of the flapping panel, in orderto reduce the leak rate from the atmosphere. The interface

Q1 =Q10 +C·(P2 -P1 )(1)

Qz = Qzo - C · ( Pz - P1)(2)

On the other hand, as the pumping speed is constant, thepressure is proportional to the out-gassing in each chamber.
Therefore, ratio of the pressure after to before connecting isexpressed as

P1 = Qw ·Qzo +C·Pzo ·Q1o +C·Pzo ·Qzo
Pw Q1o · Qzo + C · Pzo · Q1o + C · Pw · Qzo(3)

As the difference of the pressure between "ultra-high-294-

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

vacuum part" and "sub vacuum part" is usually more than
two orders, the equation (3) is simplified to~=d+P10

CPzo·Qzo
Qw ·Qzo +C·Pzo ·Q1o(4 )reduce the displacement and the stress. Although the backside wall at the feeder position moves by 0.3 mm toward theinside, the displacement can be compensated by stroking thefeeder. Although the side wall is bent by 1.5 mm at themaximum point, shift of the resonant frequency can be also
compensated by the trimmer or flapping panel. The inclineof the rotating rod of the flapping panel was estimated about
3 mrad, which is within the allowance of the bearing. It wasalso confirmed that the stress of the support material
between side wall and neighboring magnet chambers wasallowable.

One can say that this ratio of P 1 to P 10 gives an effectivenessof the differential pumping method.
In the case of the IRC main resonator, we estimated thatthe out-gassing in the "ultra-high vacuum part" Q 10 =2.3x10.5 Pa·m3/s, in the "sub vacuum part" Q 20 = 6.6xl0·5Pa·m 3/s and the pressure of the "sub vacuum part" P 20 ==5x10'4 Pa. According to equation (4), the conductancewhich causes P 1 going up to 150 % of P 10 can be calculated
about 0.03 m3/s, and the gas flow from the "sub vacuum
part" to the "ultra-high vacuum part" can be calculated
about 1.2xl0·5 Pa·m 3/s in that case. The total out-gassing of
the "ultra-high vacuum part" goes up to 3.5x10·5 Pa·m 3/s as
the result. This value is still less than half of the total outgassing of the resonator (=Q 10 +Q20 ) and is also about half of
the RRC main resonator. The conductance value of 0.03
m3/s is equivalent to a circular hole of 20 mm in diameter.
We have thus decided to adopt such a duplicatedstructure, controlling the total conductance less than 0.03
m3/s. It is possible to control this value of the conductanceby means of only metal touching seal.

We have designed a structure of the main resonator for
the RIKEN IRC. A duplicated structure, which consists ofan "ultra-high vacuum part" and a "sub vacuum part", hasbeen investigated. As each part is sealed by only metal
touching, a "differential pumping method" has been adopted.
A suitable value of the conductance between the two partsin order to achieve an ultra-high vacuum has beenconfirmed. The out-gassing in the "ultra-high vacuum part"has been reduced less than half of the total out-gassing in
the resonator. Furthermore, we have confirmed that thedisplacement by the atmospheric pressure is allowable.

4. Structural Analysis

References

The atmospheric pressure is loaded against the outerwalls made of 45 mm-thick stainless steel plates. As theside walls of the resonator is not strong enough to support
the atmospheric pressure, it should be supported by theneighboring magnet chamber. Figure 5 shows displacementof the resonator by the atmospheric pressure, calculatedusing FEM (Finite Element Method) computer code.

[1] T. Mitsumoto et a!., "Design Study of the IRC RIKEN
RI-Beam Factory", RIKEN Accel. Prog. Rep. 32,(1999) 195.
[2] Y. Yano et a!., "Progress of RIKEN RI Beam Factory
Project.", RIKEN Accel. Prog. Rep. 30, (1997) 195.
[3] H. Saito et a!., "RIKEN Ring Cyclotron", SHI Tech.
Rev. 35, (1987) 103.
[4] I. Miura et a!., "The RF system for the RCNP RingCyclotron", Proc. 7th Symp. on Ace. Sci. and Tech.,(1989) 89.
[5] T. Fujisawa et a!., "Design of Radio Frequency System
for the RIKEN Separated Sector Cyclotron", Sci. Papers
I.P.C.R., 79, (1985) 12.
[6] A Shimizu et a!., "Vacuum System of the RCNP RingCyclotron", Proc. 7th Symp. on Ace. Sci. and Tech.,(1989) 142.

5. Conclusion

Fig. 5 Displacement of the resonator by the atmosphericpressure, calculated using FEM code.
Four ribs were added on the back-side wall in order to-295-