The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

RF CAVITIES OF THE RIKEN CHARGE-STATE MULTIPLIER
0. Kamigaito 1, N. Sakamoto, M. Kase, E. Ikezawa, S. Kohara, Y. Miyazawa,T. Chiba, M. Hemmi, Y. Chiba, A. Goto, andY. Yano,
RIKEN, Wako-shi, Saitama, 351-0198, Japan

Abstract

Charge-State Multiplier (CSM)

The RII<EN heavy-ion linac has been upgraded fornuclear physics experiments, by adding six booster
cavities followed by a new beam line with six
experimental courses. Using the booster, intense beams ofheavy ions having mass-to-charge ratio of less than 5.6
will be accelerated up to 5.8 MeV/u. A decelerator cavity
has been also installed in the injection line to the ringcyclotron. The detailed structure of the cavities, as well as
the rf characteristics, are described in this paper.

( 5.8 MeV/u ( A/q < 5.6) )···r,-1 INTRODUCTION
In the RI-beam factory project, a system called "chargestate multiplier (CSM)" has been proposed to extend theenergy range of heavy-ion beams[1]. It is a combinationof a booster linac, a charge stripper and a decelerator linac,
which is placed between the heavy-ion linac (RILAC) a00
the ring cyclotron (RRC). By use of this system, themost ptobable charge-state of the beam after the stripper
is shifted upwards, and the beam energy from the RRC
can be extended to a higher value than that obtained withthe present stripper. The total accelerating voltage required
for the booster is 16 MV and that for the decelerator is 8MV.
The design study of the CSM started in 1997. The
most significant problem was that the area for theinstallation is quite limited, whereas the required energy
gain is considerably high as mentioned above. Firstly,therefore, the frequency of the CSM was chosen to be
twice the RILAC frequency in order to shorten the rfcavities. Secondly, the separated-type drift tube structure
has been adopted for the cavities so that the rf power
losses could be reduced; all the focusing elements are put
outside of the cavities. Under these conditions, theoptimization has been done by iterative simulations bothfor the beam dynamics and for the cavities.
The final configuration is illustrated. in Fig. 1. The
booster and decelerator are divided into three "units", eachof which contains two rf cavities of the booster and one
cavity of the decelerator. The three cavities in the same
unit have the same shape and dimensions except for thedrift tubes.
·In 1999, three cavities of the low-energy part wereconstructed, which have shown good rf characteristics
such as a wide variability of the frequency and high Qvalues[2]. In the same year, construction of the booster
1r· :·3·., (D3) ~(designed)··!

~~>-<;-i :2 :(D2);!
-·
Sl~- '·rDl~:

J-36 -76.4 MHz.JRRCFigure 1: Conceptual drawing of the CSM. Thecavities are represented by circles. The numbers in thecircles indicate the ''unit" of the CSM.
cavities of the high-energy part started, prior to making
the full structure of the CSM. The aim of this decision isto initiate the experiments as soon as possible, using very
intense beams from the booster with energies around thecoulomb barrier of atomic nuclei. Since the beam energyfor these experiments should be as high as possible, thefour cavities of the booster are operated at almost thehighest frequency.
The present status of the CSM is also indicated in Fig.
1; six booster cavities and one decelerator cavity havebeen installed between the RILAC and the RRC[3,4]. The
detailed structure of the cavities, as well as the rf
characteristics, are described below.

2 DESIGN

2.1 Main Parameters
The acceptable mass-to-charge ratio (Aiq) of the booster
was chosen to be the same as that of the RILAC; using
the CSM frequency f, Alq of the booster is expressed as
Alq=32,000(f. On the other hand, that for the decelerator
is given by Alq=16,000/f.

kamigait@postman.riken.go.jp-229-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Table 1: Design. parameters ofth e cavities.
C.ma)
Coutb)
Cavigap
l{J.
v.av
LgaP.
(kV)
(mm)
(de g)
ty
Al
5.88
450
80
5.00
-25
8
A2
450
5.88
6.78
87-25
8
Dl
450
6.78
5.00
83
8-155
A3
7.71
470
6.78
-2593
8A4
7.71
470
8.65
-25
99
8
D2
6.78
470
968.65-155
8A5104
8.65
9.40-25500
610.16 500
A69.40108-256D3106
10.16 8.65
500-1556a) The mput energy IS gtven byE;n(MeV/u) = C;n xF X 10·4,wherefis the CSM frequency in MHz.
b) Same as a) but for the output energy.

plates as small as possible. The surface electric field on
the drift tubes was also taken into account.
The number of the mesh points adopted in thesimulations were 90,000 - 190,000, depending on theresonant frequency. The drift tubes are approximated by aoctagonal cylinder as shown in Fig. 2. For the drift tubegaps, real values were used in the simulations.
Figure 3 shows the cavity structure. Although thevariable-frequency cavities with the shorting plates havebeen designed for the high-energy units, only the lower
parts were constructed as mentioned above. They areequipped with block tuners on the ceiling plates for theprecise adjustment of the resonant frequency.

Table 1 summarizes the main parameters of thecavities. Based on the beam dynamics calculations, theinner diameter of the drift tubes is chosen to be 35 mm.
The outer diameter is 55 mm, which has been detenninedby numerical simulations with MAFIA code. The gap
length between the drift tubes is kept constant througheach cavity, which is about one half of the cell length at
the middle of the cavity. The number of gaps of the thirdunit cavities is chosen to be six so that the cavity length
can be made less than 1.3 m. The required gap voltages
are approximately 500 kV, as shown in Table 1, and thesurface electric field is about 16 MV/m at maximum. The
input and output energies of each cavity are proportional
to the square of the frequency. The coefficients of thisrelationship are also listed in Table I.

2.2 Simulations
Side View

The designed structure is based on the quarterwavelength resonator with a movable shorting plate.
Using the computer code MAFIA, the dimensions wereoptimized so that the frequency range from 36 to 76.4
MHz could be covered, while keeping power losses aiXlthe electric current on the inner boundaries of the shorting,..RS41111--- - r~--- t------·111111

fll:t%111•.• I• Lgap ·I------~-------t------1111

IIIII

Figure 2: The real drift tube and its substitute.
The octagonal cross section is discretized withthree radial cells.

A3 andA4

Figure 3: Schematic drawing of the CSM cavities.

2.3 Mechanical Structure
Almost all the components are made of oxygen-freecopper. A remarkable point is that the side wall is cut
from a single piece of copper block in every cavity of thehigh-energy units, in order to avoid possible vacuum
leakage and to keep the machining accuracy. The baseplates are made of steel and plated with copper by 50 Jlm.
The drift tubes are aligned within an accuracy of +1- 0.15mm.
The water channels are arranged based on the heatanalyses. The total water flow per cavity is 600 1/min.
Each cavity is equipped with a turbomolecular pump of520 Us and a cryogenic pump of 4000 1/s. The vacuum
stays in the range of 0.5- 1.0 x 10·7 Torr in the cw-mode
operation.

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60

3 RF ASPECTS

40

3.1 Low Power Tests!"-""'20i

Table 2 summarizes the rf characteristics of thecavities. From the measurement, it was found that themeasured frequencies are greater than the predicted values
by the MAFIA code by 3 %.
The measured Q-values are 60- 80 % of the calculatedones, which is considerably good in spite of thecomplicated structure of the cavities. One of the reasonsfor this is that we have inserted rf contacts of spring type
into every joint between the metallic components. The
estimated power losses per cavity are 60 - 90 kW atmaximum.

-~--0I
I~" ·20"'I-40Iii..6()llJ-8010100

1000f(Hz)

110000100000

IIIII0III.;J·90II

J-180sTable 2 · Rf characteristics of the cavities
pb>
Qratioa>
Q-value
Cavity(k:W)akz)
800.6419,000Al
76.4630.72
22 70036.0860.6418,50076.4A2
640.7222 50036.0840.6318,60076.4D1
620.7322 60036.0
67
0.78
25,000
75.5
A372
24200
0.78
75.5
A4
630.76
23,70075.5
AS670.77
23 100A675.5a) Ratto of the measured Q-value to thecalculated one.
b) Power loss estimated at the maximum gap
voltage given in Table 1.

~

1:1

PM: 32deg.

1\~ -270-360-450
10100

1000
f(Hz)

10000

100000Figure 4: Bode diagram of the open-loop transfer
function of the amplitude modulation, measured for
the A3 cavity at the gap voltage of 180 kV. The
horizontal axis represents the modulation frequency inHz. The phase margin is 32 degree.
voltage, the ion beams having mass-to-charge ratio of lessthan 5.6 will be accelerated up to that energy.

5 ACKNOWLED GMENT
3.2 High Power Tests
High-power tests of the cavities have been performed
since September 2000, using the power amplifier of 100
kW[S]. So far, 70 % of the maximum voltage has beenachieved in the cw-mode operation without any significantproblems.
The open-loop transfer function of amplitude
modulation was measured in the cw mode operation at
various gap voltages. The measurement has shown thatthe system meets the requirements of the stability
condition; the phase margin is around 30 degree, as shownin Fig. 4.

4 ACCELERATION TESTS
The first beam test of the booster was performed usinga carbon beam on May 22, 2001. Since then, carbon andargon beams have been accelerated up to 5.8 MeV/u. The
maximum intensity of the Ar11 + beam ever achieved is 4.8pJlA, which corresponds to the beam power of 1 kW.
When the boosters are fully excited at the maximum

The fabrication and heat analyses of the cavities, aswell as the design and fabrication of the rf amplifiers,
were done by Sumitomo Heavy Industries, Ltd. (SHI).
The authors are grateful to Mr. K. Uno at SHI for thedetailed design of the cavities. They are also indebted toMr. Y. Murakami at SHI and to Mr. T. Takai at
Thamway Co. Ltd. at the commissioning of the If
systems. This work has been done in collaboration withthe Center for Nuclear Study, Tokyo University.

REFERENCES[1] Y. Yano et al.: Proc. 1997 Particle Accelerator
Conference, Vancouver, May 12-16, 1997, p. 930.
[2] 0. Kamigaito et al.: Proc. 12th Symp. on Accelerator
Science and Technology, October 27-29, 1999, p.
233.
[3] 0. Kamigaito et al.: Proc. 26th Linear Accelerator
Meeting in Japan, August 1-3, 2001, p. 43.
[4] E. Ikezawa et al.: in this proceedings.
[5] Y. Murakami et al.: Proc. lth Symp. on Accelerator
Science and Technology, October 27-29, 1999, p.
239.

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