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

Frequency Variable RFQ Operated with TE 110 ModeKouichi UNO, Shin-ichi WATANABE*, Takeshi KATAYAMA*
Sumitomo Heavy Industries, Ltd.(SHI)
5-2 Soubiraki-cho, Niihama-shi, Ehime, 792-8588, Japan*Center for Nuclear Study, Graduate School of Science, The University of Tokyo
3-2-1 Midori-cho, Tanashi-shi, Tokyo, 188-0002, Japan

Kenji SAWADA,Abstract
A frequency variable RFQ linac (RFQ) has been proposed
as an injector in a future CNS accelerator facility, and thebeam dynamics, resonant structure, and radio-frequency (RF)characteristics have been studied. The preliminarily designed
RFQ has so long vanes that new resonant structure operated
with a TE 110 mode has been chosen as the candidate. The RF
characteristics have been calculated with MAFIA to decide themain component sizes, and the validity of this structure has
been verified successfully by the performance test of the 114
model cavity.
1. Introduction
A preliminary design study about a future CNS accelerator
facility has been started in 1997, and the conceptual design
report has been drawn up on July in 1998 [1]. The mainfeature ofthe facility is to accelerate intense heavy ion beams
to the wide range of the energy.
The main accelerator is a relatively small separated sector
cyclotron (SSC) , which accelerates I pJ,JA Xe 31 + beam up to 6
MeV/nucleon and 100 pf,LA 0 5+beam up to 10.5 MeV/nucleon.
The longitudinal acceptance of the sse has been estimated tobe 40 degrees in phase and 1 % in energy spread in case of
injecting 650 keY/nucleon 0 5+ beam. Thus, the very small
longitudinal emittance is required to the injection beam in thewide range of the intensity. An RFQ is presumed the best
candidate to satisfY above requirements, because it capturesand bunches an injected DC beam with high efficiency and thelongitudinal emittance of the accelerated beam isn't seriously
affected by the space charge force.
Therefore, the RFQ has been chosen as the injector, and thebeam dynamics, resonant structure and RF characteristics havebeen studied preliminarily. Further, a 1/4 model cavity has
been manufactured, and RF characteristics of the model cavity
have been measured to verify the injector design. This paper
describes the injector design and the model test .
2. Preliminary beam dynamics design of the injector
Resonant frequency, input energy, and output energy are themost basic parameters defining linac dimensions (diameterand length), and these parameters have been decided
according to the following considerations. For the resonant
frequency, if the RFQ is operated at the half frequency of theSSC cavity, the longitudinal emittance of the RFQ output
beam must be half of the value which is required in being
operated at the same frequency. On the other hand, if the RFQ
is operated at the double frequency ofthe sse cavity, the sse
can't capture half of the injection beam. Therefore, we haveconsidered that the RFQ should be operated at the same
frequency as the sse cavity, and we have chosen thefrequency range from 17.25 MHz to 34.5 MHz which is usedin existing similar facilities. For the output energy, a higher

energy is more desirable to obtain enough space to installvarious components in the center region of the SSC, but RFQs
have a disadvantage that the acceleration field decreases as thebeam energy increases. Thus, we have chosen the energy of650keV/nucleon for 0 5+ beam as the output energy, which isjust above the minimum energy to arises no problem of themechanical design in the SSC center region. For the input
energy, we have chosen the energy of5.4 keY/nucleon for 0 5+
beam, because the energy is high enough to avoid the serious
emittance degradation in an injection beam transport line evenfor the 100 PILA 0 5+beam.
Table 1 shows the dimensions of the RFQ designed withRFQUIK and PARMTEQ codes, and table 2 lists the operation
parameters. The RFQ has a long shaper section of 4.6lm toobtain the small longitudinal emittance, and also has twenty
cells longer than 10 em in the accelerator section. Thus, theRFQ has long vanes in spite of the relatively low acceleration
voltage about 2 MV.
Table1 Dimensions of the RFQ
8.71
Vane length (m)26.79Characteristics bore radius (mm)8.43
Minimum aperture radius (mm)4.469
Maximum modulation
230
Cell number
Table 2 Operation parameters of the RFQ
os+
Xe 31 + XeJI+
Ion species
l/3.2
1/4.26
1/4.26Charge to mass ratio
27.93
21.17
17.25
Frequency (MHz)
6.40
3.68
2.44
Input energy (ke V/n)
650
373
248
Output energy (ke V/n)
2080
1589
1056
Output energy (keV/q)
7.0
7.0
7.0
Focusing strength
130
99.3
66.0
Inter-vane voltage (kV)

D+1/2
34.50
9.77
992
1984
7.0
124

Figure 1 shows the longitudinal emittance of the output
beam when the 0 5+beams of 0 rnA and 1 rnA are injected into
the RFQ. The sufficient transmission ratios above 95% havebeen obtained, and ~E!E and~~, which are about 3% and 14
degrees in both calculations, are independent on the beam
current. Since the product of the two values is almost same as
the longitudinal acceptance area of the sse, it is possible toinject the beam within the longitudinal acceptance through agood bunching system. Then, we have succeeded in the beam
dynamics design of the injector.
However, we are not completely satisfied with this design.
If we construct the injector with this cell parameters, we haveto overcome many problems. For example, we have toconsider how to manufacture and align so long vanes withinsufficient accuracy. It is also necessary to consider how toremove the RF power consumed on the cavity wall, since thewall loss density of the RF power is concerned to be so highbecause of the high inter-vane voltage of 130kV. Further
design study of the beam dynamics is indispensable to reduce-60-

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

the difficulties to overcome these problems.
J!O

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-22.
E-BS VS. PHI-PHIS-.l50_4.S.

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OmA injection
1mA injection
Fig.l Longitudinal emittance of the 0 5+ beams from the RFQ.
3.
3.1

tune the quadrupole field with some kinds of tuners. Further, if
the both end regions have the same local resonant frequency as
it in the center region, the field distribution along the beamaxis always becomes flat.
Compared with 4-vane structure operated with the TE210
mode, this structure has two features. The first one is that thefrequency of the TEuo mode never corresponds to the othermode, because the frequency is the lowest in all modes. We
have been released from a problem of the mode mixing. The
second is that a higher Q-factor and a higher shunt impedance
are expected because the current path in the cavity operated
with the TE 110 mode is shorter than it with the TE210 •Study of the resonant structure

Comparison ofthe various resonant structures

4-Rod structure and folded-coaxial structure (FC structure)are supposed to be the candidates for the resonant structures
operable in a wide frequency range, but we have studied toapply new resonant structure operated with the TEuo mode
according to the following considerations[2][3][4]. 4-Rodstructure has the disadvantage of the relatively low quality
factor (Q-factor), because the structure is not a cavity resonator.
If we construct the injector in this structure, so much RF
power must be required to generate the designed vane voltage.
Since the disadvantage may lead a fatal problem, we excluded
4-Rod structure from the candidates. On the other hand, theexisting FC structure has a difficulty to be applied to a long
RFQ. Since a pare of opposite vanes is supported at the bothends, it is very difficult to align the long vanes in highaccuracy. Then, we have to consider how to assemble andalign the long vanes, if we construct the injector in the FCstructure. This difficulty may become a serious problem, tlms,
we tried to adopt a new structure to the injector.

Fig. 2 Scheme of the RFQ operated with the TEuo mode.
Figure 2 shows the scheme of the new resonant structure
operated with the TEuo mode of which fundamental idea isbased on the author's patents [5][6]. This cavity is mainlycomposed of four vanes, two ridges, and many intermediaries.
One pare of opposite vanes is joined to the upper ridge by the
one group of intermediaries and another is to the lower ridge
similarly. When the TEuo mode is excited, the voltages of theopposite vanes always become the same, and then the voltagesof the adjacent vanes always become the same absolute value
in the opposite polarity. Thus, if there is no dimensional error
in the distance between the adjacent vanes, a uniform
quadrupole field is obtained along the beam axis. Thesecharacteristics lead to the advantage that it isn't necessary to\rting plate

Fig. 3Intermediary

Half cross sections used in the MAFIA calculation.

3.2 RF characteristics calculated with MAFIA
In order to apply the new structure to the injector, we havestudied the RF characteristics with MAFIA simulations.
Figure 3 shows an example of half cross sections at the two
axial positions, which were used in a simulation. The full crosssection are square shapes of 1.6 m in width and 1.5 m in height,
and two ridges are positioned at the horizontal center. One pairof the horizontal vanes is joined to the upper ridge and anotherpair of the vertical is to the lower. Movable shorting plates will
be put at the upper and lower boundaries in an actual cavity.
In the study, we tried MAFIA simulations in the two step. In
the first step, we calculated resonant frequencies of the two
axial infinite models, which have the same structure except forthe cavity heights. Here, we decided main component
configuration, and calculated the cavity height satisfying thecondition that the resonant frequency becomes almost same as
17.25MHz and 34.50MHz. Table 3 shows the calculation
result. The resonant frequencies of 35.4MHz and 17.4MHz
have been obtained under the condition that the cavity heights
are 0.7m and 1.5m respectively. Then, the required frequency
range will be obtained by moving the each shorting plate by
400 mm in an actual cavity. The required RF power iscalculated assuming that n+ is accelerated at 35.4 MHz andXe31 + is at 17.4 MHz. The RF power for the D+ acceleration isrelatively large but permissible.
Table 3 RF characteristics calculated with MAFIA
1.5
Height of the cavity (m)0.7
1.6
Width of the cavity (m)1.6
17.4
Resonant frequency (MHz)
35.4
18000
8700
Quality factor
66.0Inter-vane voltage (kV)124
19.5286
Required RF power (kW)In the second step, we calculated resonant frequencies of thetwo fmite models of which axial lengths are 1.27m and cavity
heights are 0.7m and 1.5m respectively. The main component-61-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

configuration in the axial center region is same as it decided in
the first step. Here, we tried many simulations and optimized
the ridge configuration in the end region to satisfy thecondition that the resonant frequency corresponds to it
calculated for the same height cavity in the first step. As
described above, we decided the center configuration in thefirst step and the end configuration in the second step.

4. RF characteristics ofthe model cavity
4.1

1OOmm. This frequency variability almost agrees with thetheoretical calculation. Figure 5 shows the field distribution
along the beam axis at 68.87 MHz. Very good uniformity andflatness have been obtained without any field tuning by use oftuners. This tendency is independent of the resonant frequency.
Concretely, field uniformity is within ±2.4% at 68.87 MHz
and ±3.5% at 139.62 MHz. We have succeeded in obtaining
the sufficient frequency variability and sufficient field
uniformity by the new structure operated with the TE 110 mode.

Structure ofthe model cavity

In order to verify the new structure, a model cavity havebeen manufactured with brass material and the RFcharacteristics have been measured. The scale of the crosssection is 1/4 and the axial length is 1 m. Figure 4 shows theinside view of the cavity. The two horizontal vanes are joined
to the upper ridge by the 18 intermediaries and the two verticalvanes are to the lower by the 19 intermediaries. The width of
the intermediary located at the axial center is 50 mm which isfive times wider than the others. A capacitive circular coupler
is located at the side wall facing to the center intermediary, andRF power is fed into the cavity through the coupler. Movable
shorting plates as the upper and lower wall of the cavity aredivided into the three parts in each side, which can bemanually moved independently. The cavity has a lot of tuners
to examine the effect on the field distribution. For example, six
cylindrical block tuners are installed on each side wall, andtwo same tuners are on each end wall.

7I..-.
+'e•Ii··•::;t::;tc 6:I..ci,_ 5ro'--'4-c.~_ . _ , st quad runt

3--..a.-- ·2nd quadrunt(.)·-,_ 2---3rd quadrunt+'(.)- - -o- - -4th quadrunt

LL..

.! 1-lIIUJ0897563420Fig. 5 Field distribution along the beam axis at 68.87 MHz.

5. Conclusions
Beam dynamics, resonant structure, and RF characteristics
have been studied about the frequency variable RFQ proposedas the injector in the future CNS accelerator facility. The
preliminarily designed RFQ has so long vane that the newresonant structUre operated with the TE 110 mode has beenchosen as the candidate. The RF characteristics have been
calculated with MAFIA to decide the main component sizes,
and the validity of the structure has been verified successfullyby the performance tests of the 1/4 model cavity.
Fig. 4Inside view of the model cavity.

Acknowledgements

4.2 Alignment ofthe vanes
As described before, if this structure has no dimensional
error in the distance between the adjacent vanes, a uniform
quadrupole field is always obtained. This means that if somedimensional errors exist in it, asymmetricity arises in the field
because the field strength is inversely proportional to thedistance. Thus, the vanes have been aligned so carefully thatthe error has been kept within ±90f1m.

4.3 Measurement ofthe RF characteristics
The resonant frequencies and the field distributions havebeen measured setting the movable shorting plates at various
positions. Table 4 lists the typical results of the frequency
measurement. Though the frequencies are slightly lower than
the theoretical values, the frequency range more than one
octave has been obtained by moving the shorting plates by

We would like to thank Dr. J. S. Chai and Mr. S. I. Mun in
Korea Cancer Center Hospital (KCCH) for their helpful
collaboration in the performance test of the model cavity.

References[I] "Conceptual Design Report about the Cyclotron
System", Private Report in Japanese (1998).
[2] A.Schempp, Nucl. Instr. and Meth. B40/41 (1989) 937.
[3] O.Engels, et a!., Proc. 5th Europ. Particle AcceleratorCon£, Sitges (1996).
[4] O.Kamigaito eta!., Jpn. J. Appl. Phys. 33, L537 (1994).
[5] K.Sawada, "Linear Accelerator Operable in TE 11 N Mode",
U.S. Patent No. 5,334,943, May. 19 (1992).
[6] K.Sawada, "Linear Accelerator Operable in TE 1iN Mode",
E.P. PatentNo.514 832, May. 19 (1992).

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