The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Acceleration Tests of the Folded-Coaxial RFQ Linac for the RILAC
Osamu KAMIGAJTO, Akira GOTO, Yoshitoshi MIYAZAW A, Toshiya CHIBA,Masatake HEMMI, Masayuki KASE, Shigeo KOHARA, Takahide NAKAGAWA,
Eiji IKEZAWA, Yuri BATYGIN and Yasushige YANOThe Institute of Physical and Chemical Research (RIKEN),
Wako-shi, Saitama, 351-01 JAPANAbstract
A new injector for the RIKEN heavy-ion linac (RILAC)
has been installed, which consists of a variable-frequency
RFQ and an 18-GHz ECR ion source. The RFQ, based on afolded-coaxial resonator with a movable shorting plate,accelerates ions with mass-to-charge ratios of 6 to 26 at upto 450 ke V per charge in the cw mode by varying theresonant frequency from 18 MHz to 39 MHz. Acceleration
tests of the RILAC and the ring cyclotron (RRC) weresuccessfully performed with the new injector. The beam
intensity from the RRC as well as the transmission
efficiency through the RILAC has been greatly improved.

1 Introduction
The RIKEN heavy-ion linac (RILAC) is If frequencytunable between 17 MHz and 40 MHz, which accelerates
various kinds of ions with mass-to-charge (rnlq) ratios up to28 in a wide energy range[l]. A 450-kV Cockcroft-Walton
accelerator with an 8-GHz electron-cyclotron resonance ion
source (ECRIS) has been used as the injector of the RILAC.
In order to increase beam intensities in the RILAC, anew injector has been constructed[2], which consists of an
18-GHz ECRIS[3] and a variable-frequency RFQ linac. This
injector was installed in the RILAC beam line in August,
1996, as shown in Fig. 1, while keeping the beam linefrom the Cockcroft-Walton injector alive.

Table: 1 Main Parameters of the FCRFQ~ - 07.1997

i 08.1997-····F~q·~~~~y:·(Mfu)·························r·17:7·~··39:2T.i7·:4·~··39:o··

Mass-to-charge ratio (rnlq)
Input energy (keV/q)
Output energy (keV/q)
Input emittance (mm•mrad)
Vane length (em)Intervane voltage (kV)
Mean aperture (r 0:mm)
Min. aperture (amin:mm)
Max. modulation (m)Focusing strength (B)Max. defocusing strength
Final synchronous phase!,'=,,=,,

6- 26
10450
145 1t
142
33.6
7.70
4.17
2.70
6.8
-0.30
-25°

i6- 2620450
145 1t
153
36.8
8.08
4.67
2.41
6.8
-0.30
-30°

Figure: 1 Photograph of the RILAC along with the RFQ.

2 FCRFQ linac
The RFQ resonator is based on a folded-coaxialstructure[4]. The distinct features of this folded-coaxial RFQ(FCRFQ) are that it can be operated in a low frequency
region and the frequency range is quite large. The mainparameters are listed in Table 1.
Figure 2 shows a schematic layout of the FCRFQ
resonator whose details are given in the reference[4]. The
resonator is separable into upper and lower parts, as shownin the figure. All the vanes are rigidly fixed in the lower
part. The upper part containing the stem and the movable
shorting plate can be removed as a unit. This separable
structure permits accurate alignment of the vanes and easy
maintenance. The lower part of the tank-wall is made ofsteel (SS400) whose inside is plated with copper to a
thickness of 100 J.lm, while the other parts such as thevanes and the stem are made of oxygen-free copper (C1020).
The vanes are three-dimensionally machined within theaccuracy of ±50 J.lm.

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is 2 x w- 4 , welded with copper-tungsten ~Hoy on its bothsides. This welding is possible because both materials have
similar values of the coefficient of the linear thermalexpansion. Since there is no electric field concentration in
the ceramic material, it is quite stable even in the operation
at the maximum voltage. The power loss in one pillar isestimated to be 20 W at maximum.

MovableShortingPlate

3 Acceleration Test
Gap Distance

60-850mm

Ceramics
PillarPick-up
MonitorVerticalVane

3.1 Preliminary Test

Horizontal
Vane

Figure: 2 Schematic drawing of the RFQ resonator. The
inner volume of the resonator is about 1700mm (length) x 700 mm (width) x 1150 mm(height).
The channels for water cooling are arranged based on aheat analysis. The total water flow is 155 1/min at thepressure of 7 atm. The resonator is evacuated by two turbomolecular pumps (1500 lis) on its both sides. The vacuum
stays in a range of 5 - 8 X 1o-8 Torr at the pump head
during the operation.
The rf power is supplied through a capacitive feeder
with an rf power source based on an Eimac 4CW50000E,
which has a cw power of 40 kW at maximum between 16.9
and 40 MHz. A capacitive tuner for the fine tuning is placedon the other side of the feeder.
The resonant frequency varies from 18 MHz to 36 MHz
by changing the position of the shorting plate by a stroke of
790 mm, when the lower stem is out of the resonator.
When the lower stem is used, the frequency varies from 30
MHz to 39 MHz. The maximum power losses are 6 kW at
18 MHz and 26 kW at 39 MHz in the cw operation.
The key to the stable operations of this RFQ is theceramics pillar, which stands up to the high rf-voltage.
Figure 3 shows the structure of the pillar installed in theresonator. It consists of Al203, whose nominal loss-tangent~

05 !Ocm(Cross Section)

Figure: 3 Schematic drawing of the ceramics pillar.

Prior to the installation in the RILAC beam line, wetested the new injector alone by using Oxygen, Neon,Argon, Krypton and Tantalum beams[5]. They are indicatedby the closed circles in Fig. 4. The maximum transmission
efficiency was 88 % for an A£8+ beam of 120 eJJ.A.
The input beam emittance from the ion source wasmeasured to be 150 - 300 1t mm•mrad, which decreased as
the extraction voltage and/or the charge states of the ionsincreased. The output beam emittance was almost
independent of the acceleration condition and was inagreement with the PARMTEQ simulation.
The energy distribution of the output beam wasmeasured with an electrostatic deflector placed downstreamof the RFQ along with a scanning wire probe. The beamenergy was deduced from the beam position measured withthe probe, and the voltage applied to the deflector. The
measured distributions were found to be well reproduced by
the PARMTEQ simulation. The energy spread of the output
beam was also measured with the same device. The result
was 2-3% at FWHM and was consistent with thesimulation.

3.2 Test of the RILAC
The beam matching section between the RFQ and theRILAC consists of two quadrupole doublets and one
rebuncher operated in the fundamental harmonics[6]. Therebuncher resonator is a quarter-wavelength type with fourgaps, which is driven by a 1-kW wide-band amplifier. A
capacitive phase probe, placed in a chamber before . theRILAC, was shown to be important in adjusting the tf
phase of the rebuncher.
The ions accelerated through the RILAC with the new
injector so far are N2,3+, Ar2,4,5+, Fe7+, Ni8+,Kr5,11,18+, Te18+ and Xe7,17,19+ at various frequencies.
They are indicated by the diamonds and the open circles inFig. 4.
The transmission efficiency of the RILAC has increasedto 70 %, while the original value for the beams from theCockcroft-Walton injector was 30 %. In the tests, however,
the transmission efficiency of the injector section was about
70 %, which was lower than that obtained in thepreliminary tests. Therefore, about one half of the ions
extracted from the 18-GHz ECRIS were accelerated by theRILAC. This overall efficiency is three times larger than theoriginal value of 15 %. The maximum beam intensity ever-93-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

accelerated through the RILAC with the new injector is 13pJ.lA. for a N3+ beam of 2.5 Me VI nucleon.

0<>~

3.3 Test of the Ring Cyclotron (RRC)'-'>.

In December, 1996, we started the acceleration tests ofthe ring cyclotron (RRC) with the upgraded RILAC. The
ionsacceleratedsofarare Ar5+, Fe7+, Ni8+, Te18+, Kr18+
and Xel7,19+at the frequencies of 18.0, 18.8, 19.1, 20.5,
22.9 and 28.1 MHz. They are indicated by the open circles
in Fig. 4.
Inthe first test using an 36Ar5+ beam, we achieved thebeam current of 1 pJ.lA. out of the RRC for the first time,
where 20 % of the beam extracted from the ion source wasaccelerated to the final energy of7.5 MeV/nucleon.
The charge strippers after the RILAC has become
unnecessary for the low-energy beams mentioned above,
because highly charged ions are now available with the 18GHz ECRIS. This is an advantage from the viewpoint ofthe beam intensity and the stability.

i:.ll~....
;::3&;::30600
450
300
150

~~~~~~~~015

20253035Frequency (MHz)o RFQ

Ions accelerated through40~ RFQ + RILAC

0 RFQ + RILAC + RRC

3.4 Test with the New Vanes

Since August, 1997, the maximum extraction voltageof the ECRIS has been raised to 20 kV in order to further
upgrade the beam intensity. The parameters of the newvanes for t.lJ.e upgraded beams are listed in Table 1. As apreliminary test, we accelerated Argon ions through theRILAC at 18.8 MHz, and found that the overall
transmission efficiency improved to be 62 %. The
transmission efficiency of the injector section was 83 %.
More tests with the new vanes will be performed in thisyear.

4 SummaryWe installed the new injector for the RILAC, which
consists of the 18-GHz ECRIS and the FCRFQ. The
transmission efficiency of the RILAC exceeds 70 % now.
Moreover, we achieved the beam current of 1 pJ,LA out of theRRC for the first time.
The maximum extraction voltage of the ECR ion
source has been raised recently, and further acceleration tests
will be performed in this year. In the future, this newinjector is expected to play an important role in the RI-beamfactory project[?].

5 Acknowledgments
The authors are grateful to Dr. N. Tokuda at KEK andDr. S. Arai at INS for the usage of the program for vanecutting as well as for the fruitful information about the vanedesign. They are also grateful to the operation staffs of theRILAC and the RRC for their efforts in the installation andthe improvement. The resonator was fabricated by
Sumitomo Heavy Industries, Niihama Work, the rf power

Figure: 4 Performance of the RFQ linac. The abscissa andthe ordinate represent the resonant frequency andthe intervane voltage, respectively. The output
energy, which is proportional to the intervane
voltage, is also indicated. The hatched area shows
the region where the RFQ has ever been operated
in the cw mode. The ions accelerated so far areindicated by the closed circles, the diamonds, andthe open circles. The solid curves represent theacceleration condition of ions, each of which isindicated by the m/q-value. The dashed curveshows the maximum attainable voltage with thepresent power source (40 kW).
source by Denki Kogyo, and the ceramics pillars byKYOCERA Corporation.
References[1] M. Odera et al., Nucl. Instrum. and Methods, 227, 187(1984).
[2] A. Goto et al., Proc. 14th Int. Conf. on Cycrotrons andTheir Applications, 1995, Cape Town, p. 237 (1995).
[3] T. Nakagawa et al., "Recent Progress of RIKEN 18
GHz ECRIS", 1997 Particle Accelerator Conference,1997, Vancouver (1997).
[4] 0. Kamigaito et al., Jpn. J. Appl. Phys. 33, L537
(1994), and Jpn. J. Appl. Phys. 34, 5799 (1995).
[5] 0. Kamigaito et al., Proc. 18th Int. Linear Accelerator
Conference, 1996, Geneva, p. 863 (1996)[6] 0. Kamigaito et al., RIKEN Accel. Progr. Rep. 30,p.191 (1996).
[7] Y. Yano, "RIKEN RI-Beam Factory Project", 1997
Particle Accelerator Conference, 1997, Vancouver(1997).

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