RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

Developments of linear accelerators at RI Beam FactoryO. Kamigaito ∗1 for the Accelerator Groupsuch as 136 Xe20+ and 238 U35+ , up to an energy of670 keV/nucleon. Therefore, it was possible to reduce
the total length of the linac structure to less than 8 mby choosing an rf frequency of 36.5 MHz, instead of
the RILAC frequency of 18.25 MHz, whereas the prebuncher is operated at 18.25 MHz in order to matchthe beam bunch interval to the rf system of the RRC.
We have placed the new injector in the AVF cyclotron
vault, as shown in Fig. 2, which made the length of
the beam transport line to RRC 36 m, less than halfthat of the RILAC injector. The main parameters ofRILAC2 are listed in Table 1.

The injection of very heavy ions such as uranium inRIBF 1) was initially performed with RILAC.2) However, the beam current of 238 U35+ , which was directlyinjected into the RRC 3) without charge stripping, wastoo low to meet the demand of RIBF users becauseof the limited performance of RILAC. In addition,a strong demand for more RILAC beam time wasmade by those engaged in research projects on superheavy elements (SHE) started at the RILAC facility in2002.4) It was obvious that such experiments conﬂictwith the BigRIPS experiments at RIBF, as shown inFig. 1(a). Therefore, a new injector was required to resolve these problems. By 2006, a basic design study ofthe new injector involving beam dynamics calculations
had been conducted.5) Fortunately, the project was approved by the supplemental budget in late FY2008,
and we quickly constructed the new injector by theend of FY2009.

Table 1. Main parameters of RFQ and DTL cavities.
Resonator
RFQ
DTL1 DTL2
Frequency (MHz)36.5←←Duty(%)
100
←
←m/q
6.8
←
←Ein (keV/u)
3.28
100
220
Eout (keV/u)
100
220
450
Length (m)
2.3
0.8
1.1
Gap number
93∗
1010
Gap voltage (kV)
42110
210
Aperture (mm)8∗∗17.5
17.5-29.6†-25-25φs (deg.)
Power (kW)18713∗
Cell number ∗∗ Mean aperture (r0 )†Final synchronous phase(a) 2006-2010
vgain=42

1.5

1.5BigRIPSfRC
RRCGARIS
RILAC Booster

IRC345 MeV/uSRC(b) 2011RILAC2fRCRRCGARISRILAC BoosterBigRIPS
IRC345 MeV/u

The RFQ linac used in RILAC2, which is based on afour-rod structure, is the one originally developed foruse in an ion implantation device in the 1990s.8) We
modiﬁed it to yield a resonant frequency of 36.5 MHz
by adding tuner blocks between the posts that support
the vanes based on ﬁne electromagnetic simulations.9)
We also diverted one of the cavities in the Charge StateMultiplier 10) developed for RIBF to the last cavity ofthe DTLs in RILAC2.
In order to reduce the construction cost and space,
we adopted a direct coupling scheme for the rf coupling
between the DTL cavities and ampliﬁers. Since the resonators could not be designed independently from thetetrode ampliﬁer, the frequency shift of the cavities aswell as the strength of the coupling were carefully calculated based on computer simulations with the help of
a lumped circuit model.11) This calculation exhibited
excellent agreement with the measured results.
The ﬁrst beam test of RILAC2 was successfully conducted in December 2010, and the device was fullycommissioned in October 2011 using a uranium beam.
Since then, RILAC2 has provided intense uranium andxenon beams for nuclear physics experiments at RIBF
very stably.12) The completion of RILAC2 has also

SRC

Fig. 1. Accelerator chain of RI Beam Factory. The upper panel (a) shows the uranium acceleration scheme inRIBF and metallic ion beam acceleration for the GARIS
experiments until 2011. Both schemes used RILAC.
The lower panel (b) shows the acceleration scheme since2011. Now the uranium beam is injected through RILAC2, while RILAC is almost entirely dedicated toSHE experiments.

The new injector, named “RILAC2,” is schematically shown in Fig. 2. It mainly consists of the28-GHz superconducting ECR ion source,6) a lowenergy beam-transport (LEBT) line 7) including a prebuncher, an RFQ linac, and three DTL cavities. An
important point in designing RILAC2 was that therequired total voltage is as low as 4.6 MV, which isless than one third of that of RILAC; it is suﬃcientto accelerate ions with a mass-to-charge ratio of 6.8,
∗1DTL3←←←4506701.38260
17.5-25
20

Director of Accelerator Group, RIKEN Nishina Center(2008–present)

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Research Activities 1991–2016

Ion Source Bldg.

RIKEN Accel. Prog. Rep. 50 (2017)

Shield Wall

SC-ECRISLEBTBM

4.3 m

AVF Hall

BUN

DQ DQ

RFQ

REB
DQ

DTL

DQ

TQ

HEBT
TQ

TQ

BM SQ

BM

Solenoid

Faraday-Cups

U10

B12

B12

B20

B22

B31

B41

B50Fig. 2. Side view of the RILAC2 injector. The beam is provided with a 28-GHz ECR ion source and isaccelerated through the RFQ and three DTLs. A pre-buncher and a rebuncher are implemented inthe linac.

allowed us independent operation of the SHE experiments at the RILAC facility, as shown in Fig. 1 (b).
We think that RILAC2 contributed to the third eventof the element [113] in 2012 13) to a certain degree.
When the name of the element [113] was proposed
as “nihonium” in 2016, a supplemental budget wasapproved for further investigation of heavier new elements, such as [119] and [120], through a major upgrade of RILAC. The upgrade program includes theconstruction of a 28-GHz superconducting ECR ion
source and two cryomodules that substitute the lastfour cavities of the RILAC booster. We expect tohave very intense metallic beams of vanadium andchromium of more than 5 pμA at 6.5 MeV/nucleon.
This upgrade program also aims at mass productionof 211 At, a candidate medical isotope for radionuclide
therapy in the future. For this purpose, an intense
beam of 4 He at 29 MeV is necessary.

Fig. 3. Conceptual drawing of the cryomodule. Each cryomodule contains four quarter-wavelength resonators(QWRs).

Figure 3 shows a conceptual drawing of a cryomodule, which contains four quarter-wavelength resonators(QWRs). Each QWR operates at a resonant frequencyof 73.0 MHz at a gap voltage of approximately 0.8 MV

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with a power consumption of 4.0 W. The corresponding
acceleration gradient is 4.5 MV/m. The inner radiusand height of the QWR are 1,090 mm and 300 mm,respectively.
We have investigated the same structure of a superconducting resonator in a proposal of an RIBF upgrade
program.14) A similar structure was also studied, constructed, and tested in the framework of the ImPACT
program, a) and we achieved a very high voltage in a
vertical test.15,16)
The construction of the new devices is in progress.
We expect that the upgraded RILAC facility will start
operation in FY2019. It will contribute to the intensity
upgrade of RIBF in the future.
References
1) Y. Yano, Nucl. Instrum. Methods Phys. Res. B 261,1007 (2007).
2) M. Odera et al., Nucl. Instrum. Methods Phys. Res. B
227, 187 (1984).
3) H. Kamitsubo, Proc. CYCLOTRONS1986 (1986),
p. 17.
4) K. Morita et al., J. Phys. Soc. Jpn 73, 1738 (2004).
5) O. Kamigaito et al., Proc. 3rd PASJ and 31st LAM(2006) p. 502.
6) T. Nakagawa et al., Proc. ECRIS2008, MOCO-B01(2008).
7) Y. Sato et al., Proc. IPAC10, THPEB023 (2010).
8) H. Fujisawa, Nucl. Instrum. Methods Phys. Res. A
345, 23 (1994).
9) K. Yamada et al., Proc. LINAC10, TUP070 (2010).
10) O. Kamigaito et al., Rev. Sci. Instrum. Meth 76,013306 (2005).
11) K. Suda et al., Nucl. Instrum. Methods Phys. Res. A
722, 55 (2013).
12) N. Sakamoto et al., Proc. LINAC12, MO3A02 (2012).
13) K. Morita et al., J. Phys. Soc. Jpn. 81, 103201 (2012).
14) N. Sakamoto et al., Proc. SRF2015, WEBA06 (2015).
15) K. Ozeki et al., Proc. LINAC16, TUPLR061 (2016).
16) K. Yamada et al., Proc. LINAC16, THPLR040 (2016).
a)

Impulsing PAradigm Change through disruptive Technologies Program of Council for Science, Technology and Innovation (Cabinet Oﬃce, Government of Japan)