PRESENT STATUS OF THE RIKEN HEAVY-ION LINAC
O. Kamigaito1, M. Kase, E. Ikezawa, S. Kohara, N. Sakamoto, M. Fujimaki,T. Nakagawa, M. Kidera, Y. Higurashi, Y. Chiba, M. Hemmi, Y. Miyazawa, T. Chiba,
T. Aihara2, T. Ohki2, H. Hasebe2, H. Yamauchi2, A. Uchiyama2, K. Oyamada2, and Y. Yano,RIKEN, Wako-shi, Saitama, 351-0198, JapanECRIS[3], installed in 1996. The output voltage of theRFQ is the same to that of the Cockcroft-Walton injector.
Owing to the high performance of the 18-GHz ECRIS, thebeam intensity has increased remarkably.
The booster, consisting of six resonators, wasconstructed in 2000 in order to increase the beamenergy[4]. The first two resonators are frequency-variable,
whereas the other four are fixed-frequency type. The
resonant frequency is set twice as that of the main linac.
The designed value of the voltage gain is 16 MV. The
commissioning of the booster started in 2001, and we got
the license for the upgraded operation in March 2002.
Figure 1 also shows the beam transport line and theexperimental apparatus. The transport system has one
main line, transferring the beam from the RILAC to thering cyclotron (RRC), and six branches (e1 - e6 in Fig. 1)
for various experiments. Among the six beam lines, the e3
beam line is dedicated for the gas-filled recoil isotope
separator (GARIS), where the experiments of search forsuper-heavy elements are carried out.
The most important feature of the RILAC is that theacceleration is performed in the continuous wave (cw)mode. Another important property is the variability of thefrequency, which makes it possible to accelerate variouskinds of ions. Available beam energies are summarized inTable 1, along with the maximum mass-to-charge ratio
acceptable by the RILAC.

Abstract
The RIKEN heavy-ion linac (RILAC) was successfully
upgraded, by adding a booster consisting of six resonators.
Various heavy ion beams have been accelerated to theenergies around 5 MeV/u with very high intensities. For64
example, Ni beam of more than one particle-microampere is available in the cw mode. The stability of the rfsystem, as well as the transmission efficiency, have beenimproved. On the other hand, we experienced serious
problems mainly due to the high rf-power required for theacceleration. Using the upgraded beams, systematic study
on search for super-heavy elements is under way;elements 110 and 111 have been produced so far.

1 OVERVIEW
The layout of the RILAC facility is illustrated in Fig. 1.
The main linac[1], which has been operated since 1981,
consists of six variable-frequency resonators (RILAC 1 6). The voltage gain of the main linac was designed to be
16 MV in the frequency range from 17 to 45 MHz. Therf-amplifiers for the last two resonators (RILAC 5, 6)
were replaced in 1999.
There are two injectors for the main linac. One is theCockcroft-Walton injector of 450 kV, which was installedin 1980. On the high voltage terminal, an ECR ion source(ECRIS) of 8 GHz (NEOMAFIOS) is placed. The other isa variable-frequency RFQ (FC-RFQ)[2] with an 18-GHz

Magnetic Rigidity

Prebuncher
4 - 38.2 MHz

5.8 MeV/u
M/q < 5.6e1
4.0 Tme2 e3
2.1 Tme4 e5 e6 1.2 Tm

Re-buncher
18 - 38.2 MHz
RCA4648
110 kW17 - 45 MHz

RCA4648
150 kW
17 - 45 MHz

RCA4648
180 kW
17 - 45 MHz

RCA4648
220 kW
17 - 45 MHz

RS2058CJ
100 kW
36 - 76.4 MHz

RS2042SK
250 kW
18 - 42 MHz

GARIS

e3
e2

e4

18GHz ECRIS

e5
Solenoid
Booster-A5, 675.5 MHz

FC-RFQ18 - 38.2 MHz

Stripper
12

3

4

5

6

RILAC-1
17 - 45 MHz

RILAC-2
17 - 45 MHz

RILAC-3
17 - 45 MHz

RILAC-4
17 - 45 MHz

RILAC-5
17 - 45 MHz

RILAC-6
17 - 45 MHz

e1

4CW50000E
40 kW
From Cockcroft-Walton

Booster-A1, 236 - 76.4 MHz

Booster-A3, 475.5 MHze6Buncher
17 - 45 MHz

Buncher
34 - 90 MHz

0510 mFig. 1: Schematic drawing of the RILAC facility.
12

kamigait@postman.riken.go.jp
SHI Accelerator Service, Ltd.

RS2058CJ
100 kW
36 - 76.4 MHz

to RRC
Decelerator-D1
36 - 76.4 MHz

Table 1: Beam energy in the RILAC facility
Resonators
f
Eout
M/q**
(MeV/u)(MHz)*
2
2
8000 / f
RILAC
18 - 420.00202×f(Main linac)
2
RFQ+RILAC
18 - 38.2
0.00271×f+A1+A2
RFQ+RILAC
37.75
5.8
5.6
+A1~A6*Resonant frequency of the main linac (RILAC 1 - 6).
**Designed value of the acceptable mass-to-charge ratio.

2 OPERATION STATUS
Table 2 shows the statistics of the RILAC operationfrom January 1 through December 31, 2002[5]. Although
we met a lot of problems, the beam time amounts to 4,500
hours.
Table 3 summarizes the number of days allocated todifferent research groups[5]. Since we got the upgradedlicense, nuclear physics experiments have become majorpart of the beam time. The percentage of the beam timeused by the RRC was 32 % of the total.
The accelerated ions in the last year are the followings:
42+ 408+, 11+ 48
11+ 5813+ 589+ 64
13+, 14+ 84
8+, 17+, 19+,He , Ar, Ca , Fe , Ni , Ni
, Kr
20+ 13618+, 20+, 27+ 181
20+
209
20+
20+ 86, Kr , Xe, Ta , and Bi . For more than
95 % of the beam time, the ions were fed by the 18-GHz64ECRIS. The beam time of Ni amounts to 52 days, which8648
is about 30 % of the total. The Kr and Ca ions wereaccelerated by the RILAC together with the booster (A1and A2) and injected into the RRC with a harmonicnumber of 8.
Table 2: RILAC operations from January 1 throughDecember 31, 2002.
No. of Days%
Beam time187
51.2
Overhaul and improvement77
21.1
Scheduled shutdown
68
18.6
Periodic inspection & repair
33
9.0
Total
365
100
Table 3: Beam time allocation.
No. of Days
Nuclear physics
83
Beam transport to RRC
60
Radiation chemistry
13
Material analysis
12
Atomic physics
11
Accelerator research
8Total
187

%
44.4
32.1
7.0
6.4
5.9
4.3
100

3 IMPROVEMENTS AND TROUBLES
The transmission efficiency of the beam through thelinac, from the 18-GHz ECRIS down to the exit of thebooster, was not so good. The typical value stayed around30 - 40 %. The beam loss mainly takes place in theinjector section, from the ion source to the entrance of themain linac.
One of the reasons for the beam loss is considered to bea mismatch of the transverse emittance to the acceptanceof the beam line in the injector section. However, the onlydevice with which we could control the beam profile had
been the solenoid magnet between the analyzing magnet
and the RFQ.
To improve the transmission efficiency, a new solenoid
magnet was installed between the 18-GHz ECRIS and theanalyzing magnets[6]. The maximum magnetic field on
the axis is 0.65 T. As a result, the beam intensity
increased by about 50 % at the exit of the analyzingmagnet. The transmission efficiency through the linac has
also increased; typical value is 50 % now.
We had another problem on the output beam from theRILAC; the longitudinal phase of the beam center was
sometimes quite unstable and the injection of the beam
into the RRC was difficult. From the measurement for therf-system of the main linac, it was found that the phaseoscillation was large in the region close to the carrier
frequency (∆f < 100 Hz). Therefore, all the phase control
units for the main linac were replaced into newly designed
ones[7]. After this replacement, the phase oscillation has
improved to be less than ± 0.1° in the whole frequency
range. The longitudinal excursion of the beam has become
negligibly small.
On the other hand, new problems appeared as the rfpower increased. One of them is that the thin copper
sheets (10 cm wide, 7 cm long, 0.3 mm thick), used forthe electric contact in the resonators of the main linac,
sometimes melted due to the large rf-current. We areplanning to replace them with thicker ones in this summer.
Another problem is a self-oscillation of the rf-amplifierof the last resonator of the main linac (RILAC-6); it
oscillates at 105 MHz with very high power when theplate stub is set in a certain position-range. A
measurement has shown that there is a coupled resonantmode between the grid circuit and the plate circuit, bywhich the output power is fed back to the input circuit in
the positive phase. Although this oscillation is temporary
suppressed by mounting dumping resistors in the platecircuit, the problem is not fully solved yet.

4 EXPERIMENTS WITH RILAC
In this section, two topics are briefly represented
among the experiments performed with the RILAC.

4.1 Super-heavy element search
Research experiments on super-heavy elements began inMarch 2002 with GARIS at the e3 beam line. First, the4048
58
beams of Ar, Ca, and Fe with energies of 4.6 – 4.9
MeV/u were used in order to study the characteristics ofthe GARIS.
In July 2002, a confirmation experiment on the synthesis
64of element 110 started using a Ni beam with an energy of208

64

5.0 MeV/u, through the fusion reaction Pb + Ni →271
110 + n. The total service time for the experiments was
1486.5 hours. The 18-GHz ECRIS with the MIVOC(Metal Ions from Volatile Compounds) method[8] and thebooster resonators worked actively to provide thesebeams with high intensity for a very long time. Finally, 14271
candidates of decay chains originating from 110 wereobserved, and the excitation function of the reaction wasmeasured successfully[9].
The typical beam intensity had exceeded one particlemicro-ampere on the target throughout this experiment.
The voltage gain of the booster was 10.6 MV, which was
65 % of the designed value.
From February to May 2003, a confirmation experiment64
on the element 111 was performed using a Ni beam on
209
Bi targets. The total service time was 930.5 hours. Theexcitation function of the reaction was also measured
successfully, where 14 candidates of decay chains wereobserved[10].

4.2 H=8 mode acceleration of the RRC
The harmonics of the rf-system of the RRC is designed
to be 9 when the RILAC is used as an injector. The
additional velocity gain through the booster makes it
possible to operate the RRC in the harmonics of 8, whichindicates that the extraction velocity can be boosted by afactor of 9/8 at the same rf-frequency[12]. This energy
gain is very useful to produce more intense radioactive
beams far from stability. Based on this acceleration48
scheme, more than 100 particle-nano-ampere of Ca beam
was accelerated up to 63MeV/u in order to apply to newisotope search, resulting in discovery of the new isotopes
34373486
Ne, Na, and Si[11]. Recently, Kr beam of 63 MeV/u
was extracted from the RRC with the beam intensity of100 particle-nano-ampere.

5 FUTURE PLAN
As mentioned above, most of the beams are provided
with the 18-GHz ECRIS. The ion source, however,requires periodic cleaning especially in the MIVOC
method. Consequently, the beam time is stopped during
the maintenance.
To make this intercept as small as possible, we areplanning to construct another ECRIS and install it inparallel with the present one. The beam line from theCockcroft-Walton injector will be removed at the sametime. A planned layout is shown in Fig. 2. Two analyzing
magnets are connected back to back and they are rotatedby 90 degree without opening the vacuum chamber. Thisconfiguration will allow us to test one of the ion sources
while the other is in use for the beam time. The magnets

Solenoid

PresentECRIS

Solenoid
RILAC-1

New ECRIS

FC-RFQ
01

23m

Fig. 2: Planned layout of the new injector with two ionsources.
have been already fabricated. The installation will becarried out in the next year.

6 ACKNOWLEDGMENT
We are grateful to the Safety Center, RIKEN for thecooperation in monitoring the radiation in theexperimental room. The fixed-frequency resonator (A3 ~A6) of the booster was funded by Center for Nuclear
Study, University of Tokyo.

REFERENCES[1] M. Odera et al., Nucl. Instrum. Methods 227, 187(1984).
[2] O. Kamigaito et al., Rev. Sci. Instrum. 70, 4523
(1999).
[3] T. Nakagawa and Y. Yano, Rev. Sci. Instrum. 71,637 (2000).
[4] O. Kamigaito et al., Proc. 26th Linear Accelerator
Meeting in Japan, Tsukuba, p. 43 (2001).
http://conference.kek.jp/LAM26/LAM26PDF/1C5web.PDF[5] E. Ikezawa et al., RIKEN Accelerator Progress
Report 36, 3 (2003).
[6] Y. Higurashi et al., RIKEN Accelerator Progress
Report 36, 277 (2003).
[7] N. Sakamoto et al., RIKEN Accelerator Progress
Report 36, 283 (2003).
[8] H. Kovisto et al., Nucl. Instrum. Methods Phys. Res.
B 94, 291 (1994).
[9] K. Morita et al., to be submitted to European PhysicalJournal A.
[10] K. Morita, private communications.
[11] M. Notani et al., Phys. Lett. B 542, 49 (2002)[12] M. Kase et al., RIKEN Accelerator Progress Report36, 5 (2003).