FEATURE ARTICLE

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

First Beam from SRILAC

RILAC upgrade
overview

project—Background

and

RILAC and beam intensity to initiate the project ofsynthesizing the new element.
Large accelerator facilities have contributed to society in the recent year in addition to promoting basic
science such as research on super heavy element (SHE).
At RIBF, several eﬀorts have been invested in producing and distributing radioisotopes for research. Oneof the most popular isotopes is 211 At, and its production using AVF cyclotrons is being actively pursued for
future medical applications.10) 211 At has a short halflife of approximately 7 h, and a high-intensity beamof particles is essential for mass production. However,
the technology for the mass production of 211 At has
not been established. Therefore, the establishment ofa technology for the mass production of useful isotopes
has been added to the objectives of the RILAC upgrade
plan.
The RILAC upgrade project (Fig. 1), which aims tosynthesize new elements and develop technology for themass production of useful radioisotopes, was approved
with a supplementary budget in FY2016. Indeed, thisenhancement plan will lead to a considerably higher
intensity of the beam to the RRC, especially the metal
ion beam, compared to that before.
The target performance expected in this upgrade
program is to accelerate ions with M/q = 6 to E =6.5 MeV/nucleon at more than 2.5 particle µA. To
achieve this goal, we ﬁrst built a new ion source. The
structure of the ion source is almost identical to the

The synthesis and naming of the 113th element, nihonium, is one of RIKEN’s most noteworthy achievements; further, it has had a social impact in that the
new element from Japan was added to the periodic
table of elements.1–3) This element was produced by
the RIKEN heavy-ion linac (RILAC),4) which has beenin operation since 1980 as an injector to the RIKEN
ring cyclotron (RRC) and for stand-alone applications
such as those in atomic physics. In 1996, RILAC was
upgraded to increase the beam intensity in RRC,5–7)
and in 2001, a booster linac 8) was installed as part of
the RI Beam Factory (RIBF) project 9) under ﬁnancial support from CNS, the University of Tokyo. Thisﬁnal step allowed nuclear physics experiments to be
conducted in the RILAC facility.
Following the success of nihonium, research collaborations led by RIKEN scientists have set the next
project of synthesizing new elements of atomic numbers greater than 118. To this end, metal ion beams
such as vanadium and chromium need to be accelerated. However, the production cross section is expected to drop to less than a quarter of that for producing nihonium, and therefore, beam intensities need
to be increased. In addition, it was estimated that theacceleration energy will need to be increased. Therefore, we need to increase the acceleration voltage of

Fig. 1. Overview of RILAC upgrade project. 18-GHz ECRIS was replaced by R28G-K, and a downstream
part of RILAC was replaced by SRILAC.

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28-GHz superconducting electron cyclotron resonanceion source (SC-ECRIS) that has been operational since2010.11) There is a risk of damage to SC cavities inthe downstream caused by the high-intensity beams.
Therefore, we decided to limit the emittance in the
low energy beam transport (LEBT) section up to theradio frequency quadrupole (RFQ) by measuring thebeam emittance eﬀectively.
Second, we replaced the last four normal conducting
cavities in the RILAC booster with ten SC cavities toincrease acceleration energy. Quarter-wavelength resonators (QWRs) were used as the SC cavities. A frequency of 73.0 MHz, which is four times the fundamental frequency of the RIBF, was selected for the SC cavities. This frequency is selected considering its futureapplication as an RIBF injector. For the SC-QWR,
we developed a 75.5 MHz QWR that cooperates with
KEK under the ImPACT program since FY2014.12)
This experience has been of great help in the upgrade
project.
In addition to the ion source, SC cavities, and refrigerator, test facilities of the cavities and a new hotlaboratory building for processing radioisotopes wereconstructed. Further, a gas-ﬁlled recoil ion separator(GARIS) -II was moved from the RILAC facility to theE6 room in the Nishina Memorial Building to start theSHE experiments as soon as possible.13) Meanwhile,
the RRC resonator was modiﬁed to solve the problem
of insuﬃcient voltage at the RRC,14) which had beena bottleneck in increasing beam intensity. This modiﬁcation greatly contributed to the large increase in theuranium beam intensity for the BigRIPS experiments.
The construction of the ion source completed in
March 2018 and the installation of the SC cavity in
March 2019; this was followed by the construction of
the control system of the SC cavities and the com-

Fig. 3. Schematic cross section of the R28G-K. The large
volume plasma chamber is surrounded by six SC
solenoidal and one SC hexapole magnets. Both the 18and 28-GHz microwaves are injected from the right sideof the plasma chamber. The HTO is mounted on theinjection side of the plasma chamber.

missioning of the refrigerator in September 2019. The
ﬁrst beam was delivered on January 28, 2020 and thefacility inspection was conducted on March 30, 2020,
which marked the completion of the construction planon schedule and within budget. This project could
not have been accomplished without the help of therelated manufacturers, KEK for the collaborative research, administrative staﬀ for the many orders, andthe management of the Nishina Center. We thank everyone who supported this project.
28-GHz Superconducting ECR Ion Source for
RILAC
We constructed a 28-GHz SC-ECRIS as an essential
part of the RILAC upgrade that began in FY2016. An
ECRIS conﬁnes a hot electron plasma in which electrons are heated by microwaves through the ECR in a“magnetic ﬁeld well.” That is, a “magnetic mirror ﬁeld”
created by a combination of solenoidal and hexapole
magnets known as the minimum-B conﬁguration.15)
Multiple charged heavy ions are created by countlesscollisions of ions with suﬃciently high-energy electronsin plasma. In the next projects at RILAC such as
the search for new SHE with atomic number Z > 118
and RI production, it is necessary to provide unprecedented high-intensity ion beams. This means that itis necessary to conﬁne the electron plasma, which hasa higher density and a higher temperature than theplasma generated in the ECRIS used in RILAC, ina stable manner. Therefore, as shown in Fig. 2, new
SC-ECRIS named RIKEN 28-GHz SC-ECRIS “KURENAI” (R28G-K) equipped with fully SC magnets and ahigh-power microwave generator system was launchedin 2018.

Fig. 2. Photograph of R28G-K and microwave generators(18-GHz klystron amplifier and 28-GHz gyrotron system). Extracted ion beams are analyzed by the dipole
magnet and transported to the accelerator cavities of
RILAC.

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R28G-K is equipped with six SC solenoidal magnets
and a SC hexapole magnet as shown in Fig. 3. The arrangement of the SC magnets was set by the Ion source
team of RIKEN Nishina center 11,16) based on previous
research.17–19) The SC coils are fabricated from Nb-Ti
alloy and immersed in liquid He (more than 300 L)
maintained in a high-power cryostat system combinedwith two 10-K GM (RDK-408S, SHI ltd.) and one 4-K
GM (RDK-408D2, SHI ltd.) cryocoolers. Further, a
GM-JT cryocooler (4 W at 4 K, SHI ltd.) is used to remove the excess heat induced by the large amounts ofhighly energetic X-ray radiation from the plasma. The
maximum magnetic ﬁeld of microwave injection side
(Binj ), beam extraction side (Bext ), and radial magnetic ﬁeld at the plasma chamber surface (Br ) are 3.8,
2.4, and 2.1 T, respectively. Otherwise, the bottomof the “ﬁeld well” Bmin changes from 0.5 T to 1.0 T,

Fig. 4.

which makes the microwaves with both frequencies of
18 GHz and 28 GHz available to induce ECR with suitable mirror ratios such as Binj /Bmin ∼ 4, Bext /Bmin∼ 2–3, and Br /Bmin ∼ 2–3. Further, the shape of themirror ﬁeld around the Bmin area can be changed arbitrarily by using the six solenoidal magnets. In otherwords, the ﬁeld gradient and size of the ECR region can
be changed to study the eﬀects on the ECR plasma
to optimize the beam intensity with a higher charge
state. Since the SC magnets can generate a strong
and large mirror ﬁeld, the plasma chamber is a largecapacity cylinder with an inner diameter of 150 mm
and a length of 575 mm. A total of 10 L/min or moreof cooling water ﬂows through the chamber wall. The
large volume of the plasma chamber increases not only
the volume of plasma but also the conﬁnement time of
the ions in the plasma to achieve a higher charge state.

Side and top views of the layout of SRILAC. Apparatuses described in the text are labeled.

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The high-power microwave system contains a 1.5-kW
klystron ampliﬁer and 5-kW gyrotron for microwaves
with frequencies of 18 GHz and 28 GHz, respectively.
The high-power microwaves heat the electron in theplasma to remove the electrons bound deep inside theatoms or ions.
Recently, we have been developing a method to evaporate a solid material with a high melting point suchas vanadium or calcium oxide using the high temperature oven (HTO).20) The tungsten crucible of the HTO
is heated to 2,000 K by the Joule heat caused by thehigh DC current of 700 A. For supplying the vanadium
beam to search for the new SHE, the experiment lasted
for about a month; therefore, we modiﬁed R28G-K
so that two ovens could be used as described in the
progress report this year.

Fig. 5. Photographs of DPS (Left) and gas-jet curtain(Right).

Overview of SRILAC

Figs. 4 and 5) was completed in December 2019. In
operations during the past year, we conﬁrmed thatthe DPSs achieved the desired pressure reductions ina stable manner without serious problems such as ﬁeld
emission problems.
During operations with GARIS-III for SHE research,
He leakages to the SRILAC became an issue although afour-stage large DPS upstream of the GARIS (DPSG)
was used for the windowless accumulation of He gas
up to ∼70 Pa. The pressure of DDPS is increased to∼10−7 Pa with the gas-ﬁlled GARIS-III when we use
the narrow oriﬁces of ϕ15 mm in the DPSG, whichmakes it diﬃcult to transport high-intensity heavy-ion
beams.
We introduced a N2 gas-jet curtain downstream of
DPSG (Figs. 4 and 5). The gas-jet curtain method is
a new method introduced in the DPS of the He gas
stripper.22) This method suppresses the leakage of Hegas by hitting a N2 -gas jet perpendicular to the He gas
ﬂow, and it converts the leaked gas into N2 gas, which
can be evacuated eﬀectively with the DPS.
The gas-jet curtain system has a beam aperture
with the diameter of ϕ25 mm. The nozzle is a twodimensional Laval nozzle designed to generate an N2 gas jet with a Mach number of 3.5 by optimizing with
CFD calculations (Fig. 5).
In the tests with the GARIS team, we conﬁrmedthat there was no serious backﬂow of N2 gas to the
GARIS-III when the gas-jet curtain was under our operating conditions using a quadrupole mass analyzer.
The eﬀect of charge exchanges and energy losses of
the injected beams by N2 gas jet were conﬁrmed to be
negligible.
The installation of the gas-jet curtain was ﬁnishedin September 2020. The leakage of He gas was signiﬁcantly reduced with the N2 gas-jet curtain and thepressure of the DDPS was ∼10−8 Pa even with larger
oriﬁces greater than ϕ25 mm in DPSG, which reduced
the diﬃculties with high-intensity beam transport to
the GARIS-III.

The superconducting-RILAC (SRILAC) comprises
three cryomodules (CM1, CM2, and CM3, see “SRILAC cryomodule” for detail), with room-temperature
medium energy beam transport (MEBT) between
them as shown in Fig. 4. The CMs contain neither SC magnets nor cold diagnostic devices. For
the beam transport line connecting the CMs, the socalled MEBT, a newly designed beam energy position monitor (BEPM) is employed instead of traditional wire scanners and Faraday cups to avoid thegeneration of particulates that cause the degradationof the cavity performance (See “Beam energy position
monitoring system for SRILAC ” for detail). Further,to prevent the contamination of the SC parts wherethe vacuum pressure level is 1 × 10−8 Pa, it is important to have an isolation system between the existing room-temperature part where the vacuum pressurelevel is several 10−5 Pa and the SRILAC (See “Compact particle-removal diﬀerential-pumping system andN2 gas-jet curtain for SRILAC ” for detail).
Compact particle-removal diﬀerential-pumping
system and N2 gas-jet curtain for SRILAC
A compact non-evaporable getter-based diﬀerential pumping system (DPS) with electrostatic particle
suppressors 21) has been developed to mitigate the large
diﬀerence in the vacuum and clean conditions between
the SRILAC and existing old RILAC with beamlines
built almost four decades ago. The three-stage DPS
was designed to achieve a pressure reduction from theexisting beamline vacuum (10−6 –10−5 Pa) to an ultrahigh vacuum of less than 10−8 Pa in SC cavities withina very limited length of only 75 cm, which ensures abeam aperture greater than 40 mm. All components(chambers, pumps, valves, vacuum gauges, etc.) werecleaned and assembled in an ISO class-1 clean room.
The installation of the DPSs (UDPS and DDPS, See4- S4 -

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Table 1. Design parameters of SC-QWRs for SRILAC.
The surface resistance is conservatively assumed to be
22.4 nΩ in the calculation.
Item
Operating temperature
Frequency at 4.5 K
Duty
Einj
Eout
Maximum gap voltage
Cavity typeNo. of SC cavities
Synchronous phase
βopt
TTF
ApertureG
Rsh /Q0
Target Q0
Qext
P0
Eacc
Vacc at Eacc = 6.8 MV/m, β = 0.078
Epeak /Eacc
Bpeak /Eacc
Amplifier output
Beam current

wards lowering the resonant frequency, and therefore,
the frequency variation is reduced by fine tuning thestem of each cavity and the total length of the outerconductor during assembly.25)
The inner surface of the cavity was chemically polished to 110 µm after assembly, annealed at 750◦ C,
and then chemically polished to 20 µm to maximize
the performance of the SC cavity. The cavity was thenhigh-pressure washed with ultrapure water from eachport and vacuum sealed in an ISO class-1 clean room.
After vacuum sealing, it was baked at 120◦ C, and performance tests were conducted on the single unit. As
shown in Ref. 24), all ten actual cavities fabricated exceeded the target Q0 value of 1 × 109 in the test. The
cavity must be operated in compliance with the High

Value
4.5 K
73.0 MHz100%
3.6 MeV/nucleon
6.5 MeV/nucleon
2.4 MVQWR (TEM)
10
−25◦
0.078
0.9
ϕ40 mm
22.4 Ω
579 Ω
1.0 × 109
(1.0–4.5) × 106
8W
6.8 MV/m
2.16 MV
6.2
9.6 mT/(MV/m)
7.5 kW∼100 electric µA

SRILAC cryomodule
The SRILAC comprises three CMs that contain atotal of ten SC cavities.23,24) CM1 and CM2 each contain four SC cavities, and CM3 contains two SC cavities (Fig. 4). The SC cavity of SRILAC is based on a
coaxial QWR made of pure niobium. The resonant frequency is 73.0 MHz and the cavity is operated at 4.5 K
cooled by liquid helium. The SC-QWR is fabricatedusing 3.5 mm and 4 mm pure niobium plates pressed
together to make the top plate, stem, outer conductor,
and bottom plate, and these are then assembled byelectron beam welding. This is the first actual QWR
made of pure niobium in Japan. Each QWR is designed to provide an acceleration voltage of 2.4 MV or
higher at 8 W per unit. The design parameters of thecavity are summarized in Table 1.
Cross-sectional views of the QWR and CM are
shown in Fig. 6. A pure titanium jacket is attachedoutside the cavity to store liquid helium. Permalloyis inserted into the space between the cavity and titanium jacket for magnetic shielding. The resonant frequency shifts because of external disturbances such as
fluctuations in the liquid helium pressure. The tuner
slightly pushes and deforms the cavity’s beam port,
which changes its capacitance and tunes its frequency
to compensate for the slow shift. However, the frequency adjustment range of this tuner is 14 kHz to-

Fig. 6. Cross-sectional views of SC-QWR (top) and cryomodule that contains four SC-QWRs (bottom).

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Table 2. Requirements from cryomodules for SRILAC.

Pressure Gas Safety Act in Japan, and therefore, it
was fabricated after preliminary evaluation and thencommissioned for inspection.
A fundamental power coupler (FPC) and RF pickup
are mounted on the bottom plate of the cavity. The
FPC is a coaxial capacitively coupled type, vacuum
sealed with a single ceramic window in the normal temperature section; it can handle the RF input power of
5 kW or more.26) The FPC inner conductor is fabricated from oxygen-free copper, and the outer conductor from stainless steel with a copper plating on
the inner surface. The thickness of the copper plating
was determined by balancing heat penetration with theheat transfer to the low temperature side and heat generation caused by RF power.
The vacuum vessel of the CM is made of steel anddivided into the top plate, top box, middle box, bottom box, and bottom plate. Electroless nickel plating
was applied to prevent corrosion. The cavity is builtwith four glass-epoxy pillars on the bottom plate. One
support is ﬁxed in place, while the other three slide
horizontally to allow for repositioning during cooling.
Liquid helium buﬀer pipe is installed at the top of
the cavity to control the liquid level for stable cooling. An aluminum plate is used for the 80-K thermal
shield, which is cooled by liquid nitrogen ﬂowing in series through three CMs. Cernox sensors are used asthermo sensors for the cavity, Pt-Co for the 4.5 K region, and Pt100 for the 80 K region. The heat loadof the CM is estimated to be about 18 W for heat intrusion and about 32 W for RF loss in the cavity during steady state operation in a module containing four
cavities. However, a heat load of 100 W per CM was
set as the design standard value for operation duringperformance degradation.
The ﬁrst step in assembling the CM is to move thebottom plate with the cavity and the middle box into
the clean room and to fasten the cavity and beam pipe

Item
No. of SC cavities
No. of cryomodules
Temperature of SC cavities
Temperature of thermal shield
Weight of cold mass
Heat load to 4.5 K cold mass
Dynamic
StaticHeat load to thermal shield
Cooling power
Inventory
Design pressure of the He vessel
Allowable pressure fluctuation

Value
10
2+1
4.5 K
80 K
1475 kg
120 W
30 W
120 W
≥500 W
313 L
0.125 MPa(A)
4.0 × 10−4 MPa

to seal their vacuum so that no dust can enter thecavity. Then, helium piping, liquid nitrogen piping,tuners, thermal shields, super insulators, etc. were installed, and the upper and lower boxes were covered
after moving to the installation site (Fig. 7).
A total of ten sets of digital low-level RF (LLRF)
circuits and 7.5 kW transistor ampliﬁers were introduced to excite the cavities. A digital low-level circuit
is equipped with an FPGA that stabilizes the RF voltage and phase by PID control and obtains the tuning
phase from the directional coupler signal. Internal parameters of the digital low-level circuit can be set remotely from a programmable logic controller (PLC).
Transistor ampliﬁers can operate with full output reﬂection, and they are equipped with ﬁve 1.5 kW output units with built-in isolators to continue operationin the event of unit failure. A control system using
the PLC is provided for each CM to control these RF
equipment.
Cryogenic system for SRILAC
The requirements from CMs for the SRILAC aresummarized in Table 2. SC cavities work at 4.5 K.
The thermal shield is designed to be cooled down byliquid nitrogen, and the total weight of the cold massis 1.5 ton. Heat load to 4.5 K and 80 K are 150 and
120 W, respectively. The inventory of liquid helium is313 L and the allowable ﬂuctuation of pressure in the
SC cavities is as small as 4 hPa because a change inpressure can change the resonance frequency of the SC
cavities. The design pressure of the vessel is as low
as 0.125 MPa(A) because the cavity has bellow, whichcannot sustain higher pressure.
The cryogenic system was designed to have a cooling power of 500 W at 4.5 K. This system comprises ahelium compressor, refrigerator, and buﬀer tank. The
cooling power is four times that of the heat load because we like to have a big margin to ensure contin-

Fig. 7. Photographs of cryomodule. (Left) After assembling and sealing the cavities and beam pipes in theclean room. (Right) After alignment at the installation
site.

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uous operation even if the condition of the surface on
the cavities is not so good that the cavities need considerably more cooling power. Figure 8 shows a cooling diagram of the cryogenic system. The compressed
helium gas is introduced to a cold box and liqueﬁed.
Liquid helium is then supplied to the three headersin the CMs from the top. The evaporated cold helium gas is returned to the cold box and the helium
compressor via heat exchangers. During pre-cooling,helium gas is supplied from the bottom to cool down
the cavities eﬃciently. Green lines are used for liquid
nitrogen supplied for shield cooling. A backup compressor for the superconducting ring cyclotron (SRC)
was used for this cryogenic system to reduce the total cost. The required pressure ﬂuctuation of 4 hPa
looks tight; however, from experience, it is possible
by control return pressure by the return valve witha precise pressure gauge. The Air Liquide company
supplied devices for the cryogenic system other thanthe compressor. Radiation-resistant valve positioners
were adopted because we install the refrigerator in theaccelerator room.
The stand-alone operation of the helium refrigerator
demonstrated a cooling capacity of 700 W at 4.5 K before the ﬁrst cooling of the SRILAC. Figure 9 shows thecooling down curves. The cold mass was successfully
cooled down to liquid helium temperature in four days
although pre-cooling was halted because of the trouble in the thermal insulation vacuum system for halfa day. The heat load was estimated from the heaterin the helium refrigerator to be 150 W for 2 + 1 CMs.
The pressure ﬂuctuation was hit within 4 hPa.

Fig. 9. Temperature of cold masses in the three CMs in thefirst cooling down.

Beam energy position monitoring system for
SRILAC
It is crucial to monitor a beam to accelerate it stablemanner. When the SRILAC beam is accelerated, beamloss must be reduced to under 1 W/m. Furthermore,
destructive monitors generate outgassing; if they areused, it becomes diﬃcult to maintain the Q-value andsurface resistance of the SC cavities over a long periodof time. Therefore, we developed a new BEPM system that can simultaneously measure the beam position and energy by measuring the time-of-ﬂight (TOF)
to continuously monitor the beam nondestructively. A
great advantage of this system is that it can handle
a time-chopped beam by synchronizing the measurement system with the beam-chopping signal. At thestart of commissioning, the beam was chopped to several percent duty cycle to protect the SC cavity frombeam loss. Although the beam intensity was 20 electric nA, we measured the beam position and energy
to accuracies of ±0.1 mm and several 10−4 precision,respectively.
Depending on the installation location, three typesof BEPMs (Types I, II, and III) were designed and
11 BEPMs were fabricated. The BEPMs are installedin the centers of the quadrupole magnets located between the CMs. Photographs of three types of BEPMs
and a cross section of a BEPM are shown in Fig. 10.
The ideal linear response of the quadrupole moments isrealized by using a parabolic design while maintaining
good linear position sensitivity.
A block diagram of the BEPMs and data-acquisition(DAQ) system is shown in Fig. 11. The DAQ system
can synchronize the RF reference with a beam chopper
signal. The ampliﬁed pickup signals are transmitted
through coaxial cables to the signal processing devices.
The upstream and downstream signals are switched bymultiplexers and digitized by digitizers (PXIe-5160).
Although the sampling speed is 1.25 GS/s, a consider-Fig. 8. Cooling diagram for SRILAC.

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ably higher sampling speed of 50 GS/s can be achievedby using the random interleave mode under the condition that the signal is repeating continuously. Thus,
highly accurate TOF measurements can be realized.
All modules are integrated into a PXI express chassis. The signal process procedures are controlled by
the LabVIEW 2019 graphical programming language,
and module drivers are supported by the National Instruments Corporation. The obtained data are sharedusing the CA Lab that is a user-friendly, lightweight,
and high-performance interface between the LabVIEW
program language and EPICS-based control system.
It allows easy reading and writing of EPICS process
variables (PV). Measurement results are displayed ona remote desktop. Further, once these data are saved,
the Control System Studio (CSS), which is an Eclipsebased tool to operate a large-scale control system, candisplay the results anywhere in the control room.
The ﬁrst 40 Ar13+ beam acceleration commissioning
was conducted using nine SC cavities. Figure 12 shows
the displayed results of the BEPM measurements immediately after the beam was successfully accelerated
to 6.2 MeV/nucleon (2020/01/28 21:02). The waveforms, beam positions, and beam energies at each station are displayed; the value circled in red indicatesthe ﬁnal accelerated energy.
The BEPM system worked extremely well when determining phases of SC cavities and conﬁrming beamenergies and positions, which enabled stable beam acceleration. The measured beam energy is plotted inFig. 13 as a function of the phase of the ﬁnal SC cavity. In the focusing region, the lower energy beam gains

Fig. 11. Block diagram of BEPMs and DAQ system.27)

energy, whereas in the defocusing region, the lower
energy beam loses energy. This measurement clearlyshows us the region we should select to accelerate thebeam stably. The phases of SC cavities are currently
set to −25◦ . Each phase of the SC cavity was measured
and set in sequence from upstream to downstream (See“Beam Commissioning of SRILAC” for detail).
Overview of Control System for SRILAC
Project
A distributed control system that uses Experimental Physics and Industrial Control System (EPICS)
should be adopted as in RIBF to operate the SRILAC
project eﬃciently, and a higher-level application protocol needs to be integrated to the EPICS Channel
Access (CA) protocol. Further, the SRILAC control
system requires corrections and upgrades to the shortcomings of previous RILAC control system, for example control protocol for an electromagnet power supply
and a machine protection system.
In this project, the R28G-K was installed at the frontend of RILAC to increase the beam intensity. The
new SC-ECRIS control system consists of PLCs embedded with EPICS.28) The ion source control systemdiﬀers from other control systems in that control stations with digital and analogue modules need to beinstalled on the high-voltage stage. At RIBF, a reliability of the conventional SC-ECRIS control system
is not so high because interlock signals are exchanged
via TCP/IP between the safety system and control stations on the high-voltage stage. The new SC-ECRIS
control system uses two diﬀerent types of CPUs (sequence CPU and Linux CPU) in the main PLC station, and it was connected to ﬁve PLC substations witha star-topology ﬁeld bus communication using opticalﬁbers for insulation. As a result, the interlock signal

Fig. 10. Photographs of the three types of BEPMs: (a)Type I, (b) Type II, and (c) Type III. (d) Cross section
drawing of a BEPM. (e) Schematic of Type I.27)

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exchanges the control station on the high-voltage stage
through field bus communication on the optical fiber;
system reliability was improved compared with the system that uses the TCP/IP-based interlock signal.
With this SRILAC project, the layout of the beamline has changed and new electromagnet power supplies have been introduced for the LEBT electromagnet. Old electromagnet power supplies were utilized
for MEBT electromagnets. These electromagnet power
supplies control used GPIB as the communication protocol, and their performance was relatively low compared with the modern protocol. Therefore, we upgraded the power supplies to replace the GPIB communication with the standard method of RIBF control
system such as FA-M3 PLC and NIO.29) In beam commissioning, approximately 30 modified electromagnet
power supplies were controlled via EPICS CA protocol
without problems.
With the installation of the SRILAC downstream of
the normal conducting RILAC, a machine protection
system has become necessary. For the previous RILAC control system, the machine protection system,
which comprises a simple electromagnetic relay circuit has been implemented. The machine protection
system is called the Beam Interlock System (BIS) at
RIBF; further, we newly developed and implemented
BIS for SRILAC. The BIS has a mechanism that activates the beam chopper for a signal triggered by anerror from the control equipment and stops the beam.
The speed at which the trigger signal is transmitted to
the CPU is improved compared with that of the conventional BIS by adopting the field bus connection forinter-station communication. In addition, by adopting
a ring-topology, we designed it to increase redundancy.
In the previous RILAC control system, hard-wired

Fig. 13. Measured beam energy plotted as a function of thephase of the ﬁnal SC cavity.27)

control that directly connects the accelerator room andcontrol room with a wire was already replaced and all
operations were based on remote control. However,various systems such as data collection and RF control were not integrated with the EPICS CA protocol,and these were stand-alone systems. When the control system was updated in the SRILAC project, allcontrol protocols for the higher-level application were
integrated in the EPICS CA protocol. For example,
BEPM was a LabVIEW-based system using National
Instrument PXI, and by introducing CA Lab in the
middle layer, we succeeded to operate it using the
EPICS CA client.27)
Since we succeeded to access to almost all control equipment including the above-mentioned electromagnet power supply, BIS, BEPM, and RF using
the EPICS CA protocol, it had become possible tounify the development method of the operator interface(OPI) and realize various OPIs using CSS (Fig. 14).
In addition, the Archiver Appliance has been newly
introduced as the EPICS-based data archive system;
it has become possible to store nearly 10,000 signal
points as data at a cycle of 10 Hz or higher.30) Thissystem has the ability to search for arbitrary data by
the timestamp and visualize the data on a Web-based
application. During SRILAC beam commissioning, theArchiver Appliance was very useful for clarifying the
behavior of SC cavity and the cause of troubles, and it
is currently one of the essential tools for SRILAC beamtuning. In the RILAC control room (Fig. 1), consoles
for hard-wired control system have been removed andnew consoles ideal for the remote-control system havebeen introduced (Fig. 15).
In the near future, we plan to improve the performance of BIS and implement another type of machine
protection system driven by the behavior of electri-

Fig. 12. Displayed BEPM measurement results immediately after the 40 Ar13+ beam was successfully accelerated to 6.2 MeV/nucleon (2020/01/28 21:02). Positions
at station 4 were oﬀ scale, and it was corrected after theﬁrst trial acceleration.27)- S9 -

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cal current for the electromagnet 31) because of the increase in the beam intensity from SRILAC.
Beam Commissioning of SRILAC
The design parameters of the SRILAC are listed inTable 1, and its layout is shown in Fig. 4. The TEM
cavities for the SRILAC are designed to operate in the
CW mode. The gap length of the cavity is optimized
for β = 0.078 particles with a transit time factor of
0.9. The maximum gap voltage is 2.4 MV, which corresponds to an acceleration gradient Eacc of 6.8 MV/mwith a synchronous phase of −25◦ .
The FPC is designed with a tunable coupling so thata Qext range from 1 × 106 to 4.5 × 106 can be achievedby changing the insertion distance of its antenna. The
maximum beam current is about 100 electric µA for
the SHE synthesis experiments; therefore, beam loading is negligible. A low Qext as small as 106 was selected to broaden the resonance curve with a ±60 Hz
operational-frequency-range achieved by an RF input
power of 7.5 kW. In October 2019, the ﬁrst RF testwith an operational temperature of 4.5 K was performed without signiﬁcant trouble, except SC05 encountered a vacuum leakage from the ceramic windowof the coupler. During the RF test, amplitude feedback
and phase-lock loop parameters of the newly developed
digital LLRF were optimized and a stable RF-ﬁeld was
successfully obtained.
After a successful RF test, in January 2020, thebeam acceleration test was conducted for the ﬁrst time.
An 40 Ar beam was accelerated based on the requirement for SHE synthesis experiments. However, one of
the SC-QWRs (SC05) was not available; therefore, theFig. 14. Screenshot of CSS-based OPI for Faraday cup operation.

10

Fig. 15. New consoles for remote operation in the RILAC
control room.

acceleration energy was lowered to 6.2 MeV/nucleon
from the designed energy of 6.5 MeV/nucleon.
The 40 Ar13+ beam with an intensity of approximately 23 electric nA (duty 3%, chopper frequency
1 kHz) was accelerated to 6.2 MeV/nucleon with a
gap voltage of 1.13 MV. For the SC-linac tuning, SCQWRs were energized one-by-one and the beam energy was measured with a systematic variation of the
RF-ﬁeld phase (Fig. 16). The beam energy was precisely obtained by a TOF measurement with a pair of
BEPMs with a low beam current as described above.
The phase-scan plot, where Eout is plotted as a function of the RF-ﬁeld phase for each cavity, is shownin Fig. 16. The synchronous phase ϕs = −25◦ was
obtained by shifting the RF-ﬁeld phase from the zeroacceleration/bunching phase by 65◦ towards the topof the sine curve. The beam position remained almost
at the center of beam aperture during the phase scanbecause of the geometrical correction caused by thesteering eﬀect of the QWR cavity.23) The accelerated
beam energy successfully reached 6.2 MeV/nucleon at
9 PM on January 28, 2020.
After elaborate tuning, the transmission eﬃciency
from FC6A1 to FCe11 in Fig. 4 reached 100% witha beam current of 6.11 electric µA. The vertical andhorizontal beam positions monitored by BEPM were
centralized and the beam loss which occurred at the
MEBT was minimized keeping the deterioration of thevacuum pressure below 1 × 10−7 Pa.
A 51 V13+ beam with an energy from 4.2 MeV/nucleon
to 6.3 MeV/nucleon was delivered for a user service
time. To achieve a beam current of 10 particle µA,
the transmission eﬃciency of the low-energy part of
the RILAC and optimization of the beam transport of
the SRILAC are currently being analyzed. The dataarchive system 30) for many accelerator parameters in-- S10 -

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RIKEN Accel. Prog. Rep. 54 (2021)

cluding not only the RF-field of the RF cavities, butalso the excitation current of the magnets, vacuum,
temperature of the cooling water, and BEPMs, is very
helpful to understand what occurs during beam tuning. Each cavity achieved the acceleration gradient of
5.7 MV/m that accelerates ions with M/q of 5.4 to
6.5 MeV/nucleon with ten cavities. The newly developed digital LLRF will be adopted to each RF systemof the RT-DTLs, RFQ, and bunchers to improve thestability of their RF-field.
Beam transport optimization from SRILAC to
GARIS-III
This section reports on beam transport. Heavy-ion
beams accelerated to approximately 6 MeV/nucleon
by SRILAC are transported to GARIS-III through thetransport line called the high energy beam transport(HEBT) line. The configuration of the beam line is
TQ–TQ–D–SQ–SQ–DQ (TQ = Triplet Quadrupole,
D = Dipole, SQ = Singlet Quadrupole, DQ = Doublet Quadrupole) as shown in Fig. 4. Further, there
is one dipole magnet between the second SQ and DQ,
but this is not excited when the beam is transported
to GARIS-III. The duct diameter is 60 mm in mostplaces; however, it is narrowed to 15–25 mm just before the target because of DPSG.
The requirements for the beam transport of theHEBT line are (A) beam loss less than a few percent
and (B) the adjustability of beam spot shape on the
GARIS-III target depending on the experimental conditions. The first requirement is important for maintaining the outside radiation dose. Since the side wallof GARIS-III is relatively thin, it is necessary to measure and confirm the dose while optimizing beam transport. For the second requirement, a horizontal ellipse
is desired in the production run to prevent the local de-

Fig. 16. Phase scan plot for first 40 Ar 6.2 MeV/nucleon
acceleration test.

Fig. 17. Data of emittance measurement after phase-ellipse
adjustment at an object point e00. (Left) Measured
values (red dots) and fit results (black dots) of beamwidths at the point denoted as e01. (Right) Phase ellipse estimated by fitting results. Top and bottom figures correspond to horizontal and vertical directions,
respectively.

pletion of the rotating target, while a large circle shapeis desired in the calibration run. To satisfy these requirements, the beam envelope should be narrower justbefore the target, where there is a relatively smaller acceptance, and it should be wider at the target.
Detailed optical calculation is required according to
the phase ellipse of the beam at that time to realize
the ideal optics with a limited number of optical elements. The optics are adjusted in the following threesteps. First, the phase ellipse of the beam at e00, which
is defined as the object point in the optical design,
is measured using the wire scanners at e00 and e01(Fig. 4). The beam width at e01 is measured withseveral optics between e00 to e01 by changing the TQ
magnets. Comparing the obtained widths and transfer
matrices, the phase ellipse at e00 is estimated. Second,
the phase ellipse at e00 is adjusted to be upright by the
TQ upstream of e00 based on an optical calculation.
Third, the optics from the object point to GARIS-III
are tuned using optical simulation 32) based on an adjusted phase ellipse at e00 and an experimental requirement for beam spot shape. In the default optics, theposition magnifications from e00 to GARIS-III are 1.0
in both of the horizontal and vertical directions. Basedon the measured phase ellipse and experimental requirements, position magnifications are adjusted while
suppressing the beam envelope in the entire beam line.
Finally, the optical system is optimized with a few particle µA beam by fine-tuning the quadrupole magnets- S11 -

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References

Fig. 18. Beam spot picture on viewer at GARIS-III target taken by CCD camera. The beam spot shape is ahorizontal ellipse as expected.

and steerers based on the baﬄe inside the diﬀerential
pumping system and viewer on the target.
Figure 17 shows an example of the estimated e00
phase ellipse after step 2, which causes the phase ellipse to be upright. This measurement was performed
in a commissioning run in June 2020, in which Ar11+
accelerated to 5 MeV/nucleon with intensity of 30 electric nA was utilized. The graphs shown on the left represent the beam widths (rms) measured by the e01 wire
scanner in each optics, the red dots represent the measurements, and black dots represent ﬁt results. The
contour plots shown on the right are phase ellipses at
e00 estimated from the ﬁt results. Here, the phase ellipses are assumed to be two-dimensional Gaussian distributions. As shown in the ﬁgures, the phase ellipses
were adjusted to be upright as calculated. The horizontal 4 rms emittance was ϵh = 3.1 (πmm·mrad), andthe vertical 4 rms emittance was ϵv = 6.5 (πmm·mrad),respectively. These values were consistent within 20%
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Optics in the HEBT line was optimized as expectedbased on the phase ellipse measurement and opticalsimulation. A new method to estimate the phase ellipse with non-destructive detector BPEMs 33) is underdevelopment.

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