Proceedings of IBIC2012, Tsukuba, Japan

MOPB57OVERVIEW OF BEAM INSTRUMENTATION AND TUNINGAT RIKEN RI BEAM FACTORY
N. Fukunishi*, M. Fujimaki, M. Kase, M. Komiyama, J. Ohnishi, H. Okuno, N. Sakamoto,
H. Watanabe, T. Watanabe, K. Yamada, O. Kamigaito, RIKEN Nishina Center for AcceleratorBased Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
R. Koyama, SHI Accelerator Service Ltd., 1-17-6 Ohsaki, Shinagawa, Tokyo 141-0032, JapanThe RIKEN Radioactive Isotope Beam Factory (RIBF)
has been successfully producing the world’s most intense
medium-energy heavy-ion beams, such as 0.42-pμA-48Ca
and 24-pnA-124Xe beams. Several types of beam monitor
have played a vital role in attaining current performancelevels, though many are of conventional design. This article gives an overview of RIBF beam instrumentation andintroduces beam-tuning methods adopted by RIBF.

RI BEAM FACTORY
The Radioactive Isotope Beam Factory (RIBF) [1,2]started operation at the end of 2006 as the first secondgeneration radioactive beam facility. The RIBF experimentally investigates unknown areas of the nuclear chart,
especially candidate nuclei essential to nucleosynthesis,by producing the world’s most intense RI beams. For thispurpose, beam intensities of 1 pμA are required for all
stable ions, from hydrogen to uranium.
Table 1: Beam Intensities Obtained by RIBF
Ion

Energy(A MeV)

Beam
Intensity

Acceleration Mode

4

He

320

1 pμA

Variable energy

18

O

290

0.45 pμA

AVF injection

18

O

345

1 pμA

Variable energy

48

Ca

345

0.42 pμA

Variable energy

124

Xe

345

24 pnA

Fixed energy

238

U

345

3.6 pnA

Fixed energy

The RIBF accelerator complex consists of two injector
linac complexes (RILAC [3] and RILAC2 [4]), an injector AVF cyclotron [5] and four ring cyclotrons (RRC [6],fRC [7], IRC [8], and SRC [9]), including the world’s
first superconducting ring cyclotron SRC. Three acceleration modes are available in RIBF. The first is AVF injection mode, which is suitable for accelerating very lightions such as 4He and 18O. Here, AVF, RRC, and SRC areused in series. For medium-heavy elements such as 48Ca
and 70Zn, a variable energy mode using RILAC, RRC,
IRC, and SRC is employed. The third mode is a fixedenergy mode where the new injector RILAC2 is utilizedwith all four ring cyclotrons, producing high intensity,
very heavy ion beams such as 124Xe and 238U. Table 1_________________________________________

summarizes recent beam intensities. We have alreadyextracted 1-pμA beams from SRC for light ions, and aresteadily approaching 1 pμA for 48Ca. The new RILAC2
injector was recently commissioned, and various RIBF
upgrade programs are in progress to improve 124Xe and238
U beam intensities.

RIBF BEAM INTENSITY RANGE AND
BEAM INSTUMENTATION
The typical beam intensity from SRC, the final-stage
RIBF accelerator, is several microamperes. This corresponds to several tens of microamperes in the injector andintermediate-stage accelerators, because RIBF employs
two-step charge-stripping schemes [10]. RIBF beam monitors also measure beams with intensities of 100 nA orless, because we begin beam tuning with an intensityattenuated beam to avoid unnecessary hardware damage
caused by poor accelerator optimization. The maximum
beam power already obtained is 6.9 kW for a 345MeV/nucleon-48Ca beam, which requires careful beamtuning.
RIBF beam monitors are required to cover a wide
beam-intensity region ranging from a few tens of nanoamperes to 100 μA. RIBF mainly uses conventional destructive beam monitors because they are suited to lowintensity beam operations. RIBF beam monitors are divided into two groups: monitors specific to cyclotrons,
which include phase probes and radial probes, and monitors installed in beam lines and the two linac compexes,
such as Faraday cups and beam profile monitors.

PROBES USED IN CYCLOTRONS
A typical cyclotron beam tuning procedure is as follows. Ions are injected into the cyclotron through a bending magnet, magnetic channels, and an electrostatic channel. Baffle slits at channel entrances detect beam loss,
allowing best-fit determination of an ion injection orbit.
Ions are accelerated if proper phases of accelerating RF
fields are chosen under reasonable starting values for the
cyclotron’s isochronous magnetic fields. Step by step,ions are accelerated to higher energies by adjusting magnetic and RF fields. A radial probe plays an essential rolein this procedure. The ions at last reach the cyclotron design energy, but the isochronous magnetic fields are still
imperfect and are significantly improved by using a phaseprobe. After magnetic field tuning, RF phases are optimized with the use of the radial probe. Finally, ions are
extracted from the cyclotron through an electrostatic*fukunisi@ribf.riken.jp

ISBN 978-3-95450-119-9
Overview and Commissioning of Facilities1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPB57

Proceedings of IBIC2012, Tsukuba, Japan

channel, magnetic channels, and extraction bending magnets.

Phase Probe
A large cyclotron requires isochronous magnetic fields
with better than 5 ppm accuracy, because total RF phases
experienced by ions during acceleration are nearly 1 million degrees. Such accuracy can only be realized by iterated updates of the magnetic fields, based on precise
measurements of beam phase using a “phase probe.” Aphase probe is a set of electrostatic pickup probes installed adjacently from the inner to outer radius, covering
almost the entire acceleration region of the cyclotron.
Table 2 summarizes values related to the pickup electrodes installed in RIBF cyclotrons.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Table 2: Phase-Probe Related Quantities
RRC

fRC

IRC

SRC

Inner Radius (cm)

89

156

277

356

Outer Radius (cm)

356

330

415

536

Harmonics

9

12

7

6

Number of Pickup
Electrodes

20

14

15

20

Typical Turn Numbers

300

180

180

340

RF Phase Advance

0.97

0.78

0.45

0.73

6

(10 degrees)
Maximum RF Voltage (kV)

270

450

600

600

Signal Analysis

Osc.

LIA

LIA

LIA

Signals induced by ions passing through the phaseprobes are pre-amplified (20 dB) and analyzed by a digital oscilloscope (RRC) or lock-in amplifiers (LIA, SR844[11]). Within fundamental and higher harmonic beamcomponents, the harmonic giving the best signal-to-noise
ratio is chosen in LIA analysis [12].
Figure 1(a) shows an example of an optimization procedure making isochronous magnetic fields of fRC. Magnetic field updates are performed as follows. We madenumerical simulations and prepared a database of time
shifts induced on all the pickup electrodes by given current changes in the cyclotron’s main or trim coils (Fig.
1(b)). Using the database, new coil currents are determined by a least-square fitting method to minimize theRMS deviation of time shifts. The present method is not
only valid for the cases where the first-order perturbation
works well, but also effective for magnetic fields deviating sizably from an ideal isochronous magnetic field. In
the latter case, it is sufficient that we introduce a reduction factor of 0.7 or so when applying the results of firstorder calculations to an actual magnetic-field update.
Several updates are required until isochronous magnetic
fields with an RMS deviation of 0.1 ns are obtained. This

Figure 1: (a) Fitting procedure for fRC isochronous
magnetic fields. Lines show the time of data acquisition. (b) Estimated time shifts caused by given current
changes (0.03 A for the main coil and 3 A for trim coils)
for fRC.
corresponds to a few RF degrees, and never degrades
beam quality. This fitting procedure requires 1–2 h, depending on initial conditions.

Radial Probe
Our radial probes can measure beam intensities at any
acceleration stage, that is, at any radial position in thecyclotrons, simultaneously giving turn patterns of accelerated ions. Integral and differential electrodes aremounted on the front end of a stainless-steel shaft(Fig. 2). These electrodes are inserted into a vacuum
chamber of the cyclotron and are driven by a pulse motor
from inner to outer, or vice versa.
An integral electrode measures beam intensity and performs initial adjustment of isochronous magnetic fields,
as described previously. The integral electrodes used in
RIBF are made of tantalum and cooled by water. Eachelectrode is designed to be sufficiently thick to stop completely accelerated ions, except for very light ions having
a comparatively long range. For accurate beam-intensity
measurements, it is better to avoid escape of secondary
electrons produced by heavy-ion bombardments from theprobe. If a magnetic field with sufficient strength acts on
the probe, secondary electrons cannot escape the elec-

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Proceedings of IBIC2012, Tsukuba, Japanthe integral electrode. This design works well for thesetwo cyclotrons due to the existence of sufficient magnetic
fields. We attempted to use the same design for fRC and
SRC, but were unsuccessful. We have hence modified thedesign for fRC and SRC (Fig. 2). The modified design
works well for SRC by placing the differential electrodebefore the integral electrode. In the case of fRC, the modified design allows us to obtain essential information on
beam quality, but is still insufficient in that we find a decrease in zero current level of the turn pattern in the outerradius region (Fig. 4). This is further evidence of insufficient suppression of secondary electrons.

Figure 3: Magnetic fields calculated for the lines on
which radial probes move. All the results are for 238U
acceleration. The “R = 0 cm” point corresponds to theinjection radius for each cyclotron.

Figure 4: Effect of secondary electrons on fRC turnpattern.
Figure 2: Radial probe designs of RRC, fRC, and SRC.
A differential electrode measures the turn pattern of accelerated ions. The turn pattern, the radial dependence ofthe beam density distribution, is directly related to beam
quality, and hence reveals any existence of poor beam
behavior. Secondary electrons are a potential problem fordifferential electrodes, as with integral probes. If secondary electrons emitted from an integral electrode sidewall
hit the differential probe, it is impossible to obtain a realistic turn pattern because the secondary electrons may
dominate the ion contribution of interest. Hence, the angle of the integral probe sidewall relative to the ion orbit
and the position of the differential electrode relative to theintegral electrode must be carefully designed. Conventional design is adopted for RRC and IRC, in which theangle of the tantalum block sidewall is parallel to the ion
orbit and the differential electrode is placed just behindAmong the most important roles of the differential
electrode is selecting the optimal RF field phases. Ions
are accelerated at the top phases of accelerating RF fields
to avoid unnecessary energy spread. In addition, a flattopping cavity is introduced in fRC, IRC, and SRC tofurther reduce energy spread, and its phase must be closely matched. Figure 5 illustrates the sensitivity of beam
quality according to FT phase settings. The RF phase optimization procedure is one of the most time-consuming
beam tuning processes, owing to limited moving speedsof the differential electrodes.
One recent problem was serious damage to RRC’s radial probe, caused by high-power beams, nearly breaking
the integral electrode (Fig. 6). As mentioned previously,
beam tuning begins with an intensity-attenuated beam,and then beam intensity is increased step by step to its
maximum. In this process, the space charge effect andISBN 978-3-95450-119-9
Overview and Commissioning of Facilities3

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○trode, and are finally absorbed by the top or bottom plate.
Hence magnetic-field strength at the probe is important.
Figure 3 summarizes magnetic field distributions alongprobe-moving lines for all the RIBF ring cyclotrons. Ouroperating experiences indicate that integral electrodes
installed in RRC and IRC do not overestimate measured
beam intensity in their outer radius region, suggesting thatsuppression of secondary electrons is sufficient for thesetwo cyclotrons. On the contrary, enhancement by a factorof two or more is observed for fRC and SRC, caused bymagnetic fields decreasing in their outer radius regions.
Insufficient suppression of secondary electrons is a resultof limited space available for the radial probes in fRC andSRC.

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Proceedings of IBIC2012, Tsukuba, Japan

beam loading effect are expected to change the optimal
RF phases. We therefore frequently measure turn patterns
with relatively high-intensity beams. Because the existingradial probes do not have an protection system with thehigh-power beams, we failed to avoid serious damage tothe radial probe. Increased cooling capacity or the introduction of an interlock system is required.

monitors installed for all RIBF beam lines is less than thesum of the numbers listed in Table 3.
Wire scanner beam profile monitors (PF) are commonly used to measure beam position and profile. Three
measuring wires are mounted in the horizontal and vertical directions, and a middle line between them for information redundancy. PFs are used for beam centering andemittance analysis, as usual [2]. PFs cover the beamintensity region ranging from 0.1 nA to 1 μA using 4channel amplifiers.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Table 3: Summary of Beam Line Monitors
L (m)

PF

FC

PS

PP

RILAC–RRC

71

19

10

5

4

RILAC2–RRC

39

11

6

3

2

AVF–RRC

39

12

6

3

3

RRC–fRC

81

18

11

3

3

RRC–IRC

89

20

10

3

4

fRC–IRC

120

25

10

3

4

RRC–SRC

137

28

12

3

4

IRC–SRC

54

11

5

2

2

Figure 5: Comparison of IRC turn patterns with different
parameters of FT phase in the case of a 114MeV/nucleon-48Ca beam.

Figure 6: Damaged RRC radial probe. High-power beams
melted two bolts used to fix the integral electrode to its
cooling plate. The resulting heat deformed the top andbottom plates attached to the integral electrode.

BEAM LINE MONITORS
Table 3 summarizes the beam monitors used in the
RIBF beam transport lines with the length of the beamlines. Some lines partly overlap, so the total number of

Figure 7: Standard Faraday cup design.
Faraday cups (FC) stop beams, allowing for not only
beam intensity measurements, but also section-by-section
tuning. FCs are capable of measuring a beam current
ranging from 1 nA to 100 μA. Figure 7 illustrates thestandard design, in which a shallow cup is adopted due tothe importance of compactness. A compact cup is easily
inserted into or removed from a beam path, but the priceof compactness is again increased difficulty in completely
suppressing secondary electrons. To mitigate overestima-

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Proceedings of IBIC2012, Tsukuba, Japan

Figure 8: Results of a correlation study between PP signals and RF power for a 28 GHz ECR ion source.
Phase pickup (PP) electrodes are also installed in the
beam lines. Signals induced on PPs are pre-amplified (20dB ~ 40 dB) and analyzed by digital oscilloscopes orlock-in amplifiers similarly as the phase probes of thecyclotrons. PPs are used to determine correct phases ofthe four rebunchers installed in the beam lines. They arealso used for stability analysis of the whole RIBF accelerator complex. Time-varying PP signals are analyzed incorrelation with various time series data relevant to accelerator stability, such as magnetic fields, RF fields, power

supply currents, cooling water and air temperatures, andcommercial electricity. These are continuously monitored
and displayed in real time by an integrated system developed by one of the authors [12]. Figure 8 shows an example correlation analysis in which insufficient stability of agyrotron power supply used in the 28 GHz superconducting ECR ion source [15] drove beam intensity fluctuations. These fluctuations were detected as voltage fluctuations in the PP signals measured at the beam line’s S71
position (just before RRC injection). The power supply
causing the problem has been replaced.

SUMMARY
RIBF has been steadily upgrading beam intensity for all
atomic mass regions by using conventional beam monitors, mainly destructive ones. The maximum beam powerobtained is 6.9 kW, near the limits of destructive beam
monitors. Development of nondestructive monitors is an
important issue for future RIBF upgrade.

REFERENCES[1] Y. Yano; Nucl. Instr. Meth. B261 p. 1009 (2007),
doi:10.101b/ j.nimb.2007.04.174.
[2] N. Fukunishi et al., PAC09, Vancouver, May 2009,
MO3GRI01.
[3] M. Odera et al.; Nucl. Instrum. Methods A227, p.
187 (1984).
[4] O. Kamigaito et al., Proc. of PASJ2-LAM30, WP78,(2006). K. Yamada et al.; Proceedings of IPAC’10,
Kyoto, Japan, MOPD046 (2010).
[5] A. Goto et al., Proc. 12th Int. Cyclo. Conf. p.51 andp. 439 (1989).
[6] Y. Yano; Proc. 13th Int. Cyclo. Conf. p. 102 (1992).
[7] T. Mitsumoto et al.; Proc. 17th Int. Conf. on Cyclotrons and Their Applications, p. 384 (2004).
[8] J. Ohnishi et al., Proc. 17th Int. Conf. on Cyclotrons
and Their Applications, p. 197 (2004).
[9] H. Okuno et al., Proc. 17th Int. Conf. on Cyclotrons
and Their Applications, p. 373 (2004). H. Okuno etal., IEEE Trans. Appl. Supercond., 18 (2008) 226.
[10] H. Ryuto et al., Proc. 17th Int. Conf. on Cyclotrons
and Their Applications, p. 307 (2004).
[11] http://thinksrs.com/products/SR844.htm,
Stanford
Research Systems.
[12] R. Koyama; Proc. of EPAC08, Genoa, Italy,
TUPC052 (2008).
[13] T. Watanabe et al., Proc. of BIW10, Santa Fe, New
Mexico, US, WEIANB02 (2010). T. Watanabe etal., Proc. of IPAC’11, San Sebastian, Spain,
PP.1260-1262 (2011).
[14] T. Watanabe et al., RIKEN Accel. Prog. Rep. 43, 135(2010).
[15] T. Nakagawa et al., Rev. Sci. Instrum. 79 02A327(2008). T. Nakagawa et al., Rev. Sci. Instrum. 81
02A320 (2010).

ISBN 978-3-95450-119-9
Overview and Commissioning of Facilities

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○tion caused by insufficient suppression of secondary electrons, a long electron suppressor producing a nearly uniform suppression field was introduced, instead of conventional ring-type suppressors. However, there remains a
10–20% overestimation, which is strongly indicated bycomparing measured beam intensities with those measured by the radial probes of RRC and IRC. In addition,
FCs are a major source of residual radiation in the beamlines, so nondestructive beam current monitors with higher precision are required. A high-Tc SQUID beam current
monitor was therefore innovated by one of the authors[13], and long-term testing is scheduled for next fiscal
year.
Plastic scintillators (PS) are also introduced in the
beam lines. Six pairs of appropriately spaced plastic scintillators are useful to measure ion time of flight (TOF).
The beam energy as determined by the TOF monitors has
resolution of 0.1%. PSs installed just before the cyclotrons are mainly used to investigate beam phase width.
These TOF and plastic scintillation monitors played essential roles in the beam commissioning of RIBF and
RILAC2. In our case, intensity-attenuated beams of
106pps or so are directly measured using plastic scintillators with a data acquisition system newly developed on
LabVIEW architecture [14]. Due to limitations on maximum beam intensity allowed for PSs, it is currently impossible to quantify the space charge and beam loading
effects on longitudinal beam emittance of high-power
beams. Overcoming this limitation within a reasonable
cost remains a topic for further study.

MOPB57