Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

BEAM-PHASE AND RF-FIELDS MONITORING SYSTEMUSING LOCK-IN AMPLIFIER FOR RIKEN RI BEAM FACTORY1, A), B)

A)

A)

A)

Ryo Koyama, Masaki Fujimaki , Nobuhisa Fukunishi , Masatake Hemmi , Osamu Kamigaito
A)A), B)A)
A)
A)
Masayuki Kase , Yasuteru Kotaka, Naruhiko Sakamoto , Kenji Suda , Tamaki Watanabe ,A)A)
Kazunari Yamada , and Yasushige YanoA)

A),RIKEN Nishina Center for Accelerator-Based Science
2-1 Hirosawa, Wako, Saitama, 351-0198
B)
SHI Accelerator Service Ltd.
1-17-6 Osaki, Shinagawa, Tokyo, 141-0032

Abstract
We have developed beam-phase and RF-fields monitoring system using lock-in amplifiers in order to obtain stable
operation of RIKEN RI Beam Factory (RIBF). Our system monitors constantly fluctuations of voltages and phases of
totally 25 RF-cavities and also output signals of 11 phase pickup probes placed at beam transport lines. Using this system, we have started to investigate stability of our accelerator complex and relations between fluctuations of RF-fieldsof accelerators and observed beam-phases instabilities. In addition, lock-in amplifiers are also used to obtain good
isochronous magnetic fields of three cyclotrons newly constructed in the RIBF.

理研RIビームファクトリーにおけるロックインアンプを用いたビーム位相及び加速RF監視システム
１．Introduction
The RIBF is a new-generation facility, which provides
RI beams far from the stability valley[1]. The RIBF consists of three newly built ring cyclotrons in cascade together with the existing ring cyclotron and its injectors. Itaccelerates all kinds of heavy ions over 345 MeV/nucleon
with a high beam-intensity, the goal of which is 1 pµA.

２．Developed monitoring system
2.1 Uranium acceleration in RIBF
The layout of the RIBF with the operational parametersin the case of 345 MeV/nucleon uranium acceleration isshown in Fig.1. In the uranium acceleration, ions are accelerated by the RIKEN heavy-ion linac (RILAC), theRIKEN ring cyclotron (RRC), a fixed-frequency ring cyclotron (fRC), an intermediate-stage ring cyclotron (IRC),
and a superconducting ring cyclotron (SRC). In addition,
we have an injection buncher, a RFQ linac, and four rebunchers as shown in Fig.1. Two charge strippers areplaced downstream of the RRC and the fRC.
In such a multi-stage acceleration system, one of themost important factors for stable operation is to maintainthe matching of beam-phases between accelerators. However, drifts of beam-phases have been frequently observed,
reasons of which might be fluctuations of RF-fields,
1variation of magnetic fields of the cyclotrons, and so on.
Hence, it is important to monitor stabilities of acceleratorsconstantly in addition to beam-phases, and we have developed a monitoring system for both beam-phases andRF fields using commercial RF lock-in amplifiers (LIAs)
SR844 manufactured by Stanford Research Systems[2].
The system monitors beam-phases at 11 phase probes(PPs) installed in the beam transport lines and RF-fieldsof 25 cavities in the case of a uranium acceleration asshown in Fig.1. Using the system, we can investigate correlations between observed beam-phase instabilities andRF-field fluctuations.
2.2 Configuration of monitoring system
Both beam-phase and RF-fields monitoring system isdivided into 5 sections, which are around RILAC, RRC,fRC, IRC, and SRC. Each section has the monitoring system with basically the same configuration shown in Fig.2.
The beam-bunch signals and RF-fields are analyzed by
the LIA via switching modules. All the systems are controlled by the LabVIEW program developed by one of theauthors (R. K.).
It should be noted that the selection of the reference frequency of the LIA is important to obtain good signal-tonoise (S/N) ratio when we measure beam-phases. Fig.3
shows the frequency component of the signal from PPS71 which is installed in the injection channel of the RRC
as an example. The peaks observed under the beam-off

E-mail: rkoyama@riken.jp-268-Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

Fig.1: Schematic layout of RIBF with operational parameters for 345 MeV/nucleon uranium acceleration. The positionsof phase probes (PPs) used for a beam-phase monitoring are also shown. PPs which are radially mounted on the acceleration region of cyclotrons (PPs-RRC, fRC, IRC, and SRC) are used for the tunings of isochronous field of cyclotrons.
bunch signal are rejected by numerically subtracting theI/Q component measured under beam-off condition fromthat measured under beam-on condition using the developed program of the LIA system. The reference frequencies of the beam-phase monitoring system for six sectionsin the uranium acceleration are summarized in Table 1.
Table 1: Reference frequencies of beam-phasemonitoring system for 5 sections in uranium acceleration.
The frequencies of the main background (leakage RF) arealso listed in the third row.
Fig.2: Block diagram of developed monitoring system.

2.3 Phase resolution of LIA

Fig.3: Frequency component of signal from PP-S71 observed using spectrum analyzer under a) beam-off, and b)
beam-on conditions.
condition correspond to leakage RF-fields of the RRC,
third re-buncher and their higher harmonics. The beam
bunch signal has a much larger harmonic component thanthe leakage RF-fields. Therefore, if we select a frequency
with a high S/N ratio, we can perform measurements withhigh precision. In addition, signals other than the beam

We investigated the phase resolution of LIAs using acommercial function generator AFG3252 manufactured
by Tektronix. AFG3252 has two output channels and isable to generate two output signals with precisely thesame frequency and an arbitrary phase shift. Fig.4-a)shows the output phases of the LIA when the output
phase of channel 1 of AFG3252 is shifted with 0.01 - 0.10
degree steps for every 3 minutes using the setup shown inFig.4-b). Fig.5 summarizes the 3 minutes average of output phase plotted in Fig. 4-a) as a function of the phaseshift generated by the AFG3252. The resolution of theLIA is determined to 0.03 degree in the present investigation. We plan more accurate measurement using an RFphase shifter (trombone type) instead of the use of-269-

Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

AFG3252, because the phases of the two output signals
generated by the AFG3252 might be fluctuate each other.

Fig.6: Relative phase drift between two output channelsof PD.
Fig.4: Investigation of phase resolution of LIA; a) is results of measurement and b) is measurement setup.

was observed at PPs-e11, X51 and S71 during 6 hours. At
the same time, relatively large phase drift of RILAC-#5
was also observed among RILAC 6 cavities. The correlation between the observed beam-phase drift and instabilities of RF-field of RILAC-#5 is clearly understood asshown in this figure. For more detail analysis on the observed beam-phase instabilities and RF-field fluctuations,see reference [3]. In order to reveal reasons of instabilitiesof the RF-field, the time series analysis using various datasuch as electric power, cooling water temperature, roomtemperature and so on, is planed.

Fig.5: Every 3 minutes average of output phase plotted inFig. 4-a) for each step of set phase.
2.4 Phase stability of multi-channel power divider
In the RIBF, a chain of multi-channel power dividers(PDs) is used for distributing the reference signal fromthe master oscillator. Some signals from other output
channels of PD are also used as reference signals of theLIA system. Hence, we have to investigate the relative
phase stability between output channels of PDs in order toestimate the phase accuracy of LIA system. Fig.6-a)shows the relative phase drift between two output channels of the PD measured using the setup shown in Fig.6c). This PD is the first PD just downstream of the master
oscillator of the RIBF. The observed phase drift was
about 0.06 degree. Fig.6-b) shows the large phase drift of0.08 degree when the equipment hatch near the PD wasopened. This phase drift may be correlated with air temperature around PD, so it is planned to measure correlation between the phase drift and air temperature.

Fig.7: Phase drift of beam-phases and RILAC-RF#5.
Fig.8 shows the periodic arrival time of cyclic beam ineach cyclotron (fRC, IRC, and SRC) observed by the PPs
placed in the acceleration region. The results indicate that
good isochronisms were obtained except for the SRC.
Since the beam intensity was too low to obtain the precise
data set with high S/N ratio, the isochronism of the SRC
was poor. When the beam intensity will increases in future, the better isochronism of the SRC will be obtained.

Fig.8: Isochronous condition of new cyclotrons in uranium acceleration.

３．Examples of acquired dataReferences

Fig.7 shows an example of the phase drift of uranium
beam and RF-fields, which were observed on April 28,
2008 using the LIA system. The sizable beam-phase driftof 0.6 nsec, which corresponds to 4 degrees in RRC-RF,[1] Y. Yano, Nucl. Instr. Meth. B 261 (2007) 1009.
[2] R. Koyama et al., Proceedings of PASJ4-LAM32, August
2007, Wako-shi Saitama, WP38.
[3] K. Suda et al., “Stability of Radio Frequency system at
RIBF”, in these proceedings.

-270-