THE CONSIDERATION OF RF PHASE REFERENCE DISTRIBUTION SYSTEM FOR THE INJECTOR LINAC OF SUPERKEKB*N. Liu†, T. Miura1, T. Matsumoto1, F. Qiu1, S.Michizono1, S. B. Wibowo,
SOKENDAI, Kanagawa 240-0193, Japan
D. Arakawa, H. Katagiri, Y. Yano, 1KEK, Ibaraki 305-0801, Japan
LINAC is shown in Fig. 1 [2]. The J-shaped linac comprises 124.8 m long and 488.3 m long straight beam lines
which consist of 8 sectors (sector A-C and 1-5). Sector A
consists of two sub-harmonic bunchers (SHB1 is operatedat 114 MHz and SHB2 is operated at 571 MHz), an S-band(2856 MHz) pre-buncher and a buncher [3]. Other sectors
are operated at 2856 MHz as the regular accelerating sectors which comprise sub-booster klystron (SB)/solid state
amplifier (SSA), high power klystron, pulse compressor
and normal conducting accelerator structure. The reference
signals of three different frequencies (114 MHz, 571 MHz
and 2856 MHz) are generated by the Master Oscillator(MO, 571 MHz) system and delivered to each sector in thetwo beam lines through long coaxial cables and opticallinks. The RF reference phase stability depends on theshort-term RF phase noise jitter and the long-term phasedrift due to the temperature and humidity variations in environment, especially for the S-band signal. Taking into account the large amount of the RF devices to be synchronized, necessity of delivering different frequencies in longdistance, the long-term phase drift of the RF/optical components and long cables, the stabilization of the RF phasereference becomes one of the critical issues for the large
accelerators. A phase monitor and feedback system is being
considered to monitor and compensate the phase drift ofthe optical links.

Abstract
Stabilization of RF phase reference for long distancetransmission is very important for stable RF operation, especially in the large accelerator facilities. Coaxial cables
and optical links as the distribution medium without phasefeedback control are used for the present system of theSuperKEKB injector LINAC. A more stable RF phase distribution system using single-mode optical links with feedback control is required to improve the phase stability. For
SuperKEKB PHASE-2 commissioning, the phase stability
requirement is within 0.1 deg. rms including the short-termand long-term stability. This paper describes the main design considered issues and shows the current test results of
the optical devices, transmission medium and RF components. A high precision phase monitor system with twotone calibration is being considered.

INTRODUCTION
The SuperKEKB injector linac is utilized as a multi-purpose injector, which not only delivers 7 GeV electron
beams to the KEKB high-energy ring (HER) and 4 GeV
positron beams to the low-energy ring (LER), but also provides electron beams of 2.5 GeV and 6.5 GeV for the Photon Factory (PF) and the Photon Factory Advanced Ringfor pulse x-rays (PF-AR), respectively [1]. Layout of theRF reference distribution system for SuperKEKB Injector
124.8 mSector A

Sector B
B-5

B-6

SB-B

SHB1
ACC. Buncher Prebuncher SHB2
Guide 2856MHz 2856MHz 571MHz 114MHze-

RF GUN for lowemittance 5nC beamCoaxial cable line
Optical fiber line
e- beam linee+ beam line

10kW Amplifier
I ΦA

40MW Klystron
I ΦA

I ΦA

I ΦA

I ΦA

I ΦA

60kW Subbooster (SB)

Drive System

Drive System

SB-B
5m

DriveSystem

100m

90m

100m

114 MHz

TriggerSystem

508 MHz571 MHz

80m

2856
MHz

Reference Signal
Generator(FMD-30)

SB-5
E/O
O/E

400m

Oscilloscope

571 MHz
10 MHzMR

Sector R

SB-A
5m

High power
ATT/phaseshifter (I ΦA)

600W Solid State
Amplifer (SSA)

O/E
E/O
E/O

400m

RF Drive Unit

30m
Drive
System

SB-1

SB-C

SB-2

SB-3

Drive
SystemSB-4

Drive
System

I ΦA
I ΦA

I ΦA

I ΦA

I ΦA

Damping
Ring 1.1 GeV

I ΦA

I ΦA

I ΦA

I ΦA

For PF
2.5 GeV,0.1nC X 1I ΦA

For PF-AR,6.5 GeV
Bunch
Compression

e+ capture
sectionSector C

Sector 1e- bypass orbit

Sector 2

EnergyCompression

For LER
4.0 GeV,4nC X 2
Sector 3

SB-4

Sector 4

4-4

5-1

5-2
Sector 5

SB-5488.3 mFigure 1: Layout of the RF reference distribution system for SuperKEKB Injector LINAC.
___________________________________________† liuna@post.kek.jp

For HER
7.0 GeV,5nC X 2

OPTICAL TRANSMITTER & RECEIVERAs shown in Fig. 1, the electrical RF reference signals(114MHz, 571 MHz and 2856 MHz) are delivered to sector
A-C and sector 1 through coaxial cables. Compared to coaxial cable, optical link has many advantage in long distance transmission such as lower transmission loss, smaller
electrical noise. Part of the S-band reference signal (2856MHz) is converted to optical signals by an optical transmitter (E/O) and delivered through the single-mode optical fiber. The transmitted optical signal is received by an opticalreceiver (O/E) and then converted to an electrical signal atsector 2-5 as the local oscillator (LO) of Low Level Radio
Frequency (LLRF) control system. Therefore the reference
phase stability directly affects the beam performance. TheRF reference phase stability depends on short-term phasenoise jitter and long-term phase drift of the optical link
(E/O, O/E and optical fiber).

Short-term phase noise jitter
Take the phase stability requirement and S-band transmitted signal into consideration, the candidates are EOC144/OEC-1041 which is used in the sector 2-5 of theSuperKEKB injector LINAC, MP-2320 TX/RX andWSM-1/WRU-2 which is used in SuperKEKB main ring.
Due to the larger phase drift of S-band signal, the shortterm phase noise jitter of E/O and O/E was measured at2856 MHz with the Signal Source Analyzer (SSA, E5052,Agilent Technologies). Figure 2 shows the single sideband
phase-noise power spectrum of three different types of theoptical link for comparison. The result is shown in Table1.

Figure 2: Single side-band phase noise power spectrum ofthe 2856 MHz RF signal of optical links.
Table 1: RMS Jitter and Phase Noise [10 Hz-10 MHz]
Model
Signal Generator
MP-2320 TX/RX
EOC-144/OEC-1041
WSM-1/WRU-2

Jitter [fs]60.2098
94.3005
90.3788
109.0993

Phase Noise [°]0.07010.0970
0.09290.1122

In contrast to the other two optical links, EOC-144/OEC1041 has a lower phase jitter of 90 fs, corresponding to0.093 deg. rms at 2856 MHz. The specification of EOC-144/OEC-1041 is shown in Table 2. The modules are provided by TAMAGAWA Electronics Inc.
Table 2: The Specification of EOC-144/OEC-1041
Items
RF Input range
RF input level
Optical power
Wavelength
Temperature range

EOC-144/OEC-1041
10 ~ 3000 MHz-20 dBm (max. 0 dBm)
+7.8 dBm /+7.0 dBm
1310 nm (standard)+10 ~ +40 ºC

Temperature and humidity characteristics
The long-term phase drift of the RF reference signal depends on the characteristics of E/O and O/E. Owing to thequantity limitation of the temperature and humidity controlled chamber (ESPEC PR-3J), we measured the opticallink (EOC-144/OEC-1041 and 1m optical fiber) instead of
E/O and O/E individually with only one chamber. The test
bench is shown in Figure 3. The optical link was putted in
the chamber and the phase of the required rf frequency (114MHz, 571 MHz and 2856 MHz) signals was measured withNetwork Analyzer (NA, E5071C, Agilent Technologies).
In order to cancel the phase drift of NA itself and the coaxial cable outside the chamber, the 5m Andrew coaxial cable(FSJ1RK-50B) was used as the reference cable on port1
due to its excellent thermal stability [4].

Figure 3: Test bench of the temperature and humidity characteristics measurement of the optical link.
The temperature and humidity characteristics of the optical link is shown in Fig. 4 and Fig. 5, respectively. Thetemperature is changed from 25 ºC to 40 ºC (round trip)
every 5 ºC/5 hours at fixed humidity 50 %RH. And the humidity is changed from 50 %RH to 70 %RH every
10%RH/10 hours at fixed temperature 30 ºC. Table 3 summaries the temperature coefficient (T.C.) and the humidity
coefficient (H.C.). The temperature and humidity rises andthe phase drift decreases. We prefer use the test parameterof the S-band signal (2856 MHz) to estimate the phase drift
in the following paragraphs.
Table 3: T.C. and H.C. of the Optical Link
Freq.[MHz]114571
2856

T.C. [ps/ºC]-0.9004-0.7168-0.6154

H.C. [ps/%]-0.0547-0.0544-0.0382

Figure 4: Temperature dependence (left) and thermal coefficient (right) of the optical link.

Figure 5: Humidity dependence (left) and humidity coefficient (right) of the optical link.
In present system, the E/O and O/E are putted inside the The phase-stabilized optical fiber (PSOF) is required due
chamber at the temperature controlled 28 ± 0.1 ºC but the to its excellent stability. The propagation delay temperatureuncontrolled humidity. Figure 6 shows the fluctuation of coefficient of PSOF provided by Sumitomo and Furukawa
temperature and humidity inside the chamber in sector 5 is Inc. is 1 ppm/ºC and 0.4 ppm/ ºC from 25 ºC to 30 ºC meas0.22 ºC and 18 %RH, respectively, in 30 days. Therefore ured in [5]. Now only the Furukawa PSOF is available. The
the long-term phase drift of 2856 MHz transmitted signal humidity coefficient of Furukawa PSOF (100m) was measthrough E/O and O/E is -0.14° and -0.71°, respectively. ured and the result is shown in Fig. 7.
Due to the large humidity fluctuation, the humidity controlled chamber is necessary in each sector.

Figure 6: Long-term temperature and humidity fluctuation
inside the chamber in sector5 for 30 days.

PHASE-STABILIZED OPTICAL FIBER
The RF reference signal is delivered through optical fiber from MO to sector 2-5. The temperature and humidity
characteristics directly affect the reference phase stability.

Figure 7: Humidity coefficient of Furukawa PSOF.
The humidity transmission delay time is about -0.896
ps/km/%, -0.907 ps/km/%, -0.89.87 ps/km/% at 114 MHz,
571 MHz and 2856 MHz, respectively. The temperatureand humidity fluctuation of the injector linac gallery is 2ºC and 26 %RH during operation in 30 days (April 1st ~May 1st), respectively. When the thermal coefficient is +0.4
ppm/ºC, the temperature and humidity phase drift is +1.65°
and -9.48° at 2856 MHz for 400 m transmission from MO

to sector5, respectively. Thus the large humidity fluctuation has a big impact on the phase drift both E/O and O/Eand PSOF. These characteristics does not satisfy the phasestability requirement. The phase feedback control systemof optical link is necessary to compensate for the phase error in the propagation delay of the optical link.
In order to deliver the stable RF reference signal throughthe optical link over distances of a few hundred meters toseveral kilometres, several approaches have been implemented with the average uncertainties of 10 fs to a few hundred fs, such as 400-MeV proton linac of JAERI, KEKB
main ring, SACLA, LCLSII and European XFEL.

implemented [6, 7]. The simulation and implementation isin progress.

PHASE MONITOR SYSTEMThe RF reference distribution system requires phasemonitor system providing high measurement accuracy. The
detected reference signal passes through coaxial cable andmany RF components including attenuator, amplifier,mixer and Band Pass Filter (BPF). The measurement stability is mainly affected by the slow temperature and humidity variations effect on these components used in themonitor system.
Figure 8 shows the humidity dependence of 1.3 GHz RF
reference phase obtained in compact Energy Recovery
Linac (cERL) at KEK. In the figure, the unit of humidity(%RH) is voltage (V) of humidity sensor. Use the following equation to convert the voltage to %RH:y (%RH) = 26.774 ∗ x (V) − 25.659(1)Figure 9: Humidity coefficient of 1310 MHz BPF.

CONCLUSION
The temperature and humidity characteristics of opticallink were measured, and the thermal coefficient of -0.62
ps/ºC and the humidity coefficient of -0.038 ps/%RH fromwere obtained at 2856 MHz. Also the humidity characteristics of PSOF and 1310 MHz BPF were measured. Compare to the thermal phase drift, the phase drift induced by
the large humidity fluctuation makes more contribution.
Actually the humidity coefficient is not high. The phasedrift of optical link is not sufficiently low for the requirement. Therefore the phase feedback system is necessary inorder to transmit the RF reference signal over distances ofa few hundred meters. The high precision phase monitor
system independent of temperature and humidity variations are simulated and the implementation is in progress.
It will be used to measure the phase drift of optical link
between MO and sector 5 in the SuperKEKB injector linac
in September 2017.

REFERENCES

Figure 8: Humidity dependence of reference phase.
Most of RF components are putted inside the temperature controlled (28 ± 0.1 ºC) but humidity uncontrolledchamber. The 1.3 GHz reference phase is down-converted
to Intermediate Frequency (IF) 10 MHz with LO signal(1310 MHz) which is filtered by a narrow band BPF (1310± 5 MHz), then digitized and monitored with a Feedback
FPGA (Field Programmable Gate Array) board. The BPF
has high thermal coefficient -0.49 °/ºC measured in [4].
The humidity coefficient is -0.16 ps/%RH from 50%RH to70 %RH shown in Fig. 9. As shown in the Fig. 8, the total
phase error is 3°. The humidity fluctuation is 0.9 V, corresponding to 24 %RH and 1.81° phase drift (60% of the total
phase error). The desiccant (silica gel) is used in a box withBPF and the reference phase becomes more stable.
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