Proceedings of LINAC 2002, Gyeongju, Korea

RF REFERENCE DISTRIBUTION SYSTEM FOR THE 400-MEV
PROTON LINAC OF THE KEK/JAERI JOINT PROJECT
T. Kobayashi†, E. Chishiro, JAERI, Tokai, Naka, Ibaraki, Japan
S. Anami, S. Yamaguchi, S. Michizono, KEK, Tsukuba, Ibaraki, Japan
324-MHz RF sources and four 972-MHz RF sources arerequired. To achieve the requirements of both the 3-GeV
ring and the ADS, the maximum pulse width and repetition rate are 620 µs, including the cavity build-up time,and 50 pps, respectively. Because RF sources must maintain the correct accelerating field within an amplitude
error of ±1% and a phase error of ±1°, a digital feedback
and feed-forward control system with high intelligence isrequired. Therefore, for the control systems, the reference
clock signal, which is distributed to all of the power amplier stations (4 solid states and 46 klystrons), should be
more highly stable. Our objective for the RF reference
aims at within ±0.2° for the phase stability.

Abstract
For the high-intensity proton linac of the KEK/JAERI
joint project, the error of the accelerating field must bewithin ± 1 degree in phase and ± 1% in amplitude. Thus, ahigh phase stability is required to the RF reference distribution system. A 12-MHz RF reference is converted to an
optical signal, and is distributed to 50 low-level RF control systems of klystron and solid-state amplifier stations
through optical fiber links (E/O, O/E and optical fibers).
Phase-stabilized optical fiber (PSOF) will be used as an
optical transfer line. The characteristics of generalpurpose optical components (E/O, O/E, PSOF) weremeasured. Their performance is not sufficient to satisfy
the requirements. New optimized E/O and O/E for thislinac will thus be produced. The total stability of the RFreference transfer system with new E/O and O/E will beevaluated.

2 RF REFERENCE DISTRIBUTION LINE
A block diagram of the RF reference distribution system is shown in Fig. 1. The 12-MHz RF reference is distributed to 50 low-level RF (LLRF) control systems of
klystron and solid-state amplifier stations (RFQ, buncher,chopper, DTL, STDL, ACS and debuncher) through optical links (E/O, O/E and optical cables). As shown in thefigure, the electrical RF reference signal (12 MHz) generated by a master oscillator is converted to optical signals
by an E/O, and is then divided into 14 optical transfer
lines by an optical star coupler. One optical line provides
the RF reference for 4 klystron stations. The transmitted

1 INTRODUCTION
In the KEK/JAERI joint project for the high-intensity
proton accelerator facility [1], the RF source for the injector (400-MeV proton linac) will power twenty accelerating cavity modules (an RFQ, 3 DTLs and 16 SDTL modules) operated at a frequency of 324 MHz, and twentythree ACS modules operated at a frequency of 972 MHz.
For the buncher, chopper and debuncher cavities, threefrom O/E

Klystron Gallery
Temperature Controlled Chamber

12MHz

Phase Stabilized Optical Fiber (PSOF)-3dBm+10dBmE/O

12MHz+10dBmwith Pulse
Driver

CLK & RF Generator

1x14
Optical Star Copuler

MasterOscillator

960MHz 48MHz(312MHz) CLK

12MHz 972MHz
Ref.
(324MHz)

14 Optical Lines
1x4 OC

1x4 OC

DigitalFeedBack
Control
12MHz

1x4 OC-10dBmBuilt in cPCI
with Temp.
Controller

O/E

E/O: Optical Transmitter (E to O Conveter)
O/E: Optical Receiver (O to E Conveter)
OC: Optical Coupler

I
I/Q
Mod.
Q

DriveAmp

12MHz
from Cavity

Optical Signal
Electrical Signal

LLRF
Controlto KLY

x50 LLRF Stations
ADS

KLY
Ion Source
ISDTL

3m

27m

Proton Linac Beam Line
50keV

3MeV

Buncher
Chopper

SCC 972MHz(Phase IIACS

SDTL
91m

108m
Buncher

50MeV

190MeV

400MeV

Fig. 1: Block diagram of the RF reference distribution system.
___________________________________________†3Gev Ring

600MeV
972MHz324MHzRFQ

GateSignal

PulseMod.

972MHz(324MHz)koba@linac.tokai.jaeri.go.jp

188

RFQ: Radio Frequency Linac
DTL: Drift Tube Linac
SDTL: Separate-type Drift Tube Linac
ACS: Annual Coupled Structure Linac
SCC: Super Conducting Cavity Linac

Proceedings of LINAC 2002, Gyeongju, Korea

3 PHASE-STABILISED OPTICAL CABLE
Since the reference RF is delivered trough optical links,
the phase stability directly depends on the characteristicsof the optical components (E/O, O/E and optical fiber).
For the optical transfer line, phase-stabilized optical fiber(PSOF) is required. The thermal coefficient of PSOF isgenerally less than 1 ppm/°C, while that of normal fiber isabout 6 ppm/°C. In order to reduce the thermal coefficient, the silica optical fiber is coated with a liquid-crystal
polymer, of which the thermal-expansion coefficient isnegative [3]. For this linac with a total distance of about
300 meters, the dispersion and power loss in PSOF are
negligible for both 1.31 µm and 1.55 µm wavelengths of
the Distributed Feed Back (DFB) laser.
There are two types in PSOF: one manufactured by
Sumitomo Electric Ind. Ltd Japan; the other by Furukawa
Electric Ind., Ltd Japan. However, these days, only Furukawa Electric Ind. Ltd Japan manufactures PSOF.
The temperature dependence of the transmission delay
time of both the Sumitomo and Furukawa PSOF wasmeasured. The result is shown in Fig. 2. About 1 ppm/°Cand 0.4 ppm/°C were obtained at temperature from 25 °C
to 30 °C, respectively. The characteristics of all measured
fibers of the same type agree with each other. No remarkable difference was observed between 1.31 µm and 1.55
µm wavelength transmission. Our results are different
from published data in [3][4]. Although the reason for thedifference is no evident, it is considerable that the characteristic depends on whether the cable is tightly or looselyrolled on a drum.

189

The room temperature will be controlled at 27 ±2 °C
during operation. When the thermal coefficient is 0.4
ppm/°C, a room-temperature change of ±2 °C induces
about a ±0.4° phase change for 300-m transmission at 972
MHz RF. This characteristic does not satisfy the required
stability. In order to keep the phase change due to the optical cable within ±0.05°, the temperature change of thePSOF should be controlled to be ±0.2 °C by a coolingwater system.
30.0
Sumitomo
25.0Transmission Delay Time [ps/km]

optical signal is divided into 4 by an optical coupler. Eachof them is received by an O/E and converted to an electrical signal at each station. The merits of optical transfer
are: a) very low loss, b) free of electrical noise, and c)high stability against temperature change. All componentsof this distribution system will be installed into the klystron gallery.
The accelerating source RF (klystron driving signal) of
324 MHz or 972 MHz is generated by a VCXO with PLL
synchronizing with the distributed 12-MHz reference ateach local station. Although the fast-change phase jitter issuppressed by the VCXO, for very slow-change phasejitter stabilizing the temperature drifts very important.
To stabilize the amplitude and phase of the field in theaccelerating cavity, a digital feedback and feed-forward
technique is used in the LLRF control system [2]. Thissystem controls I/Q components of the RF signal as
shown in Fig. 1. The feedback and feed-forward control is
performed by a combination of FPGAs (field programmable gate array) for fast and simple processing andDSPs (digital signal processor) for slow and complicateprocessing. At the present stage, a compact PCI (cPCI)
bus module and a Windows OS system is used as an integrated development environment in order to make it easy
for us to develop the software of FPGAs, DSPs and aCPU host.

Furukawa

20.0

Sumitomo PSOF

1ppm/°C ≈ 0.5°/300m/°C~ 1 ppm/°C( 972MHz )15.0

Furukawa PSOF
10.0

5.0

~ 0.4 ppm/°C

0.0-5.0

Temperature Range in Operation-10.01820222426283032PSOF Temperature [°C]Fig. 2: Measured temperature dependence of thetransmission delay time of the PSOF.

4 OPTICAL TRANSMITTER/RECEIVER
With respect to the required stability, Ortel 3540A and4510A, which are widely used for analogue signal transfer, are candidates as required E/O (optical transmitter)
and O/E (optical receiver), respectively. There are few
other candidates matching the required performance. In
our use, however, Ortel’s E/O or O/E has some mismatches; for example, they are suitable for use withhigher frequency (12 MHz is too low), the O/E has to bevery compact in order to be installed into the cPCI module of the digital feedback system, etc. Thus new optimized E/O and O/E for this linac were designed. A trial
product E/O (λ = 1.55 µm) was produced by Graviton
Ind. Ltd Japan [6], which is called Graviton LD-1500 inthis paper.
The stability characteristics of the Ortel 3540A, 4510A
and Graviton LD-1500 were measured. Figure 3 shows
the long-term phase stability in 324 MHz signal transmission through the optical link. A phase stability of ±0.04°(±0.12° for 972MHz) was obtained for about 6 hours.
Figure 4 shows the temperature dependence of the delay
time of the E/O and O/E. The thermal coefficients of Ortel
E/O and O/E are 0.75 and 1.25 ps/°C, respectively, andthat of the Graviton E/O is 0.5 ps/°C. From these results,
it is needed that the E/O should be operated in a thermalchamber. On the other hand, an O/E is to be built in thecPCI module of the digital feedback system at every drive
station. It should be equipped with a peltier device fortemperature control.

Presently, proper E/O and O/E are under the construction. The timing jitter of the optical transmission is designed to be lower than 1 ps (rms). Their performance
will be measured in the autumn of 2002.

OpticalAmp

SGE/O
12MHz

Optical Coupler
1 x 14 ch

Proceedings of LINAC 2002, Gyeongju, Korea

Optical Delay Module
OpticalCirculator∆φ
PSOF+21dBm-∆φ

1x4 OC

25%
Return+2dBm

O/E

Fiber Grating2∆φ0.602∆φ26.0

Temperature

LLRF
Control0.40

25.00.08°° p-p0.20

0.00

23.0-0.20

22.00.14°° p-p-0.40E/O(Ortel 3450A)+ O/E(4A11A)

E/O(Graviton LD1500)+ O/E(4A11A)

Fig. 5: Proposed phase feedback system of the opticaltransfer line using a fiber grating and an optical delaymodule.

24.0

Temperature. [°C ]Phase Shift [Deg.] (324MHz)

No Optical Links

6 SUMMARY
For distributing the RF reference, the temperature characteristics of the PSOF were measured and a thermal coefficient of 0.4 ppm/°C was obtained. It is not sufficiently
low for the requirements. We will control the temperaturechange of the optical cable to be within ±0.2 °C by using
a cooling water system.
Also, the performances of the E/O and O/E were evaluated. Although good long-term stability can be expected,
because of the temperature characteristics, it is necessary
to control the temperature to be constant. An O/E is to bebuilt in the cPCI module of the digital feedback control
system at every LLRF control station. It should beequipped with a peltier device for temperature control.
The production of new optimized E/O and O/E is in progress. The performance of the total system will be evaluated soon.
The introduction of an optical feedback system and anoptical amplifier for the RRF reference distribution isproposed, if needed. The performance of the system isunder evaluation.

21.0-0.60

20.0024681012Time [hour]Fig. 3: Long term stability of the E/O-O/E link. A
phase stability of ±0.04° (324MHz) was obtainedfor about 6 hours.

60.0
50.0

Delay Time [ps]

O/E

-1/2

3540A (RF-cable Canceled)

4511A (RF-cable Canceled)

E/O (Gravition LD1500)

O/E (Ortel 4511A)
1.25 ps/°° C

40.0
30.0

E/O (Ortel 3540A)
0.75 ps/°° C

20.0
10.0E/O (Graviton LD1500)
0.5 ps/°° C

0.0-10.0-10010203040

AKNOWLEDGEMENT

50

We would like to deeply thank Dr. T. Naito, Dr. K.
Ebihara and Dr. M. Suetake (KEK), who gave me much
help and information about the optical system and components.

Temperature [°C]Fig. 4: Temperature characteristics of the E/O andO/E.

6 REFERENCES
5 FEEDBACK LOOP
When an active phase-lock system is required for thestability of the reference distribution, we plan a feedbackcontrol, as shown Fig 5. The feedback loop consists of afiber grating which returns back a part of the optical signal in the same fiber, an optical delay module for phase
control, an optical circulator which picks out the backward signal, an O/E and a phase detector. In this system,
an optical amplifier is required because the output powerof an E/O is not sufficient. A stability test of the opticalamplifier and an evaluation of the performance of each
optical component are in pregress. The 1.55 µm wavelength of the E/O is valid for the use of an optical amplifier.
190[1] URL: http://jkj.tokai.jaeri.go.jp/[2] S. Michizon, et al., “Digital RF Control System for
400-MeV Proton Linac of JAERI/KEK Joint Project”,
this conference.
[3] T. Kakuta, S.Tanaka, “LPC coated optical fiber withzero thermal coeffient of transmission delay time”,
Proc. of International Wire & Cable Symp., pp. 234240, 1987.
[4] T. Naito, et al., “RF Reference Distribution using Fiber-Optic Links for KEKB Accelerator”, Proc ofPAC2001, pp. 791-793, 2001.
[5] H. Suzuki, et al., “508.58 MHz Synchronous Universal Counter for Beam Control System of SPring-8”,
Nucl. Ins. and Meth. A431, pp. 294-305, 1999
[6] URL: http://www.graviton.co.jp/