MEASUREMENT OF THE RF REFERENCE PHASE STABILITY IN THE
SUPERKEKB INJECTOR LINAC
N. Liu†, T. Miura1, T. Matsumoto1, F. Qiu1, S.Michizono1, SOKENDAI, Kanagawa, Japan
D. Arakawa, H. Katagiri, Y. Yano, 1KEK, Tsukuba, Japan
Abstract
The SuperKEKB injector is a more than 600 m J-shaped
LINAC. The requirement of the RF phase reference stability is 0.1 degree (RMS) at 2856 MHz for SuperKEKB
PHASE-2 commissioning. In order to clarify and improve
the reference line performance, the RF reference phase stability is measured. The phase noise of the RF reference ateach sector is shown in this paper. A new phase monitor
system is implemented to measure the short-term stability
and long-term drift due to the temperature and humidity
fluctuations in the klystron gallery.

INTRODUCTION
The SuperKEKB injector LINAC is utilized as a multipurpose injector, which delivers 7 GeV electron beams tothe KEKB high-energy ring (HER) and 4 GeV positron
beams to the low-energy ring (LER) and provides electron
beams of 2.5 GeV and 6.5 GeV for the Photon Factory (PF)and the Photon Factory Advanced Ring for pulse x-rays(PF-AR), respectively [1]. The J-shaped LINAC comprisesof 124.8 m long and 488.3 m long straight beam lines,
0m80 mwhich consist of 8 sectors (sector A-C and 1-5). The layoutof the RF reference distribution for the SuperKEKB Injector LINAC is shown in Fig. 1. Sector A consists of two subharmonic bunchers (SHB1 is operated at 114 MHz andSHB2 is operated at 571 MHz), an S-band (2856 MHz)pre-buncher, and a buncher [2, 3]. The other sectors are operated at 2856 MHz as the regular accelerating sectors,
which comprise of the sub-booster klystron (SBK)/solidstate amplifier (SSA), high-power klystron, pulse compressor, and normal conducting accelerator structure. Reference signals of three different frequencies (114 MHz, 571MHz, and 2856 MHz) are generated by the Master Oscillator (MO, 571 MHz) system [4]. The 2856 MHz reference
signal (REF) is delivered to the SBK at each sector throughlong coaxial cables (sector A-C,1) or optical links (sector2-5) [4]. The requirement of the energy spread forSuperKEKB injector LINAC is 0.1% in [5] so that the RFphase reference stability is estimated with the requirementof 0.1 degree (RMS) at 2856 MHz. In order to clarify theREF phase stability, the RF phase noise, the short-term stability and the long-term phase drift are measured.

110 m

Sector B
B-6

Sector A

B-1,2,3,4,7,8

B-5

Legend124.8 me-ACC. Guide

A-2,3,4

RF GUN

Coaxial cable line

SHB1
Buncher Prebuncher SHB2
2856MHz 2856MHz 571MHz 114MHze-

Optical fiber line
DC GUNe- beam linee+ beam lineI ΦAI ΦAI ΦAI ΦAI ΦA

6 KLY.

571 MHzMR10 MHz
508 MHz

Drive System
SB-B
5m30 m114 MHz

Trigger
System

10kW Amplifier

3 KLY.

Drive System

Drive System

High power ATT/phase shifter (IΦA)

I ΦA

SB-A
5m

600W Solid State Amplifer (SSA)

100m

100m

90m

60kW Sub-booster (SB)Master2 chamber

Reference Signal
Generator(FMD-30)571 MHz2856 MHz

Test chamber
Reference Phase
monitor system
ADC4Sector R

40MW Klystron

O/E
E/O
O/E
E/O
O/E
E/O
O/E
E/O

ADC3ADC2SLED

SB-5

E/O
O/E

ADC180m

15m
SB-2

SB-1

SB-C

Drive System
8 KLY.

4 SSA

2 KLY.

I ΦA

2 KLY.

I ΦA

I ΦA

I ΦA

I ΦA

Drive System

Drive System

2 SSA Damping Ring
1.1 GeV

2 SSA

I ΦA

RF Drive Unit

SB-4

SB-3
Drive System

5 KLY.

I ΦA

8 KLY.

I ΦA

6 KLY.

2 SSA
I ΦA

For PF
2.5 GeV,0.1nC X 1

I ΦA
For PF-AR
6.5 GeV, 5nC X 1

Bunch
Compression

e+ capture
sectionC-1,2,3,4,5,6,7,8
Sector C

0m

80 m

1-1,2

1-5,6,7,8
Sector 1

e- bypass orbit

2-1,2,3,4,6

160 m

For LER
4.0 GeV,4nC X 2
3-4,5,6,7,8
DN,3-2,3
Sector 3

2-7,8,DS
Sector 2

140 m

EnergyCompression

240 m

4-1,2,3,4,5,6,7,8
Sector 4

320 m

5-1,2

5-3,4,5,6,7,6-1
Sector 5

400 m

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

480 m

For HER
7.0 GeV,5nC X 2

RF PHASE NOISE MEASUREMENT
For the optical link, the 2856 MHz REF signal is converted into an optical signal by an optical transmitter (E/O),
and transmitted by the single-mode phase stabilized opticalfiber (PSOF). Then, the optical signal is converted into a2856 MHz electrical signal by an optical receiver (O/E).
This converted electrical signal is used as the local oscillator (LO) for the Low-Level Radio Frequency (LLRF) control system and the RF monitor in each sector. The electrical and optical components are inside the temperature-stabilized chamber without humidity control, except for thelong PSOF, which is distributed in the klystron gallery. ThePSOF was provided by Furukawa Inc., and the propagation
delay temperature coefficient is 5 ps/km/ºC from −10 ºC to
35 ºC in the specification. TAMAGAWA E/O and O/E are
installed in sector 2-5.

The results are summarized in Table 1. Due to the phasechange for different injection mode, there is no filter afterthe O/E output. Therefore, the frequency offset of the phasenoise integrated from 10 Hz to 10 MHz is considered. Mostof the phase noise at each sector is 100 -200 fs. It is roughly
meet the requirement of 0.1 deg.

SHORT-TERM AND LONG-TERM PHASESTABILITY MEASUREMENT
Phase Monitor System for Sector 2 to 5
In order to monitor the 2856 MHz REF phase drift of theoptical link during the beam operation, a similar configuration of optical link is used to deliver the converted electric RF signal back from sector 2-5 to 2856 MHz MO side.
A new phase monitor system is implemented in LINAC.
The schematic diagram of the monitor system including
LO generation is shown in Fig. 3. The S-band 2856 MHz
reference signal is down-converted to intermediate frequency (IF, 14.28 MHz) by the LO (2870.28 MHz). The IF
signal is sampled by a 16-bit ADC with a sampling rate(SR) of 114.24 MSPS and processed in the FPGA board
based on µTCA. When the SR and IF signals satisfy thecondition N ∙ SR = L ∙ IF (L is an integer greater than 3 andN is an integer), the IQ component of the sampled IF signal
can be calculated by the following equation [6-7].
𝐿−1

22𝜋 ∙ 𝑁I = ∑ 𝑥[𝑛] cos(∙ 𝑛)
𝐿𝐿𝑛=0

𝐿−1

22𝜋 ∙ 𝑁Q = ∑ 𝑥[𝑛] sin(∙ 𝑛)𝐿𝐿
𝑛=0Figure 2: Single side-band phase-noise power spectrum of2856 MHz REF of the optical links.
Table 1: Phase noise for sector A-5 [10 Hz–10 MHz]
Signal2856 MHz REF
SOPT O/E output
SB-A output
SB-B output
SB-C output
SB-1 output
SB-2 O/E output
SB-3 O/E output
SB-4 O/E output
SB-5 O/E output

Power
[dBm]
+6.47
+4.39
−8.56
−9.16
−11.38−12.32−8.90−11.25−8.22−8.09

Jitter[fs]
106.08159.58103.03
98.33
90.99
93.13
134.67127.06216.35103.89Phase
Noise [°]
0.109
0.164
0.106
0.101
0.094
0.096
0.138
0.131
0.222
0.107

The phase noise of the transmitted 2856 MHz REF ineach sector was measured using the Signal Source Analyzer (SSA) (Agilent E5052B). Fig. 2 shows the single
sideband phase-noise power spectrum of the 2856 MHzREF, 1 m short optical link (SOPT) and the forward signal(O/E output) from 2856 MHz generator side to sector A-5.

where 𝑥[𝑛] is the sampled signal. Here, L = 8 and N = 1
Take the system latency and the ADC sampling rate limitation into consideration, SR = 8 ∙ IF is selected. The I/Q
signals are filtered by the digital infinite impulse response
low-pass filter (IIR LPF) with 100 kHz bandwidth to suppress the ADC noise. Then the amplitude (A) and phase (φ)
of the IF signal is calculated with A = √𝐼 2 + 𝑄2 and φ =tan−1 (𝑄/𝐼).
The injector LINAC is operated in the pulse mode andthe reference phase of the injector is changed depending on
the injection mode of HER/LER at 50 Hz. The short-termand long-term data taking should be synchronized with thebeam trigger 50 Hz, and should exclude the phase changing
timing. The optical link returned back signals from sector
2-5 (SBxxOPTR) are measured by the ADC with100 kHz
bandwidth IIR LPF. The 2856 MHz REF signal is alsomeasured to detect the monitor system drift including frequency divider, mixer, 2870 MHz band pass filter (BPF)
and the S-band amplifier. The optical link phase stability isprocessed to reject the phase drift of the monitor systemitself. All the RF components of the monitor system are inside the temperature-stabilized chamber with 0.1 °C peakpeak stability. The 1 ms short-term stability is summarizedin Table 2. All the short-stability values are within 0.1 deg.
RMS except sector 5. We are trying to find out the reason.

Master Oscillator
571 MHz

Temperature Controlled Chamber
ADC Sampling Freq. 114.24 MHzµTCA
Board1
CLKBoard counter 14.28 MHz
TRG

Master1

Digital trigger 50 Hz
from Event System

Phase Shifter
Reference Signal Generator
FMD-30ADC4ADC3ADC2ADC1

114.24 MHz

Freq. Divider
AD95181/25

571.2 MHz114.24 MHz

Freq. Divider
AD9518LPF
15 MHz1/8

14.28 MHz

2856 MHz

RF=2856 MHz

Connector

IF=14.28 MHz
IF

LPF
15 MHzIF

LPF
15 MHzIF

LPF
15 MHzIF

Short optical link

SOPTR
O/E

SB3OPTR

E/O

1m

Coaxial cable (except S-band)
Master2 chamber

SB2 chamber

SB2OPTR
O/E

REF_1

LPF
15 MHz

E/O

140 m

E/O
O/E

LO Amplifer
BPF
2870.28 MHz
20 dB

REF_xx : 2856 MHz reference for Boardxx
SOPTR : Short OPTical link O/E output
SBxxOPTR : SBxx OPTical link Returned
SBxxOPT = SBxxOPTR - REF

2856 MHz coaxial cable
2870 MHz coaxial cable
Optical link

O/E

LO for LLRF
2856 MHz SB3 chamber

E/O

220 m

E/O

LO=2870.28 MHz

O/E
SB4 chamberµTCA
Board 2

E/O

O/E

300 m

CLK

E/O

O/E

TRG

Digital trigger 50 Hz
from Event System

ADC1ADC2ADC3ADC4IFLPF
15 MHzIF

LPF
15 MHzIF

LPF
15 MHzIF

LPF
15 MHzSB5 chamber

REF_2

O/E

SB4OPTR

E/O

E/O

380 m
O/E

SB5OPTRNot used

Figure 3: Schematic diagram of the 2856 MHz RF reference phase-monitor system based on µTCA.
Table 2: 1ms short-term optical link phase stability
REF[deg.]
0.028

Sector2[deg.]
0.055

Sector3[deg.]
0.068

Sector4[deg.]
0.067

Sector5[deg.]
0.22

PSOF. Fig. 5 shows the phase drift and the temperaturedrift. Because the temperature drift trends are very different from sector to sector, the temperature dependence
is not so clear.

In order to study the 2856 MHz reference phase dependence with humidity and temperature, the long-term
phase drift of the optical link from the 2856 MHz reference to sector 2-5 (round way), the humidity fluctuationand the temperature drift in the klystron gallery are monitored for 7 days.

Figure 5: Long-term phase drift of the reference phaseand the temperature fluctuations in the klystron gallery.

CONCLUSION

Figure 4: Long-term phase drift of the reference phaseand the humidity fluctuation in the gallery.
Fig. 4 shows the long-term phase drift and the humidity fluctuations in the klystron gallery. The humidity
fluctuation is around 25%RH and almost in the same
trend for sector 2-5. The phase drifts of the short andlong optical links are 1.5° and 12°, respectively. We
found that the phase drift depends on the humidity fluctuation clearly. The short optical link (SOPT) consistsof 1m PSOF and a pair of E/O and O/E. It is in the temperature stabilized chamber but no humidity control. So
the phase drift of SOPT indicates the phase drift of E/Oand O/E module due to the humidity fluctuation. Thephase drift of the long optical link (LOPT) for each sector include 2 pairs of E/O and O/E modules and the long

The phase noise of the 2856 MHz RF phase reference
was measured, and all the values of RMS jitter were
found to be less than 220 fs. A new phase monitor system based on µTCA is implemented to monitor the RFreference phase stability for sector 2-5. All the shortterm phase stability values for sector 2-5 are less than0.1 deg. RMS, except SB5. But the reason is not clear
for us. The long-term phase drift is more than 10 degrees. We found that the phase drift depends on the humidity fluctuation clearly. A feedback control system isnecessary for the RF reference phase stabilization to fulfill the requirement of the SuperKEKB injector LINAC.
Based on the present system configuration, a feedbacksystem, which includes a wavelength-division multiplexing device and variable optical delay line, is proposed [8-9]. The performance evaluation of the feedback system is in progress.

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