LOW-LEVEL RF SYSTEM FOR THE SUPERKEKB INJECTOR LINAC
T. Matsumoto1,2†, M. Akemoto1,2, D. Arakawa1, H. Katagiri1, T. Miura1,2, F. Qiu1,2, Y. Yano1,1
KEK, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
2
SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki, 305-0801, Japan
Abstract
The low-level RF system of the KEKB injector linac
has been upgraded for the SuperKEKB. To satisfy theoperational requirement for the SuperKEKB that is 40times higher in luminosity compared with the KEKB, themain drive system in the low-level RF system of theKEKB injector linac was reconstructed. With recent technical developments in the solid-state amplifier, the configuration of the sub-booster system is expected to change.
An outline of the low-level RF system for theSuperKEKB injector linac is described herein.

INTRODUCTION
After approximately 10 years of operation of the KEKB,
the KEKB injector linac has been upgraded for use in theSuperKEKB (Figure 1). To archive 40-times higher luminosity compared with the KEKB, the electron/positron
beam intensities and their emittances required for thelinac must be upgraded [1]. To satisfy these requirements
at the linac, a photo-cathode RF gun is added for the electron injection to the high energy ring (HER), and a damping ring (DR) is introduced for the positron injection tothe low energy ring (LER).
The KEKB injector linac is used as a multi-purpose injector not only for the KEKB HER/LER, but also for thephoton factory (PF), and the photon factory advanced ringfor pulse X-rays (PF-AR). During the KEKB operation
period, simultaneous top-up injection into the three storage rings (HER, LER, and PF) was established. In thesimultaneous top-up injection, the beam injection parame-

ter of each ring is controlled with an event-based systemand switched pulse to pulse at 50-Hz repetition [2].
To satisfy the requirements of the SuperKEKB andsynchronize with the event-based system for top-up injection, new RF sources for the photo-cathode RF gun, energy compensation system at the DR, and bunch compensation system for the DR are installed. Furthermore, various
developments and changes have been planned and implemented for the low-level RF (LLRF) system comparedwith that of the KEKB injector linac [3]: LLRF control unit and rf monitor system based on
digital system [4, 5], Main drive system for compatibility between laser
oscillation for RF gun and HER/LER beam injection,
 Sub-booster system based on 600-W solid-state amplifiers.
The main drive system and the sub-booster system aredescribed herein.

MAIN DRIVE SYSTEM
The main drive system is equipped with a frequency
multiplier/divider and two-frequency (571 MHz and 2856
MHz) phase shifters. The master frequency of theSuperKEKB injector linac is 571.2 MHz. The signal isgenerated with a signal generator (Keysight; E8663DHY2), which is synchronized with the SuperKEKB master oscillator via a 10-MHz reference signal.

571-MHz Phase Shifter
For laser oscillation of the RF gun, a supply of clock

Figure 1: Diagram of the upgraded RF system for the SuperKEKB___________________________________________† toshihiro.matsumoto@kek.jp

signals of 10.38 MHz, 114 MHz, and 2856 MHz are required from the main drive system. In the KEKB injector
linac, the switching of the HER/LER beam mode and theinjection phase adjusting was controlled with the phaseshifter located in the KEKB ring, as shown in Figure 2(a).
However, we found that the laser oscillation could not be
maintained by the signal phase modulation caused by thefast (~50 Hz) beam mode switching.

(a)2856-MHz Phase Shifter
The positron beam emitted from the DR is accelerated
with RF sources located from sector 3 to sector 5, asshown in Fig.1. However, the operating frequency of theDR and the HER/LER (508.9 MHz) is not an integer
relation with the operating frequency of the linac (571.2MHz). Owing to this relation, the positron injection to theLER is limited by a certain condition. To perform thepositron injection to the LER effectively, the 2856-MHz
phase shifter was installed. This phase shifter was developed based on the LLRF control unit [4] and changes thephase during the positron injection mode. The output
signal is converted into an optical signal with an E/Oconvertor and transmitted to sectors 3, 4, and 5. The
demonstration will be performed in the near future.

Frequency Multiplier/Divider(b)Figure 2: Configuration of the main drive system
for the (a) KEKB injector linac, (b) SuperKEKB
injector linac
To establish the compatibility between the laser oscillation and the fast beam mode switching, the 571-MHz
phase shifter was developed and the main drive system
was reconfigured, as shown in Figure 2(b). The 571-MHz
phase shifter consists of two I/Q modulators located inseries and an event-receiver (EVR), and its phase modulation is controlled with the event-based system. The firstI/Q modulator adjusts the HER injection phase with thephase modulation speed (~1 degree/ms) that the laseroscillation is maintained. The second I/Q modulator isused for the injection phase switching between the HER
and LER and for adjusting the LER injection phase.

The frequency multiplier/divider generates a 2856MHz signal for the linac fundamental frequency, 571.2
MHz for SHB2, 114.24 MHz for SHB1, and clock signal
for the event-based control system, 10.385 MHz for thebeam trigger signal, and 508.8 MHz for the monitor signal to the SuperKEKB ring.
The frequency multiplier/divider was manufactured tohave a low noise performance to minimize any RF phasejitter. The block diagram of the frequency multiplier/divider is shown in Figure 4 and the measured phasenoise of each frequency is listed in Table 1.

Figure 4: Block diagram of the frequency multiplier/divider

Figure 3: Phase noise distribution of the signal fromthe first I/Q modulator (blue), the second I/Q modulator (red), and signal generator (green)
The phase noise of output signals from the first andsecond I/Q modulators are measured. The result show noadditive phase noise compared with the phase noise of thesignal generator.

Table 1: Phase noise and jitter integrated from 10 Hz to 1
MHz of the outputs of the frequency multiplier/divider
Signal114.24 MHz
571.2 MHz
2856 MHz508.8 MHz

Phase noise (jitter)
3.2 mdeg. (78 fs)
10.4 mdeg. (51 fs)
54.7 mdeg. (53 fs)
10.4 mdeg. (57 fs)

The components of the main drive system are installed
into a thermostatic chamber and placed under a temperature control of 28 ± 0.03 °C. The thermostatic chamber iscomposed of two Peltier devices with an endothermic
quantity of 250 W and a heater for precise temperature
control [6]. By adopting a Peltier component instead of acompressor, the RF components located inside the thermostatic chamber are not affected by mechanical vibrations and a long operational life can be expected.
The phase drift of the signals (114.24 MHz/571.2
MHz/2856 MHz/508.9 MHz) output from the main drive
system is measured with a sampling oscilloscope (Tektronix; TDS8000) and the measured data is recorded to theEPICS archiver. A one-day drift tendency of zero-crossposition at each frequency triggered with 10.38 MHz isshown in Figure 5. The jitter, the fluctuation of the zerocross position, of each frequency is estimated with thedata shown in Fig. 5 and is listed in Table 2.

Figure 5: One-day’s drift tendency of zero-cross position at each frequencies: 114 MHz (red), 571 MHz(light green), 2856 MHz (blue), and 508 MHz (orange)
Table 2: Jitter estimated one-day’s drift tendency of eachsignals triggered with 10.38 MHz
Signal114.24 MHz
571.2 MHz
2856 MHz508.8 MHz

Jitter (estimated)
1.71 ps
1.10 ps
1.75 ps
1.15 ps

SUB-BOOSTER SYSTEM
In the KEKB injector linac, eight sub-boosters areavailable and each sub-booster supplies power to four oreight high-power klystrons, as shown in Figure 5(a).
However, for special purposes such as ones in the electron

(a)

(b)

Figure 6: Diagram of the sub-booster system (a) in the
KEKB injector linac, (b) planned for the SuperKEKB
injector linac

bunching section, positron target and focusing section,and energy spread adjustment, one high-power klystron is
directly driven with one 600-W solid-state amplifier. The
specification of the 600-W solid-state amplifier is shownin Table 3.
Table 3: Specification of the 600-W solid-state amplifier
TypeSolid-state, AB classFrequency
2856 ± 10 MHz
Output power
600 W
Gain
57.8 dB
Pulse width
200 ns – 5 μs
Gain57.8 dB
Cooling
Water coolingPower supply
AC 100 V(1φ50/60Hz), 8.0 A
Owing to the recent technological advances and cost
reduction in solid-state amplifiers, the cost of one 60-kW
klystron and that of eight 600-W solid-state amplifiers
necessary for constructing one sub-booster system isbalanced. Therefore, 60-kW klystrons are expected to beobsoleted in the future and replaced with new sub-booster
systems of 600-W solid-state amplifiers, as shown inFigure 6(b).

SUMMARY
To satisfy the operational requirements for theSuperKEKB and synchronize with the event-based system for top-up injection, the LLRF system of the KEKB
injector linac has been upgraded.
Various developments and changes have been plannedand implemented for the LLRF system of the linac. The
main drive system was reconstructed to operate the RFgun and the DR. We intend to construct new sub-booster
system constructed using only 600-W solid-state amplifiers.

REFERENCES[1] T. Kamitani, et al., “Injector LINAC upgrade forSuperKEKB”, LINAC’10, MOP011.
[2] K. Furukawa, et al., “Pulse-to-pulse Beam Modulation andEvent-based Beam Feedback Systems at KEKB Linac”,
IPAC’10, TUOCMH01.
[3] H. Hanaki, et al., “Low-power RF systems for the KEKB
injector LINAC”, APAC’98.
[4] T. Miura, et al., “LLRF Control Unit for SuperKEKB Injector Linac”, IPAC’18, WEPAK018.
[5] H. Katagiri, et al., “RF Monitor System for SuperKEKB
Injector Linac”, IPAC’18, WEPAK016.
[6] Y. Yano, et al., “Master Oscillator for the KEKB InjectorLinac”, Proc. of 28th Linear Accelerator Meeting in Japan,TP-15.