LLRF CONTROL UNIT FOR SuperKEKB INJECTOR LINAC
Takako Miura†1, Mitsuo Akemoto1, Dai Arakawa, Hiroaki Katagiri, Toshihiro Matsumoto1,Feng Qiu1, Yoshiharu Yano, KEK, Tsukuba, Japan
Na Liu, 1Sokendai, Hayama, Japan
Abstract
The low-level RF (LLRF) control unit based on the digital system has been developed for a stable and high precision pulse modulation for the SuperKEKB. The RF pulse
is changed at a 50-Hz repetition rate for the top-up injection to four different rings by the event system. The LLRFcontrol unit has not only the pulse modulator, but also otherfunctions: VSWR meter, RF monitor, event receiver (EVR),and pulse-shortening detection.

INTRODUCTION
SuperKEKB is an upgraded machine of the KEKB e-/e+
collider and aims at a 40-times higher luminosity than theKEKB by the nano-beam scheme [1]. The injector linac
has also been upgraded to provide higher bunch charge andhigher quality beam [2] for SuperKEKB.
Furthermore, the linac needs to provide the beams with
top-up injection to four storage rings, not only theSuperKEKB HER/LER, but also the photon factory PF/PFAR. Figure 1 shows the layout and beam energy patternsof the injector linac. The beam mode is switched pulse bypulse at a 50-Hz repetition rate of the linac, according that,
the RF phase is switched for the each pulse by the event
system [3]. The RF phase-inversion timing for the pulse
compression is important to reduce the beam accelerationenergy jitter, and it should be synchronized with the beamtiming generated by the event-timing system.

new system was designed to be compatible with the previous system [6] for an easy replacement. This paper describes the LLRF control unit and its performance.

RF SYSTEM
As shown in Fig. 1, the linac is divided into eight sectors(A−C and 1−5). Figure 2 shows the RF drive-line. Eachklystron unit consists of an S-band 50-MW klystron, which
is operated at 40 MW, and a SLED [7], which is a pulse
compressor. The RF frequency is 2856 MHz, and the RFpulse width is 4 µs. The RF phase is 180° inverted at thelast 1 µs for pulse compression by the SLED. Each klystronunit feeds the RF power into four 2-m accelerating structures, and the field is 21 MV/m. Typically, each sector has
eight klystron units. The RF pulse with amplitude andphase modulation is amplified by 60-kW sub-booster klystron, and the amplified RF pulse is distributed to eight klystrons in each sector. The input RF phase and amplitude foreach klystron unit downstream of the sub-booster is adjusted by a mechanical-type high power IΦA (isolator /phase-shifter / attenuator). At the special units used for theelectron-gun, bunching, and energy adjustment, the RF iscontrolled independently at each klystron unit using anLLRF control unit and a 600-W solid state amplifier.

Figure 2: Klystron drive-line from a sub-booster klystron(left), and independent drive-line (right).
Figure 1: Layout and beam energy patterns of KEK injectorlinac.
For these purposes, various low-level RF (LLRF) components have been also upgraded [4, 5]. The new LLRF
control unit based on the digital system has been developed
for a stable and high precision pulse modulation for theSuperKEKB, and synchronization with the event system.
During the upgrade construction, the linac had to continue
providing the beam to the photon factories. Therefore, the___________________________________________† Takako.miura@kek.jp

LLRF CONTROL UNIT
The new LLRF control unit has been developed since
2011[8] as a pulse modulator with a higher precision andstability than the previous one: further, it has additional
functions such as a VSWR meter, RF monitor, event-receiver (EVR), and pulse-shortening detection.
Figure 3 shows a block diagram of the LLRF control unit.
The control unit consists of a Xilinx Virtex-6 FPGA board,an analog-to-digital (AD) / digital-to-analog (DA) board(six 14-bit ADC: AD9254, six 14-bit DAC: AD9744) thatis connected to the FPGA by an FMC connector, I/Q modulator/demodulator, and Armadillo CPU board with an

LCD touch panel. The sampling clock of the ADC/DAC is114.24 MHz, which is the same frequency as the eventtrigger system, to reduce the phase inversion timing jitterfor the beam.

external control inputs are generated by event system depending on the pulse mode changed at 50 Hz. In phase control, it has an “analog mode” and “digital mode”. In theanalog mode, which is the same as the old version, thephase is controlled by an external 0–10 V level input corresponding 0–400°. Meanwhile, in the digital mode, thephase information, which is distributed on an optical fiberfrom the event system, is received directly by the EVR implemented in the FPGA board. The digital mode is undertesting. The RF pulse is generated by the I/Q modulator
using I/Q baseband outputs from the DACs and a 2856MHz LO signal.
The correction for the nonlinearity of the I/Q modulator
is performed by calculations in the FPGA using the I/Q calibration table, which is stored in the compact flash memoryon the FPGA board. The specifications of the RF pulse
modulator are shown in Table 1
Table 1: Specifications of RF pulse modulator

Figure 3: Block diagram of the LLRF control unit
The waveform data and some information are sent
through a Gb Ethernet to the EPICS-IOC server at the 50Hz repetition rate of the injector linac.
For the LLRF system upgrade, RF monitor units [5] havebeen developed, and a similar design with the control unit
was adopted.

Frequency
RF output level
Linearity of amp.
RF pulse rise time
Phase setting-range
Linearity of phase2856 MHz+10 dBm (100%)
0.3% rms and max < ±0.5%< 35 ns (0 - 90%)
0 – 400°
0.3° rms and max < ±0.5°

Figure 5: Amplitude and phase error of RF output

Figure 4: Front/rear panel of LLRF control unit
Figure 4 shows the picture of LLRF unit. The pulse trigger and four-channel “video signals” used for the oscilloscope inputs in the front panel are output by DACs. We canset the parameters not only by the front touch panel but alsoby the VNC remotely. The parameter settings are saved in
the SD card on the CPU board, or the USB memory.

RF Pulse Modulator
The RF pulse timing and width, 0°/180° inversion timing,amplitude, and phase are controlled by external inputs. The

The typical result of the amplitude and phase accuracyof the RF output is shown in Fig. 5. The amplitude error is0.02% rms and the phase error is 0.04° rms in an amplitudesetting of 100%.
Figure 6 shows the measurement result of the temperature dependency of the RF output phase. The temperaturewas changed at 1 °C intervals, and the humidity was maintained at approximately 40% rh.
The output phase changed by approximately 4° between
22 °C and 28 °C. The temperature dependence of the phase
is severely large, and the thermal coefficient is 0.62 °/°C.
Thus, the LLRF control unit for each sub-booster has been
installed in a thermostatic chamber. However, in the caseof an independent drive unit, the temperature is not stabilized. This should be improved in the future.

Figure 8: Klystron output (top) and SLED output (bottom). Left: Normal case, right: Pulse-shortening case.
Figure 6: Temperature dependency of RF phase.

VSWR Meter
The LLRF control unit has the function of a VSWR meterfor protection of the klystron RF window. Three gate
ranges are prepared. The first two are to exclude the phaseinversion timing, and third is a special one to detect thedelayed reflection due to the discharge of the far accelerating structure. In each gate, we can set each Pf, Pb, andVSWR level, and select the type of the interlock. Furthermore, the interlock level is switched in the high- or lowlevel mode for the threshold level, as shown in Fig. 7.

SUMMARY
The LLRF control unit in the SuperKEKB injector linac
has been upgraded for a stable and high precision pulsemodulation. The RF pulse timing and phase are switchedat 50-Hz depending on the beam mode for the top-up injection to four rings. The pulse modulation is controlled by
the event system, and synchronizes with the event system
clock to reduce the phase-inversion timing jitter against thebeam.
The nonlinearity of the I/Q modulator was well correctedby the I/Q calibration. The phase drift depending on thetemperature is severely large, i.e., 0.62 °/°C. Thus, theLLRF control unit for the sub-booster has been installed ina thermostatic chamber.
Furthermore, the LLRF control unit has other functions:
VSWR meter, RF monitor, event receiver (EVR), andpulse-shortening detection, etc. All these functions areworking well.

REFERENCES

Figure 7: Settings panel of VSWR meter.

Pulse-Shortening Detector
Klystron pulse-shortening means that the width of thepulse becomes short, as shown in Fig. 8. It is caused by theinside of the klystron. In the LLRF controller, the pulseshortening is detected by comparing the pulse gate width
and the measured pulse width. When the pulse-shorteningoccurs, the VSWR interlock may occurs because of the low
or zero forward power. In the case of the pulse-shortening,
therefore, we can choose whether to disable the VSWR interlock. The information on the occurrence frequency isuseful for evaluating the klystron performance.

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