PERFORMANCE OF THE DIGITAL LLRF SYSTEMS AT KEK cERL
F. Qiu#, D. Arakawa, Y. Honda, H. Katagiri, T. Matsumoto, S. Michizono, T. Miura, T. Obina,
H. Sakai, KEK, Tsukuba, Ibaraki 305-0801, JapanS. B.Wibowo, SOKENDAI, The Graduate University for Advanced Studies,Hayama, Kanagawa 240-0193, Japan
Abstract
A compact energy recovery linac (cERL), which is atest machine for the next generation synchrotron lightsource 3-GeV ERL, was constructed at KEK. In thecERL, a normal conducting (NC) buncher cavity andthree superconducting (SC) two-cell cavities wereinstalled for the injector, and two nine-cell SC cavities
were installed for the main linac (ML). The radiofrequency (RF) fluctuations for each cavity are required
to be maintained at less than 0.1% rms in amplitude and
0.1° in phase. These requirements are fulfilled byapplying digital low-level radio-frequency (LLRF)
systems. During the beam-commissioning, the LLRF
systems were evaluated and validated. A measured beam
momentum jitter of 0.006% shows that the target of theLLRF systems is achieved. To further improve the system
performance, an adaptive feedforward (FF) control-based
approach was proposed and demonstrated in the beamcommissioning. The current status of LLRF system andthe adaptive FF approach for LLRF control in the cERL
are presented in this paper.

is just like an adaptive FF control.
In this paper, we first introduce the LLRF system in thecERL, and then present the measured LLRF stability andbeam momentum jitter during the cERL beamcommissioning. In the next stage, we describe the basic
idea of the proposed adaptive FF approach for
disturbances rejection. Finally, we present the preliminary
result of this adaptive FF approach for microphonics
rejection in the cERL commissioning.

8 kW SSA

25 kW Kly.
300
kWSSAKly.
8 kW
8 kW SSA 16 kW SSA

Vector-sumControlling

Dump
Two-cell SC

Main linac 2 Main linac 1~ 20 MeV

SCSC
Nine-cell SC~3 MV/m for Injector Cavities~8.5 MV/m for main linac Cavities

INTRODUCTION
At KEK, a compact energy recovery linac (cERL), as atest facility for future 3-GeV ERL project, wasconstructed, and the first beam-commissioning wascarried out at June, 2013 [1, 2]. The cERL is a 1.3 GHz
superconducting radio-frequency (SCRF) machine that is
operated in continuous-wave (CW) mode. As shown inFig. 1, the cERL consists of an injector part and a mainlinac (ML) part. A normal conducting (NC) cavity(buncher) and three two-cell superconducting (SC)cavities (Inj. 1, Inj. 2, and Inj. 3), were installed in theinjector, and two main nine-cell SC cavities (ML1 andML2) were installed in the main linac (ML). For lowemittance beam, the requirements of the RF fieldstabilities are 0.1% rms in amplitude and 0.1° in phase in
the cERL. This requirements are fulfilled by applying
digital low-level radio-frequency (LLRF) systems.
The LLRF system in the cERL is disturbed by various
disturbances include the 50-Hz microphonics, the 300-Hz
high-voltage power supply (HVPS) ripples and the burst
mode beam-loading [3-4]. The current LLRF system is
not sufficient to reject all of these disturbances. In view ofthis situation, we have proposed a disturbance observer(DOB)-based approach for suppress the main
disturbances in the cERL [3]. Based on this approach, thedisturbances can be reconstructed by the cavity pickup
signal and then removed from the feedforward (FF) tablein real-time. Therefore, in terms of function, this approach___________________________________________#qiufeng@post.kek.jp

Figure 1: Layout of the cavities in the cERL. The marked
values of beam energy and accelerating field indicate thecurrent state in the cERL beam-commissioning.

HLRF SYSTEM
RF power sources including 25 kW klystron, 300 kW
klystron, 8 kW solid state amplifier (SSA) and 16 kW
SSA were employed in the cERL. Figure 1 shows thelayout of the cavities and corresponding power sources inthe cERL. Table 1 gives the loaded Q value, required RFpower, and RF sources for each cavity. It should be
mentioned that, in the Inj .2 and Inj .3, a vector-sum
control method is applied. All of these RF sources arestable and reliable in the beam commissioning.
Table 1: Cavity Parameters of the cERL

f1/2
[Hz]

RF power[kW]

RF source

5700038 kW SSA

540

0.53

25 kW Kly.

2.4300 kW
Kly.

Cav.

QL

Bun.

1.1×104

Inj. 1

1.2×106

Inj. 2

5.8×1051120

Inj. 3

4.8×1051350

ML1

1.3×10750

1.616 kW SSA

ML2

1.0×10762

28 kW SSALLRF SYSTEM
A simplified schematic of the cERL LLRF system isshown in Fig. 1. The 1.3-GHz cavity pick up signal isdown-converted to a 10-MHz intermediate frequency (IF)signal at first. The 10-MHz IF signal is sampled at 80MHz by a 16-bit analog to digital converter (ADC) andthen fed into a field-programmable gate array (FPGA)
board. The baseband in-phase and quadrature (I/Q)
components are detected from the IF signal with a nonIQ-based IQ detection method. After being filtered byinfinite impulse response (IIR) filters, the detected I/Qsignals are compared with their set values, and the I/Q
error signals are calculated. The I/Q error signals arecontrolled by proportional and integrational (PI) feedback(FB) controllers and then added together with theircorresponding FF tables. Finally, the combined I/Qsignals are fed into an I/Q modulator via 16-bit digital toanalog convertors (DACs) to regenerate the 1.3-GHz RF
signal. This regulated RF signal will be used to drive thehigh-power RF source (e.g. klystron and SSA), which
drives the cavities. It should be mentioned that, toevaluate the stability of the cavity pick-up signal, we haveinstalled a pick-up monitor inside FPGA (see Fig. 2). An
adjustable-bandwidth digital filter, aims to remove theADC noises, is placed in front of the monitor.
A µTCA system is employed as the digital control
platform. Experimental physics and industrial control
system (EPICS) is selected to be the data communication
system. The detailed information about that digitalplatform can be found in [4].

SW1Q

Calib.

IIRFilFilIF ≈ 10 MHz

ADC

Vector sum Fil.
IIIR
 cos sin sin cos φbVc

Buncher-90°

Inj. 10°

0.7 MV

Inj. 20°0.65 MV

Inj. 30°

0.65 MV

ML10°

ML20°

RF stability (rms)

δA/A

δθ

0.07%

0.04°

0.006%

0.009 °

0.007%

0.025°

8.56 MV

0.003%

0.010°

8.56 MV0.003%

0.007°IQ Mod.

Down
Convertor

Pre. Amp.

RF = 1300 MHz
I_Set

ADC

CavityKly. / SSA

FPGA

LTC2208

Table 2: Status of RF Systems in the Commissioning

1300 MHzLO ≈ 1310 MHz
Analog filter

Clk ≈ 80 MHz

observed in the phase of the ML2, this component ismainly caused by the microphonics [5].
The beam energy stability is measured by the screen
monitor which is installed downstream of the bending
magnet with a 2.2 m dispersion and 62.6 µm/pixel
resolution. The beam momentum jitter is calculated basedon the peak point of the beam projection in the screen
monitor. The calibrated beam momentum jitter is about
0.006% rms as shown in Fig. 4. This value is inconsistence with the measured RF stability in the LLRF
system (see Table 2).

DIO (RF SW)
I_FF
 cos sin sin cos PIPICalib.
Q_FF

Q_Set

DAC I
DAC

Q

AD9783

bandwidth

ADC
ADC

Power PC
LinuxEPICS IOC

Monitor(pick up)

AMC Card

Waveform

CommandFigure 2: Schematic of the LLRF system in the cERL.

STABILITY
The typical LLRF system performance in the cERL
beam-commissioning are listed in Table 2. Generally
speaking, all of our LLRF systems satisfy therequirements of the cERL beam-commissioning.
Disturbances in the LLRF system are suppressed well byapplying high FB gains. However, some high intensity
disturbances still exist in the LLRF systems. Figure 3
shows the RF performance of Inj. 2&3 and ML2. It isvery clearly to see that, there is a 300-Hz fluctuation in
the vector-sum RF filed of Inj. 2&3 cavities, especially inthe phase. Investigations reveal that this 300-Hz ripples
come from the high voltage power supply [3]. On theother hand, an approximately 50-Hz component can beFigure 3: RF stability of the Inj. 2&3 (top) and ML2(bottom). The 300-Hz fluctuation in the Inj. 2&3 iscaused by the high voltage power supply ripples.

Figure 4: Beam momentum jitter measurement. The
measured beam momentum jitter was 0.0065% rms,
which is in agreement well with RF stability.

ADAPTIVE FF CONTROLAs depicted in Fig. 3, disturbances still exist in thecurrent LLRF system even with high FB gains. These
disturbance signals can be rejected clearly by applying anadaptive FF approach. The key idea of this adaptive FF
approach is as bellow.
1. Identify the nominal system model andcalculate its inverse model.
2. Estimate and rebuild the disturbance signals
based on the inverse model in the step 1 fromcavity pick up signal.
3. Remove the estimated disturbance in step 2
from the FF table inside FPGA.
The first step is performed off-line, whereas the next twosteps are carried out in real-time. Therefore, thedisturbances are removed in the cavity pickup signal. This
process can be illustrated in Fig. 5. Here, GP(s) and Gn(s)represent the actual plant (e.g., cavities and RF devices)and nominal model, respectively. Signals d and derepresent the real disturbance and the disturbance estimate,respectively. Signal FF represents the FF table output. As
shown in Fig. 5, the disturbance estimate de can be
expressed by [3]de    d  GP  s  Gn1  s    .

(1)

If the system nominal model Gn(s) is a perfectrepresentation of real system GP(s), then according to (1),the GP(s) is perfectly cancelled by Gn-1(s), therefore, thedisturbance estimate de is exactly equal with real
disturbance d, that means, the disturbance d is perfectly
rebuilt by disturbance estimate de. In practice, the systemmodel cannot be identified perfectly, this is to say, there
are some deviations between real disturbance d and de.
Fortunately, a related analytical study reveals that therobustness of this approach is rather strong, that means
the adaptive FF controller still works well even in thepresence of the model mismatch.
Micr ophonics
PS Ripple + Beamd

DACFFRemovedGP  s Cav

Pick upyGn1  s yInv.

desystems include the 300 Hz power supply ripples, thehigh intensity beam-loading, and the microphonics.
Concerning the validation of the adaptive FF controller
regards to power supply ripples rejection and beamloading compensation, the preliminary results werealready presented in [3]. For microphonics rejection for
the ML2 cavity, the performance of the adaptive FF
control is shown in Fig. 6. If we only use traditional PI
control but without the adaptive FF control (indicated bythe blue color in Fig. 6), the microphonics effect of ML2
cavity can be observed clearly in the phase of the RFfield. It is clear to see that there is a 50 Hz dominantcomponent in both waveform and spectrum in the RFphase of the ML2 cavity. After switching on the adaptive
FF control, the microphonics include the 50-Hz dominantcomponent are disappeared (indicated by the red color inthe Fig. 6).

Rebuild

Figure 5: Basic idea of the adaptive FF approach, the key
step is to reconstruct the disturbances based on the inverse
system model.
We have developed and installed this adaptive FFcontroller in the cERL µTCA-bases LLRF systems.
During the beam-commissioning, we have demonstrated
this approach for disturbance rejection. As presented
above, in the cERL, the main disturbances in the LLRF

Figure 6: Measured RF phase of the ML2 cavity pickup
signal in the case of with and without adaptive FF control.
Both waveform (top) and spectrum (bottom) arepresented.

SUMMARY
Digital LLRF systems for the injector and main linac
were constructed in the cERL at KEK. During the beamcommissioning, the LLRF systems perform well and therequired RF stability of the cERL (0.1% rms in amplitude
and 0.1° in phase) is satisfied. Furthermore, a beam
momentum jitter of approximately 0.006% was achieved.
Additionally, for R&D, we have proposed an adaptive FFbased approach aims to reject the disturbances in the RFsystem. Results in the cERL commissioning show that
this approach is very effective for the disturbancerejection.

REFERENCES[1] N. Nakamura et al., “Present status of the compact
ERL at KEK,” Proc. IPAC’14, Dresden, Germany(2014).
[2] S. Sakanaka et al., “Construction and commissioningof compact-ERL Injector at KEK,” Proc. ERL2013,
Novosibirsk, Russia (2013).
[3] F. Qiu et al., “A disturbance-observer-based
controller for LLRF systems”, Proc. IPAC’15,Richmond, USA (2015).
[4] T. Miura, “Low-level RF system for cERL”, Proc.
IPAC’10, Kyoto, Japan (2010).

[5] M. Satoh et al., “Mechanical vibration search ofcompact ERL main linac superconducting cavities incrymodule”, Proc. IPAC’14, Dresden, Germany(2014).