PERFORMANCE OF RF SYSTEM FOR COMPACT-ERL MAIN LINAC AT
KEKT. Miura# , M. Akemoto, D. Arakawa, H. Katagiri, T. Shidara, T. Takenaka, K. Nakao, H. Nakajima,S. Fukuda, H. Honma, H. Matsushita, T. Matsumoto, S. Michizono, Y. Yano, F. Qiu, A. Akiyama,
H. Sakai, K. Umemori, KEK, Tsukuba, 305-0801, Japan
Abstract
The construction of the compact Energy Recovery
LINAC (cERL) in KEK was completed at the end of 2013,
and the beam commissioning has begun. Solid-state
amplifiers were employed as the RF power sources in themain LINAC (ML). Cavity field feedback and tuner
feedback control were performed using the digital lowlevel RF (LLRF) system. Large microphonics wasobserved, but the effect was suppressed well by the digital
LLRF system.

INTRODUCTION
The compact Energy Recovery LINAC (cERL) is thetest facility for the 3 GeV ERL planned as the future lightsource at KEK. As shown in Fig.1, the cERL consists ofthe photocathode DC gun, injector LINAC, main LINAC(ML), and return loop. A normal-conducting buncher
cavity and three two-cell superconducting (SC) cavities
were installed in the injector. Two nine-cell SC cavities
were installed in the ML. The injected beam is accelerated
in the ML, and the beam is decelerated at the ML down to
the injection energy after circulating through the returnloop and then dumped. For a low-emittance beam, ahighly stable cavity field is required. The requirements forthe 3 GeV ERL are 0.01%rms in amplitude and 0.01 rms
in phase. The requirements for the RF stability of thecERL are less than 0.1% rms in amplitude and 0.1 rms inphase.

successfully transported the return loop, and the beam wasdumped after deceleration in the ML [2]. This paper
reports mainly the RF commissioning and performance ofthe ML.

HIGH LEVEL RF SYSTEM
The ML has two nine-cell SC cavities, MLSC1 andMLSC2. The RF frequency of the cERL is 1.3 GHz. The
loaded quality factors (QL) of MLSC1 and MLSC2 are
1.3107 and 1.0107, respectively. In the ML, theenergy of the beam acceleration is recovered bydeceleration; thus, the beam loading can be small. On theother hand, the large detuning caused by microphonics
was a concern before operation. The cavity unit testing
revealed that a large power source is not necessary.
Therefore, a 16 kW solid-state amplifier (SSA) and 8 kW
SSA were employed as high power RF sources for
MLSC1 and MLSC2, respectively, as shown in Fig.2. The
SSA is compact, stable, and reliable.

Figure 2: RF power sources in ML: 16 kW (right) and
8 kW (left) SSAs.

Figure 1: Plan view of cERL.
The construction of the injector was completed in April
2013 and the beam commissioning in the injector hasbegun. The RF was stabilized using a digital feedbacksystem. We already reported the RF commissioning
results of the injector [1].
The construction of the entire cERL was completed by
the end of 2013, and beam commissioning has begun. As
a result of commissioning, the accelerated beam was_________________________________________#takako.miura@kek.jp

LOW LEVEL RF SYSTEMFor RF feedback control, the digital low-level RF(LLRF) system using micro-TCA digital boards [3] isused in cERL, as shown in Fig.3. The boards consist of abase card and two daughter cards, an analog-to-digitalconversion (ADC) card and a digital-to-analog conversion(DAC) card, as shown in Fig.3. The field-programmablegate array (FPGA: Vertex5FXT), the digital I/O, and an
external trigger port are on the base card. The ADC card
is equipped with four 16-bit ADCs (LTC2208) and a clock
input. The DAC card is equipped with two dual DACs
(AD9783). Embedded Linux is installed in a PowerPC onthe FPGA. The experimental physics and industrialcontrol system (EPICS) is installed for communication
control, and each board act as an EPICS IOC. Two boards
are used for each cavity, one is for the RF field feedback
control and the other is for tuner control.

Figure 3: MicroTCA digital feedback boards.

RF Field Feedback Control
Figure 4 shows a schematic diagram of the digital
feedback system. The master oscillator (MO) frequencyof the cERL is the same as the RF frequency, 1.3 GHz.
The local oscillator frequency is 1310.1565 MHz. The
cavity pick-up signal of 1.3 GHz is down-converted to anintermediate frequency (IF) of 10.15625 MHz, which is
1/128 of the MO frequency. The sampling frequency ofthe ADC is 81.25 MHz, which is 1/16 of the MOfrequency. The sampled data are separated into in-phase(I) and quadrature (Q) components. After the phase for theI/Q are corrected using a rotation matrix, the I/Q data passthrough the infinite impulse response (IIR) low-pass filter[4], which is important because it rejects the excitation
due to parasitic modes, such as an 8/9 mode in the ninecell cavity. The feedback is performed by proportionalintegral control. The baseband I/Q signals from the DAC
card are fed to the IQ modulator. Finally, the amplified 1.3
GHz RF is fed to the cavity. The parameters such as thefeedback gain and filter were determined by parameterscanning [4].

piezo tuner are controlled by using the outputs of thedigital I/O and the DAC output of the FPGA board,respectively.
A tuner control diagram is shown in Fig.5. The
feedback logic in the FPGA is very similar to that of theRF field feedback. The phase is calibrated by setteing thephase difference () between the cavity input RF (f) andthe cavity pickup signal (c) to zero at the resonant point.
Figure 6 shows a result of a resonance search in MLSC1
by scanning the DAC-offset. After phase correction, eachphase datum passes through the IIR filter as a first-order
low-pass filter. The current set value of the cut-off
frequency is about 10 Hz. Tuner feedback is performed tomaintain  at zero. The amplitude thresholds were
established to maintain the reliability of the phasemeasurement.
As shown in Fig.5, the feedback method of piezo andmechanical tuners differ. For the piezo tuner, feedback is
the only integral control for . The center voltage ofpiezo is 250V, and the piezo is driven between 0 V and500 V by the amplified DAC output of the digital board.
When interlock occurs, the frequency feedback is turnedoff, and the piezo value is maintained using the hold value
as shown in Fig.5. Furthermore, to protect the piezo fromrapid and large changes, IIR low-pass filter is installed in
front of the DAC output. In contrast, for the mechanicaltuner feedback, pulses in the clockwise (CW) or
counterclockwise (CCW) directions are output inproportion to . For SC cavities, the piezo tuner issufficient for frequency feedback. On the other hand,frequency feedback of the normal-conducting buncher
cavity is performed by only for the mechanical tuner.

Figure 5: Schematic diagram of tuner control.
5000

TN4-piezoscan-20140206-13h12m38s.csv

Vc

4000
3000
2000
1000
-3

Figure 4: Schematic diagram of digital feedbacksystem.

 Kly - Cav

100

Each cavity has a mechanical tuner and a piezo tuner,which are used for coarse and fine tuning of the cavity
resonant frequency, respectively [5]. The tuners arecontrolled using the same boards as for the RF fieldfeedback. The stepping motor for mechanical tuner and-2-1.5-1-0.5

050
0-50-100-3

Tuner Feedback Control-2.5

4DAC Valuex 10Vc Maxpoint : Piezo= -16380 Phase offset : -11.6963 degree-2.5-2-1.5

DAC Value-1-0.5

0x 104Figure 6: Resonance search by scanning DAC-offset.
Phase is calibrated by setting =0 at the maximum
point of Vc.

The QL values of the ML cavities are around 107, so thecavity band width is approximately 65 Hz. For such alarge QL, detuning due to microphonics was a concerned.
Figure 7 shows the result of the  feedback for 3 h. 
was controlled less than a few degrees by the tuner
feedback. This corresponds a few hertz of detuning.

The phase noise was also measured with a signal source
analyzer. Figure 10 shows the phase noise spectra from 10
Hz to 1 MHz. A large phase noise caused by themicrophonics was observed from 10 Hz to around 500 Hzin the both cavities, MLSC1 and MLSC2. During
feedback operation, the phase noise due to microphonics
was suppressed, and the phase noise of Vc was almost thesame as that of the MO. The integral phase jitter from
10Hz to 1 MHz was around 0.017 rms, which isconsistent with the result for our digital LLRF system.
-40

FB5-PT-HGFB-1.5kW-8.6MV-ssa-20140521-2.csv
Pf: w/o FB
Pc: w/o FB
Pc: w FB
MO-60

Figure 7: Plot of the detuning angle of the ML cavities.

RF STABILITY
Figure 8 shows waveforms of the input RF (Vf) andcavity pick up (Vc) of MLSC2 during operation without
feedback. The stability of the amplitude of Vc does notdiffer greatly from that of Vf, however, the phasefluctuation of Vc is much greater than that of Vf because ofmicrophonics.
1.665

x 10

4

Vf: 0.10975%rms, Vc: 0.15353%rms
VfVc

1.655

-3

1.65-3.5

1.645
1.64
1.635

-4-5.5
0100

200

ms

300

400-6-100-120-140-160-180 110102103104105106HzFigure 10: Results of phase noise measurements ofMLSC2 using a signal source analyzer; green and blue:
input RF and cavity signal w/o feedback, red: cavity
signal with feedback, cyan: MO.

SUMMARY

-4.5
-5

1.63
1.625

VfVc

-2.5

Phase(deg.)

Amplitude

1.66

Vf: 0.065368deg.rms, Vc: 0.89349deg.rms-2dBc/Hz-80

0100

200

300

400ms

Figure 8: Waveforms of input RF (Vf) and cavity pickup(Vc) of MLSC2 during operation without feedback.
Figure 9 shows a waveform of Vc of MLSC2 during
operation with feedback. Stabilities of MLSC1 andMLSC2 with feedback are 0.012% rms and 0.013% rms
in amplitude, 0.014 rms and 0.015 rms in phase,respectively. These results satisfy requirement of cERL.

Figure 9: Stability of Vc of MLSC2 with feedback:
0.013% rms in amplitude and 0.015 rms in phase.

The construction of the cERL has been completed, andcommissioning has begun. SSAs were employed as thehigh-power RF sources of the ML. RF field feedback andtuner feedback were performed by a digital LLRF systemusing FPGA boards. The effect of microphonics wasdamped well by the digital feedback system. RF stabilities
of MLSC1 and MLSC2 are 0.012%rms and 0.013%rms in
amplitude, and 0.014 rms and 0.015 rms in phase,respectively.

REFERENCES[1] T. Miura et al., “Performance of RF System for Compact
ERL Injector in KEK”, Proc. of ERL2013, Novosibirsk,WG306.
[2] M. Shimada et al., “Commissioning of a Recirculation
Loop of the Compact ERL at KEK”, MOPRO111, theseproceedings.
[3] T. Miura et al., “Low-Level RF System for cERL”, Proc. ofIPAC’10, Kyoto, pp.1440-1442 (2010).
[4] F. Qiu et al., “Performance of the Digital LLRF System atcERL”, WEPME072, these proceedings.
[5] K. Enami et al., “Performance Evaluation of Compact ERL
Main Linac Tunerer”, WEPRI027, these proceedings.