PERFORMANCE OF THE DIGITAL LLRF SYSTEM AT THE cERL
F. Qiu#, S. Michizono, T. Miura, H. Katagiri, D. Arakawa, T. Matsumoto, KEK, Ibaraki, JapanAbstract
A digital low-level radio frequency (LLRF) system has
been developed and evaluated at compact Energy
Recovery Linac (cERL) in High Energy Accelerator
Research Organization (KEK), Japan. A total of three
two-cell cavities were installed for the injector, and twonine-cell cavities were installed for the main linac. The
required RF stabilities for these cavities are 0.1% rms inamplitude and 0.1° rms in phase. To satisfy theserequirements, we survey feedback parameters such as theproportional and integral (PI) gains. Furthermore, weevaluated the beam energy fluctuation due to the vectorsum controlling error between the cavities injectors 2 and3. Finally, we present the performance of the LLRF
system that was realized in the beam commissioning. This
paper describes the current status of the LLRF system.

INTRODUCTION
The compact Energy Recovery Linac (cERL) is a
prototype machine that was developed for the future nextgeneration light source accelerator 3-GeV ERL project,which is a superconducting project that is operated incontinuous wave (CW) mode [1]. With cERL, a highaccurate radio frequency (RF) field stability of 0.1% (inamplitude) and 0.1° (in phase) is required to achieve anexcellent beam quality. For the future 3-GeV project, theRF field fluctuation should be maintained at less than
0.01% (in amplitude) and 0.01° (in phase). To realize suchchallenging RF field requirements, a μTCA-based digital
low-level RF (LLRF) system has been developed in thecERL.
To improve the performance of the LLRF system, thefeedback gains such as the proportional and integral (PI)gains were optimized. Furthermore, we analyzed theeffects of vector-sum control in the cavities of injectors 2and 3. Finally, we summarize the performance of system.
This paper focuses on the current status andperformance of the LLRF systems in the cERL.
Optimization approaches and preliminary experimentresults will also be presented.

LLRF SYSTEM
There is a total of six cavities in the cERL (see Table 1).
All of them are superconducting cavities (SCs) with the
exception of the buncher cavity, which is a normal
conducting (NC) cavity. Both vector -sum control (for thecavity of injector 2&3) and individual cavity control (forthe other cavities) are adopted in the cERL [2].
Figure 1 shows a schematic diagram of the LLRF
system employed in the cERL. The 1.3-GHz cavity probe
signal is down-converted to the 10-MHz intermediate
frequency (IF) signal. The IF signal is sampled at 80 MHz____________________________________________#qiufeng@post.kek.jp

by a 16-bit ADC (LTC2208) and transmitted to a fieldprogrammable gate array (FPGA). The base-band inphase and quadrature (I/Q) components are extracted
from the digitalized IF signal. After being filtered by afirst order IIR low-pass filter, the I/Q components areregulated by a PI controller. The processed I/Q signal is
fed into the I/Q modulator by a 16-bit DAC (AD9783) tore-generate the 1.3-GHz RF signal. Finally, the LLRF
feedback loop is closed by driving a high power source tothe cavities [3].
Table 1: Cavity Parameters
CavityQL

Buncher

1.125×1046

Injector 1

1.2×105

f1/2 [Hz]Ctrl. method

57000

Individual ctrl.

540

Individual ctrl.

Injector 2

5.78×101120

Injector 351350

Vector-sumcontrol

1.3×10750

Individual ctrl.

1.0×10762

Individual ctrl.

ML1ML2

4.8×10 cos(t) sin(t) A(t)    sin(t) cos(t )

 cos sin A   sin cos Figure 1: Schematic of the LLRF system in the cERL.

GAIN SCAN
To optimize the feedback gains in the PI controller (seeFig. 1) of the system, a gain scanning experiment wasperformed in the cERL. The system performance ismeasured, and recorded via different PI gains, and theoptimal gains were determined according to the scanned
performance curves. The detailed information about thisscanning process was presented in [2] and [3].
Figure 2 shows the gain-scanning results obtains fromthe cavities of the buncher and injector 1. It should be
noted that we normalized the gain in the digital PI
controller to its analog form [2]:KIKI
 K ( s)  KP (1)K ( z)  K P 
11 zs

Figure 3: Vector-sum control with a phase/amplitude
calibration error. The red lines (M and N) indicate thevector measured by the LLRF system, whereas the bluelines indicate the real vector seen by beam.

Figure 2: Gain scan results for the cavities of the buncher(upper) and injector 1 (lower). The optimal PI gains areindicated in the red circle.
In the lower figure of Fig. 2, we observe that, theproportional gain KP is the dominant gain. Thisconclusion can also be generalized to the othersuperconducting cavities. In the case of the NC cavity,
such as the buncher cavity (upper figure in Fig. 2); ahigher KP is not available because of its wider bandwidth;
as a result, the integral gain KI is the dominant gain.
Results for all of the six cavities are listed given inTable 2.
Table 2: Gain-scanning Results at the cERL
CavityKPopt

Dominant Gain

KIopt
5KI

Buncher

0.7

1.2×10Injector 1

84

1.0×105

KP

41

1.1×105

KP

1.5×105KP

1.5×105KP

Injectors 2 and 3
ML1ML2150150VECTOR -SUM CONTROL ANALYSIS
In a digital LLRF system, the true accelerating voltages(seen by the beam) are measured and calibrated by thedigital board (FPGA and/or DSP). In practice, thevoltages seen by the beam and the voltages measured by
the digital LLRF system are not exactly the same (seeFig. 3). The difference (which can be expressed as acomplex factor in Fig. 3) between them is the calibration
error. These errors are usually insignificant for individualcontrol. However, this situation is more complicated forthe vector -sum control.

Owing to the existence of the calibration error, thedetuning in the LLRF system (e.g., microphonics, power
supply ripples, Lorentz detuning, etc.) will lead to avector-sum error in the vector sum control. The phasecalibration error in the system can be converted to anamplitude error in the vector-sum; conversely, theamplitude calibration error will leads to a phase error inthe vector-sum. In the simplest vector-sum control inwhich one klystron drives two cavities (the same situation
with the cavities of injectors 2 and 3 in cERL), therelation between the fixed phase calibration error and thevector-sum error in amplitude is given asPP P(2) 2   2 A 
4
4
where, the parameters Pc1 and Pc2 are the phasecalibration errors of the first and second cavities,respectively. The parameter Δθ is the rms value of thedetuning of each cavity. The cavities of injectors 2 and 3
are operated in the on-crest mode during the beamcommissioning in the cERL (see Table 3), thus theamplitude error in the vector-sum is dominant, andtherefore, we only consider the phase calibration error inthis paper. The detailed discussion about vector-sum
control is given in [4].
In the cERL, the difference in the calibrated phase forinjectors 2 and 3 was mainly performed by a mechanicalphase shifter that was installed in the line of injector 3 atthe outside of the shield (see Fig. 4). We adjusted thephase shifter to increase the beam energy until the beamphase was on-crest in the cavities. The estimateddeviation (ΔPc) in the phase calibration in the adjustment
is supposed to be 10°.
c1c2cvsFigure 4: Layout of the injector cavities and RF system.
The phase shifter is installed in the line of injector 3.
According to (2), the amplitude error in the vector-sum
induced by detuning can be evaluated with the given

phase calibration error. In the cERL, the detuning of eachcavity was regulated to approximately 1° (rms) by thetuner system. The measured detuning in injector 3 isshown in Fig. 5. As a result, an amplitude error ofapproximately 0.11% in the vector-sum can be calculatedon the basis of (2). The cavities of injectors 2 and 3 willcontribute about 7% of the total energy (~20 MeV); thus,
the fluctuation in the beam momentum jitter induced by
the amplitude error in the vector-sum is about 0.008%
rms. This prediction was validated after we measured thebeam momentum jitter during beam commissioning (seeFig. 6).

Figure 5: Detuning in the waveform (left) and histogram(right) of injector 3 cavities. The RMS value of thedetuning is approximately 1°.

projection in the screen monitor. The calculated beam
momentum jitter was about 0.013% RMS as shown in
Fig. 6. One of the possible reasons for the trend signal(indicated by the red line in the Fig. 6) in the beam jitteris the vector-sum error. This trend in the figure willcontribute to an rms fluctuation approximately 0.01% of
the total beam jitter. This result is in agreement with theprevious analysis (0.008% rms).

Figure 6: Beam momentum jitter. The measured beam
momentum jitter was 0.013% rms, and the possible
reason for the red line in the upper figure is the vectorsum error in the cavities of injector 2 and 3.

SUMMARY

The typical LLRF system performance and operationalparameters during beam commissioning are listed inTable 3. The feedback gains were determined by the gainscanning experiments, as stated above. Disturbing signals
such as microphonics and power supply ripples were
suppressed well by the feedback [5]. Furthermore, theparasitic modes in the nine-cell cavities of the main linac
were removed by a fourth order IIR filter [2]. The typical
RF stabilities of the amplitude and phase, for the cavities
in the main linac were, 0.012% and 0.015°, respectively,and for the cavities in the injector, they were 0.02% and0.025°. All of them satisfied the requirements in cERL.

Digital LLRF systems were installed for the injector
and main linac in the cERL. To improve the performanceof the LLRF systems, the feedback gains were surveyed
and determined by performing a gain-scanningexperiment. Furthermore, the vector-sum errors in thecavities of injectors 2 and 3 were analyzed and estimated.
Finally, we summarized the performance of the LLRF
system in the cERL, the typical RF stability values are
0.012% rms in amplitude and 0.015° RMS in phase formain linac cavities, and 0.02% rms in amplitude and0.025° rms in phase for the injector cavities. Thisperformance satisfied the required stability criteria of thecERL. Additionally, a beam momentum jitter ofapproximately 0.013% was achieved during beamcommissioning.

Table 3: Status of RF System in the Beam Commissioning

REFERENCESSTABILITY

RF stability (RMS)
CavityφbBuncher-90°

Injector 10°

0.7 MV

Injector 20°

0.60 MV

Injector 30°

0.60 MV

ML10°

ML20°

Vc

δA/Aδθ0.05%

0.06°

0.02%

0.02 °

0.02%

0.025°

8.56 MV

0.012%

0.014°

8.56 MV

0.012%

0.015°To measure the stability of beam energy, a screen
monitor was installed downstream of the bending magnet
with a 0.487-m dispersion, and a 63.7-μm/pixel resolution.
The beam momentum jitter was then calculated byextracting the peak point information of the beam

[1] T. Miyajima, “Beam Commissioning of Energy
Recovery Linacs”, IPAC’13, Shanghai, May 2013,
FRXBB201.
[2] F. Qiu et, al, “Feedback optimization in MicroTCA
based LLRF system,” RT2014, Nara, Japan, May,2014.
[3] F. Qiu et al., “Evaluation of the superconducting
LLRF system at cERL in KEK”, IPAC’13, Shanghai,May 2013.
[4] A. Brandt, “Development of a Finite State Machinefor the Automated Operation of the LLRF Control atFLASH,” PhD thesis, DESY, 2007.
[5] T. Miura et al., “Performance of RF System for
Compact-ERL Main Linac at KEK”, IPAC2014
Dresden, Germany.