STATUS OF MICROPHONICS ON CERL NINE-CELL CAVITIES
F. Qiu1,2,†, S. Michizono1,2, T. Miura1,2, T. Matsumoto1,2, D. Arakawa1, H. Katagiri1, E. Kako1,2,
H. Sakai1,2, K. Umemori1,2, T. Konomi1,2, N. Liu2, B. T. Du2
1High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
2The Graduate Univiersity for Advanced Studies (Sokendai), Hayama, Kanagawa 240-0193, Japan
Abstract
In the main linac (ML) of the compact energy recovery
linac at KEK, two 1.3 GHz nine-cell superconducting
cavities (ML1 and ML2) with high loaded Q (QL >1×107)
are operated in continuous wave mode. Because of thenarrow bandwidth of these cavities, the microphonics
detuning have a significant impact on the achievable RF
field stability. In this paper, we have analysed themicrophonics performance of the two ML cavities.
According to our study, a “field level dependence
microphonics” phenomenon is observed on the ML1 cavity.
Several frequency components higher than 500 Hz weresuddenly excited if the cavity field is above an onset field(~3 MV/m). Although the mechanism for the phenomenon
remains obscure, the onset field is probably related with thecavity quench limits. Finally, we confirmed that thedeteriorated RF stabilities (due to the deteriorated
microphonics) can be improved by applying a disturbanceobserver based control approach.

INTRODUCTION
A compact energy recovery linac (cERL) wasconstructed at KEK to demonstrate the feasibility of the
future 3 GeV ERL based light source [1]. It is a 1.3 GHz
superconducting (SC) test facility operated in thecontinuous-wave mode. In the main linac (ML) of thecERL, two nine-cell cavities (ML1 and ML2) with a highloaded Q (QL) of more than 1×107 were installed [2]. The
beam commissioning for the ML was started at Dec. 2013.
Since the cavity was operated with high QL, themicrophonics detuning seriously disturbed the cavity phase.
Figure 1 compares the cumulative microphonics
detuning as a function of the vibration frequenc of the ML1
and ML2 cavities from year 2015 to 2019. The cavities
were operated at different field according to the cavity
condition and physical requirements. It is clear to see that
there is a dominant vibration at around 50 Hz at 2015
(especially for the ML2 cavity). The main source for thisvibration is from the scroll pumps. After inserted a rubber
sheet under the pumps, the 50 Hz vibration was well
suppressed as expected. The microphonics conditions in
the past 4 years (except 2015) are in good consistence for
the ML2 cavity. On the other hand, the microphonics
detuning of the ML1 cavity increased year by year.
Especially in 2019, two large vibrations at 680 Hz and890 Hz were observed as shown in left subplot of Fig. 1.
Due to the deteriorated microphonics conditions of theML1 cavity, the measured radio frequency (RF) stabilities____________________________________________†

qiufeng@post.kek.jp

were getting worse as shown in Fig. 2 (left). The results arecalibrated by the measured cavity pick up signals which arefiltered by a 250 kHz fourth order digital low-pass filter [3].
The feedback (FB) parameters such as the proportional andintegral (PI) gains in the low level RF (LLRF) were kept
the same in all these measurements. It is clear to see that
the RF stabilities (especially the RF phase stability) of theML1 cavity deteriorated after 2016. Whereas the stabilities
of ML2 cavity always performed well due to its similar
microphonics conditions in the past five years.
In view of this situation, we have estimated thebackground microphonics of these two cavities carefully.
We found that the microphonics on the ML1 cavity
depends on its field level, but there is no obvious field level
dependence of the microphonics on the ML2 cavity. In thispaper, we first introduce the current microphonics
conditions of the cERL ML cavities. The method tomeasure the microphonics with the LLRF systems isbriefly discussed. In the next step, the field level
dependence of the ML1 cavity is presented; furthermore,
the possible reason for this dependence is investigated aswell. Finally, a potential approach aiming to improve the
RF stabilities under deteriorated microphonics detuning ispresented.

Figure 1: Cumulative RMS detuning as a function of thevibration frequency of the ML1 (left) and ML2 (right).

Figure 2: Amplitude and phase stabilities of ML1 cavity(left) and ML2 cavity (right) under FB operation in the past
five years.

LLRF SYSTEM
Figure 3 shows the digital LLRF systems and frequency
tuner system of the ML cavities. The detailed informationof these systems can be found in [4-5] and [6], respectively.
As shown in Fig. 3, the baseband in-phase and quadrature(I/Q) components of the cavity pick-up signal andfeedforward (Pf) signal are extracted in the LLRF system.
In the next stage, the phase differences ( Δφ ) between theabove two signals are calculated. A 2.5 kHz first order
digital low-pass filter is adopted to reduce the backgroundnoise as shown in Fig. 3 [6]. The microphonics detuningΔω is then calibrated based on the Δφ by the equation

Eacc is almost same in our cavity [7]. There is no obvious
dependence between the field and microphonics detuningin the ML2 cavity according to the lower subplot of Fig. 4.
The detuning of the ML2 cavity is mainly concentrated on
the frequency less than 500 Hz.
Figure 6 compares the RMS detuning of the two ML
cavities as a function of their corresponding Vc. The
intensity of the RMS detuning of ML1 cavity is increased
if the cavity field is above the onset field. On the other hand,
the RMS detuning of the ML2 cavity almost remains
invariable. This is also in good agreement with the field
detuning maps.

(1)    0.5  tan     ,where ω0.5 represents the half-bandwidth of the cavity.
To evaluate the field dependence of the microphonics
detuning, we gradually increased the cavity field bychanging the set value of the LLRF FB loop under FB
operation (see Fig. 3). The field-detuning map was thenobtained by performing Fourier analysis of Δω under
different cavity field.
CavityPf

CavityPick Up

SSA
1.3 GHz

Piezo
Low-pass
Filter

ADC
Tuner
DAC
ADC (freq. ctrl.)

Pf

LLRF (field ctrl.)

Vc
Time
IF
ADC

I/Q

SET

I/Q
Modulator

FF
PI

RF
DOB

LO

IF
ADC

Fil
2.5 kHzΔφI/Q FilPf
Communication

Internet

Figure 4: Field-detuning maps of ML1 (upper) and ML2(lower) cavity. The color in the figure presents the intensityof the microphonics detuning.

DAC
Power
PC

I

Q

DAC

PC

Figure 5: Microphonics spectrum comparison under thecavity field near the onset Vc.

Figure 3: Schematic of the digital LLRF system andfrequency tuner system of the cERL ML cavity.

MICROPHONICS OF ML CAVITIESFigure 4 shows the measured field-detuning maps ofML1 (upper) and ML2 (lower) cavities. The color code in
the map represents the detuning spectrum power. It is easy
to see that there is a boundary on the map of ML1 cavity.
The frequency components higher than 500 Hz were exited
only if the cavity voltage (Vc) is larger than a 3.1 MV onset
voltage. Figure 5 compares the microphonics spectrums of
the ML1 cavity under the voltage close to the onset Vc.
Several higher frequency components (>500 Hz) suddenly
appeared as long as the Vc is slightly larger than 3.1 MV.
The corresponding onset accelerating filed, Eacc, isapproximately 3 MV/m. The relationship between Vc andEacc in the ML cavities is expressed asVc [MV] = 1.038 [m]×Eacc [MV/m], i.e. the value of Vc andFigure 6: RMS detuning of the ML cavities as a functionof Vc.
We scanned the field-detuning maps by several times
under different LLRF conditions (switching on/off the
LLRF field FB loops or the frequency tuner FB loop). The
onset field was reproduced almost on the same value for allof these cases. A hysteresis phenomenon was observed inour experiment. The onset field was somewhat different
when we scanned the field with different directions(increasing the field or decreasing the field). Table 1 hassummarized the onset fields for all of these cases during

the experiments. We therefore conclude that the onset field
is not related with LLRF FB conditions.
We found that the value of the onset field is probably
related with the cavity quench limits. The cavity quench
threshold field was increased by performing the pulse (PS)aging before the beam operation. It was very interesting to
see that the onset field was also increased after PS aging.
We observed that the quench threshold field was slightly
decreased after one week beam operation, and the onset
voltage was decreased as well. This variation of the onset
field under different quench limits are summarized by
Table 2. The mechanism for this phenomenon remains
obscure so far. More study time will be required in the next
cERL beam commissioning. To confirm whether thequench limits is the reason or not, we plan to operate theML1 cavity with the 8π/9 mode instead of π mode [3]. We
expect that the quench limits will be changed under
different passband mode, as a result, the onset field will bealso varied if a dependence exists.
Table 1: Onset field under different LLRF conditions.
Round
No.

Onset Eacc
(field inc.)

Onset Eacc
(field dec.)

Field
FB

Tuner
FB

1

3.11 MV/m

2.80 MV/m

ON

ON

2

3.12 MV/m

2.82 MV/m

ON

ON

3

3.12 MV/m

2.80 MV/m

ON

OFF

4

3.11 MV/m

2.82 MV/m

ON

OFF

5

3.12 MV/m

Unmeasured

OFF

ON

6

3.15 MV/m

Unmeasured

OFF

ON

Figure 7: RMS VC phase stability varies with the RMS
detuning in the ML1 cavity.
Disturbance observer (DOB) based control is a possible
solution to improve the RF stabilities. It is an advanced
control approach that aims at rejecting the external
disturbances in a control system. Since the microphonics
detuning can be seen as a kind of external disturbances, wehave applied the DOB method in our LLRF systems asshown in Fig. 3. The detailed information of this method isgiven in [5-6].
Figure 8 compares the case of with and without DOB
control, and the improvement is obvious due to themicrophonics induced phase fluctuation is almost
disappeared after switching on the DOB control. The phasestability of ML1 cavity is improved from 0.028 degree rms
to 0.013 degree rms.

Table 2: Onset field versus quench limit field.
Cavity Conditions

Onset Eacc

Quench Limits

Before PS aging

3.0 MV/m

5.9 MV/m

After PS aging

3.2 MV/m

6.3 MV/m

After one week
operation

3.1 MV/m

6.1 MV/m

DOB CONTROLAs discussed above, the microphonics detuning
deteriorated in the past several years, as a result, the cavity
phase stability was getting worse. Figure 7 has compared
the RMS detuning and cavity phase stability when
scanning the field of the ML1 cavity. It is easy to find thatthe cavity RMS phase stability varies following the
microphonics detuning. As mentioned above, the 2.5 kHz
low-pass filter was applied in this measurement to reject
the effects of the background noise (see Fig. 3).

Figure 8: Measured ML1 Vc phase in the case of with andwithout DOB control. Upper: waveform. Lower: FFTresult.

SUMMARY
After a long term operation, the microphonics detuningof ML1 cavity was increased. The field dependency
microphonics maps were obtained by scanning the cavity
field. The frequency components higher than 500 Hz were
excited in the ML1 cavity if its field is above the onset field(~3.0 MV/m). This field is probably related with thequench limits although the internal mechanism is still not
well understood. Due to the deteriorated microphonics
conditions of the ML1 cavity, the performance of RF
system, especially the phase stability were getting worse.
After applying the DOB control, the phase stability of ML
cavity was improved as expected.

REFERENCES[1] M. Akemoto et al., “Construction and commissioning ofcompact energy-recovery linac at KEK”, Nucl. Instr. Meth.,vol. 877, pp. 197-219, 2018.
doi:10.1016/j.nima.2017.08.051[2] H. Sakai et al., “Long-term 0peration with Beam and Cavity
Performance Degradation in Compact-ERL Main Linac at
KEK”, in Proc. LINAC’18, Beijing, China, Sep. 2018, pp.
695-698. doi: 10.18429/JACoW-LINAC2018-THPO008[3] F. Qiu et al., “Digital filters used for digital feedback system
at cERL,” in Proc. LINAC’14, Geneva, Switzerland, Aug.Sep. 2014, pp. 227-229.
[4] T. Miura et al., “Low-Level LLRF system for cERL”,
in Proc. IPAC’10, Kyoto, Japan, May 2010, pp. 1440-1442.
[5] F. Qiu et al., “Application of disturbance observer-based
control in low-level radio-frequency system in a compact
energy recovery linac at KEK”, Phys. Rev. ST Accel. Beams,
vol. 18, p. 092801, Sep. 2015.
doi: 10.1103/PhysRevSTAB.18.092801[6] F. Qiu et al., “Progress in the work on the Tuner control
system of the cERL at KEK”, in Proc. IPAC’16, Busan,
Korea, May 2016. Pp.2742-2745.
doi: 10.18429/JACoW-IPAC2016-WEPOR033[7] H. Sakai et al., “Field emission studies in vertical test andduring cryomodule operation using precise x-ray mapping
system”, Phys. Rev. Accel. Beams, vol. 22, p. 022002, Feb.
2019. doi: 10.1103/PhysRevAccelBeams.22.022002