Characterization of microphonics in thecompact ERL main linac cavities
Feng QIU (KEK), on behalf of the cERL LLRF group

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20, 20191

Main Content
 Introduction LLRF and Tuner control system

 RF stabilities of the ML cavities
 Microphonics measurement
 System identification of piezo tuner
 Summary

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Introduction¾ The Compact ERL (cERL) is a test facility to demonstrate ERL technology. It is a
1.3-GHz superconducting system and is operated in CW mode [1].
Layout of cERL

HLRF

Injector consists offour cavities: Buncher(NC), Injector 1 (SC),Injector 2 (SC),Injector 3 (SC).
Main linac (ML)
includes two nine-cellcavities (SC).

Two 9-cellin ML (SC)Three 2-cell in
the Injector¾ April, 2013, injector commissioning. Oct. 2013, main linac commissioning.
ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Cavity and RF system¾ At present, total four kinds of power sources are applied in cERL : 8 kW SSA,
16 kW SSA, 25-kW klystron and 300 kW klystron.
RF requirement (need LLRF feedback)
0.1 % rms, 0.1 deg. rms for cERL
0.01% rms, 0.01deg.rms for 3GeV-ERL
8 kW SSA
25 kW Kly.
300 kW Kly.
16 kW SSA

8 kW SSA

Vector-sumControlling

@2015
Dump

ML1

ML2
Main linac~ 20 MeVSC

Main linac

Two-cell SCSC
Nine-cell SC~3 MV/m for Injector Cavities

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20~8.5 MV/m for main linac Cavities

CavityQL

RF power

Bun.

1.1e5

3 kW

Inj. 1

1.2e6

0.53 kW

Inj. 25.8e5

Inj. 34.8e5

ML1

1.3e7

1.6 kW

ML21.0e7

2 kW

2.4 kW

4LLRF & Tuner (Hardware)¾ MicroTCA-based digital low level radio
frequency (LLRF) system and tuner
resonance control system are applied incERL.

Shaft oftuner

FlangeSlide-jack
TunerPiezo

Pick up
ADC: LTC 2208

CavityDigital I/O

Pf

SSA

Xillinx Virtex5 FPGA

Motor Driver(CW/CCW puls e)
uTCA Digital Board

Piezo Driver-250V~+250V

Micro TCA.0 board
 FPGA vertix5-FX
 16-bit DAC × 4 ch
 16-bit ADC × 4 ch
 Dig. I/O × 12 ch
 Epics in Power PC

Pick up
Pf

PC

Tuner(frequency ctrl.)

Pf

FB(field ctrl.)

Pick upEPICS

ADC & DAC Interface

Ethernet

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

I/Q
Mod.

Piezo Tuner
0 V~500 V
Stroke = 4 μm @ 2 K
1 μm→300 Hz
Cavity BW: 130 Hz(QL~1e7)

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LLRF & Tuner (Algorithm)¾ Piezo Tuner: Integral (I) control.
¾ LLRF system: PI control. Evaluate LLRF

SETPick up

performance22­ AIQ°®
1 Q°T tanI¯

PfΔφP
IPIFil2.5 kHzΔφI/Q Fil

To I/QMod.

To Motor
PickupCavityPf∫

SSA

To Piezo

Motor Driver(CW/CCW puls e)

Pick up
Pf

PC

Tuner
(frequency ctrl.)Piezo Driver-250V~+250V

Pf

FB(field ctrl.)

Pick up

EPICS

Pick up

Pf

I/Q

FF1

Ethernet

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

I/Q
Mod.

'Z

Z0.5  tan 'M

Z0.5

f RF
2QL

Calibrate the detuning

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Main Content
 Introduction LLRF and Tuner control system

 RF stabilities of ML cavities
 Microphonics measurement
 System identification of piezo tuner Summary

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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RF stabilities (under FB)
ML120192016

After 2016

ML10.012 deg rms

0.015 deg rms

0.015 deg rms

0.029 deg rms

Before 2016

ML22015, 20162017, 20182019

¾ Performance of ML1
becomes worse in thepast 5 years, ML2
performs well.

ML20.012 deg rms

0.012 deg rms

0.012 deg rms

0.012 deg rms

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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RF stabilities of ML1¾ Some components(> 500 Hz) wereexcited in thecavity phase ofML1 in 2019.

2019

2016

ML1 phase

FFT analysis

ML2ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

9RF stabilities of ML1
Best performance @ 2016

Current (2019, April)

VcPha.
High freq.

Δω

Very clean

Very clean

2015 (8.5 MV)

2016 (7 MV)

2017 (5MV)2018 (5MV)

Cavity degrade due to
Add rubber sheet
under the scroll pump field emission and thermal
to remove the 50 Hz. breakdown [2]¾ RF stabilities becomes worse due to thedeteriorated microphonics conditions [3].
ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 202019 (6MV)

After 2016

Before 201610

Main Content
 Introduction LLRF and Tuner control system
 RF stabilities of the ML cavities

 Microphonics measurement
 System identification of piezo tuner
 Summary

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Field Scanning (cont’d)¾ In 2019, accidentally, we found that the back-ground mircophpnics in ML1
depends on its cavity field.
¾ Field-scanning (under feedback operation).
Pick up
CavityPfP

Pickup
ck u
Pf

Tuner(frequency ctrl.)

Pf
Pick upPC

Ethernet

Piezo Driver-250V~+250V

FB((fieldctrl.)

EPICS

Motor Driver(CW/CCW puls e)

SSAVc~ 1hrs

I/Q
QMod.
od.

SETPick upPf

PIFil
2.5 kHzΔφI/Q FilI/Q

To I/Q
Mod.

Increase the set
value in the FB loop.
6 MV

1 MV
Time

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

FF112Field Scanning¾ Perform the FFT analysis of the detuning under different field, thenplot the Map.

Cavity Voltage [MV]RMS ∆ωLarge ∆ωInc.Vc

Cavity Phase

Field & Detuning Maps

RMS detuning andRMS Vc phase stability

FFT of ∆ω

Small ∆ω

Frequency [Hz]Cavity Voltage [MV]ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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ML1 Field Scanning (result)¾ High frequency component's suddenly appears @ ~3.1 MV (Threshold Vc).
¾ Detuning and RF phase stabilities becomes worse under higher Vc.

Frequency > 500 Hz

Threshold Vc ≈ 3.1 MV

RMS ∆ω
RMS ‫ס‬Vc

Threshold Vc ≈ 3.1 MV
Vc [MV] = 1.038 [m]Eacc [MV/m]

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Cavity Voltage [MV]

¾ The boundary appeared
only in the ML1.
¾ Why “field dependencymicrophonics”? The
mechanism remains
unclear.

Cavity Voltage [MV]Field--detuning Map (ML1 vs. ML2)

Threshold Vc ≈ 3.1 MV

ML1No components in ML2
Frequency > 500 Hz

ML2

Frequency [Hz]ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

15Frequency [Hz]Hysteresis Phenomena

Cavity Voltage [MV]Inc. Field: 3.18 MV
Dec. Field: 2.80 MV
Hysteresis phenomena?

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Threshold Vc vs. Quench limits¾ The value of threshold Vc is probably related with quench limits (remains unclear)?
I: Before pulse (PS) aging

II: Just after PS aging

III: After 1 week operation (result in the morning)

Case Threshold QuenchVcLimits
[MV]
[MV]

I

3.07

6.16

II

3.34

6.51

III

3.18

6.30

IV

3.17

6.30

IV: After 1 week operation (result in the afternoon)

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Main Content
 Introduction LLRF and Tuner control system

 RF stabilities of the ML cavities
 Microphonics measurement
 System identification of piezo tuner
 Summary

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Requirement for system model¾ Simply increasing the FB gain is NOT a good method, some mechanicalmodes would be excited and the system therefore oscillated [4].
340 Hz oscillation¾ Advanced control methods (e.g. active compensation method [5], oractive noise control [6]) are better choice, for these cases, a systemmodel is usually necessary (or helpful). We have to know the systembetter.

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 2019

Identification of the TF Model¾ Transfer function (TF): Piezo to RF.
¾ Excite piezo with sinusoidal signal and sweep the frequency.
¾ FFT analysis → TF model.

350 Hz

5 Hz

Frequency Range: 5 Hz to 350 Hz
Frequency Step: 1 Hz
For each frequency
5 sec.
ncy point:p

Pick up
Pf(CW/CCW puls e)

Pick up
Pf

EPICS
S

Pfwith FFT analysis

Piezo Driver-250V~+250V

InSinewave

PC

Obtain the frequency response
I/QMod.

OutΔθPick upEthernet
ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20‫ܪ‬ሺ݆߱ሻFrequency [Hz]Pha. [deg]Motor Driver

SSA

Mag. [dB]

Cavity‫ܪס‬ሺ݆߱ሻ

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TF Model vs. ∆ω¾ Piezo Transfer function is probably related with ∆ω.

H sN§ M0· Td sZk2 M k.
e
¨¦
22 ¸
© W s  1 k 1 s  2[ k Zk s  Zk ¹

Microphonics and Tuner Model∆ω vs.. ‫ܪ‬ெ߱ሻ߱ெ௅ଵ ሺ݆߱ሻss. ‫ܪ‬ெ߱ሻ߱∆ω vs.
ெ௅ଶ ሺ݆߱ሻ

Overlap of ∆ω & ‫ܪ‬ሺ݆߱ሻ

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Validation of the TF Model¾ Excite the system (and model) with square wave.

¾ TF Model vs. Actual System.
¾ We will optimize tuner control with TF model.

Response
Type A

Type AaRealSys.
ysCCavity

Motor Driver(CW/CCW puls e)

50 Hz

Pick upPf

Tuner(frequencyuency ctrl.)
Pick up

Pf

SSA

Piezo Driver-250V~+250V

I/Q
Mod.

FB
(field ctrl.)

EPICS

Input

Pick up

Actual SystemmEthernet

PC

Type B
4a

Type BTF ModelBlue: Meas.
Red: model

20 HzOutIn
ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 2022

Summary¾ RF stabilities of ML1 cavity were getting worse due to thedeteriorated microphonics in the past 5 years.
¾ A “field level dependency microphonics” phenomenon wasobserved in ML1.
¾ The threshold Vc for the deteriorated microphonics is about
3.1 MV, and it is probably related with quench limits level.
¾ We have identified and validate the TF model of the piezo
tuner system and we plan to optimize the tuner control withthis TF model.

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Thank you for your attention

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 20

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Reference[1] M. Akemoto et al., “Construction and commissioning of compact energy-recovery linac at KEK”, Nucl.
Instr. Meth., vol. 877, pp. 197-219, 2018.
[2] H. Sakai et al., “Long-term operation with Beam and Cavity Performance Degradation in Compact-ERL
Main Linac at KEK”, in Proc. LINAC’18, Beijing, China, Sep. 2018, pp. 695-698.
[3] F. Qiu et al., “Status of microphonics on cERL nine-cell cavities” in Proc. PASJ2019, Kyoto, Japan, JulyAug. 2019.
[4] 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.
[5] A. Neumann et al., “Analysis and active compensation of microphonics in continuous wave narrowbandwidth superconducting cavities”, Phys. Rev. ST Accel. Beams, vol. 13, p. 082001, Aug. 2010.
[6] N. Banerjee et al., “Active compensation of microphonics detuning in high QL cavities”, Phys. Rev. Accel.
Beams, vol. 22, p. 052002, May 2019.

ERL2019, Berlin (HZB) . Sep. 15 ~ Sep. 2025