STATUS OF MICROPHONICS ON CERL NINE-CELL CAVITIESWEPH010F. 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 the narrow bandwidth of these cavities, the microphonics detuning have a significant impact on theachievable RF field stability. In this paper, we have analysed the microphonics 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 were suddenly 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 the cavity quench limits. Finally, we confirmedthat the deteriorated RF stabilities (due to the deteriorated microphonics) can be improved by applying a disturbance observer based control approach.
Microphonics detuning vs. Cavity field (cont’d)Introduction

 For ML1 cavity, the RMS detuning is increased if the cavity
field exceeds the onset field. ML2 is okay.

The cERL is a 1.3 GHz superconducting (SC) machine operated in the continuous-wave
mode. In the main linac (ML) of the cERL, two nine-cell cavities (ML1 and ML2) with ahigh loaded Q of more than 1×107 were installed [1-2].
 Before 2015, a 50 Hz vibration (from the scroll pump) is the dominant microphonics.
Resolved by inserting a rubber sheet under the scroll pumps.
 After 2016, microphonics in ML1 deteriorated. ML2 is okay.
 At 2019, high frequency components at 680 Hz and 890 Hz are excited in ML1.
 From 2016-2019, the performance of RF stability in ML1 deteriorated due to thedeteriorated microphonics.

Figure 5: RMS detuning of ML cavities as a function of their Vc.

Microphonics spectrum near the onset voltage
 Higher frequency components suddenly appeared (very fast).

Figure 1: The Cumulative RMS detuning as afunction of the vibration frequency of the MLcavities. Left: ML1. Right: ML2.

Figure 2: Amplitude and phase stabilities ofML1 cavity (left) and ML2 cavity (right) under FB
operation in the past five years. .

Figure 6: Microphonics spectrum comparison near the onset Vc.

Onset Vc vs. Quench Threshold Voltage

LLRF
 Field control: LLRF feedback (FB) system.
Resonance frequency control: Tuner FB system.
 The phase differences (Δφ) between cavity pick-up signal andfeedforward (Pf ) signal signals are calculated by LLRF system.
 Detuning is calibrated by Δφ.

Quench limit: 6.16 MV

Quench limit: 6.51 MV

SSACavity

PfCavity
Pick Up

ADC
ADC

Pf

Tuner(freq. ctrl.)

0.5 
LLRF (field ctrl.)

TimeIF
ADC

Low-passFilter  0.5  tan   DACVc
SET

I/Q

I/Q

Communication RF2QL
Quench limit: 6.30 MVRFDOB
ADC
IFQuench limit: 6.30 MV

I/Q
Modulator

FFPILOPf

1.3 GHzPiezoDAC

ΔφI

Q

Figure 7: Onset Vc varied according to the different quench threshold voltage.

DACPower PC

DOB control
InternetPC

Figure 3: Schematic of digital LLRF and frequency tuner system.

 The RF stabilities of the ML1 cavity can be improved with DOB control evenunder deteriorated microphonics [2].

Microphonics detuning vs. Cavity field For ML cavity, the high frequency components is excited if the cavityvoltage is above the onset voltage (~3.1 MV, or ~3.0 MV/m).
 No field dependency in ML2.

Figure 8: Measured ML1 VC phase in the case of with and without DOB control.
Vc [MV] = 1.038 [m]×Eacc [MV/m]

Reference[1] 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.

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

[2] 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.