# Study of laser-beam arrival time synchronization towards sub-picosecond stability level

K. Popov, A. Aryshev, N. Terunuma, H. Kaji, A. Back, D. Charlet, C Esnault, A Martens

► To cite this version:

K. Popov, A. Aryshev, N. Terunuma, H. Kaji, A. Back, et al., Study of laser-beam arrival time synchronization towards sub-picosecond stability level. 16th International Particle Accelerator Conference (IPAC'25), Jun 2025, Taipei, Taiwan. pp.THPM050, 10.18429/JACoW-IPAC2025-THPM050. in2p3-05396584

HAL Id: in2p3-05396584

https://in2p3.hal.science/in2p3-05396584v1

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# STUDY OF LASER-BEAM ARRIVAL TIME SYNCHRONIZATION TOWARDS SUB-PICOSECOND STABILITY LEVEL

K. Popov*, A. Aryshev, N. Terunuma, H. Kaji

High Energy Accelerator Research Organization (KEK), Tsukuba, Japan

A. Back, D. Charlet, C. Esnault, A. Martens

Université Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France

## Abstract

In order to achieve laser pulse to electron beam arrival time with sub-picosecond stability at the accelerator facilities, a new Low-Level Radio-Frequency system clock generators synchronization architecture is currently under investigation in collaboration between KEK (Japan) and IJCLab (France). The system is based on the 10 MHz sinewave signal generator, White Rabbit Switch module, SkyWorks Si5362-A-EVB clock generator evaluation board and IDROGEN boards. This report demonstrates the measurement results of the long-term and short-term synchronization between clock generators. Also, the architecture details are discussed in this report.

## INTRODUCTION

Precise synchronization between laser pulse, accelerating field High-Power RF pulse and electron beam arrival is essential for accelerator successful operation [1], as well as the sub-picosecond synchronization stability over long time span. The sub-picosecond synchronization level is usually achieved by the low-noise Low-Level RF signals distribution system over short and/or long distances and timing modules implementation into the accelerator facility RF infrastructure.

The Low-Level RF signals distribution system based on the IDROGEN board [2] is being tested in accelerator facility environment and infrastructure in a collaborative study between High Energy Accelerator Research Organization (KEK, Japan) and Laboratoire de Physique des 2 Infinis Irène Joliot-Curie (IJClab, France). Also, the system performance, stability and reliability in large-scale accelerator environment [3] are under investigation.

## THE LOW-LEVEL RF TEST SYSTEM ARCHITECTURE

The architecture consists of Stanford Research Systems (SRS) FS725 Rb Frequency Standard, White Rabbit [4] Switch low-jitter version module (WRS), IDROGEN board and SkyWorks Si5362-A-EVB clock signal synthesizer (see Fig. 1). SRS FS725 Frequency Standard provides 10 MHz sinewave signal and Pulse Per Second (PPS) signal for the WRS. Also, the Rohde&Schwarz SML03 low-noise signal generator reference input is synchronized by the FS725 10 MHz reference signal. The White Rabbit Switch module is configured to operate as a Grand Master [5]. Then,

the WRS output port is connected to the IDROGEN board via fiber link. The White Rabbit Switch technology utilizes IEEE 1588 Precision Time Protocol (PTP) [6] to send the timing information. Also, it has a Phase-Locked Loop (PLL) inside the WRS unit to compensate the timing jitter and fiber length extension due to humidity, temperature and air pressure jitter and/or drift.

<figure><img src="image_2.png" /><figcaption>Figure 1: The Low-Level RF Test System architecture block-diagram.</figcaption></figure>

Afterwards, the IDROGEN board is linked to the SkyWorks Si5362-A-EVB clock generator board. The IDROGEN board transfers the 125 MHz clock to the SkyWorks Si5362-A-EVB. For the Low-Level RF test system architecture, the 8th sub-harmonics of 2856 MHz was chosen. The signal is 357 MHz sinewave. At the signal generation chain last step, the Rohde&Schwarz SML03 low-noise SG generate 357 MHz sinewave too. The synchronization stability diagnostics is based on the I/Q demodulator and RedPitaya STEMlab 125-14 FPGA board [7]. The I/Q demodulator convert 357 MHz sinewave into DC signals, which have the phase and amplitude information. The data acquisition is triggered by the TTL pulse signal provided by SRS FS725 Frequency Standard PPS output. The Thorlabs T4119 feed-through terminators are dedicated to match RedPitaya STEMlab 125-14 ADC inputs High Z impedance with the I/Q demodulator output 50 Ohm impedance.

<footnote>* popovkon@post.kek.jp</footnote>
# MEASUREMENT RESULTS OF LLRF SIGNALS POWER SPECTRAL DENSITY OF PHASE NOISE

This system aims to achieve high stability and low-jitter clock signal delivery over long distances, which is critical for the timing requirements of modern accelerator infrastructures. Therefore, the Phase Noise Power Spectral Density (PN-PSD) were measured by the PN8000 Phase Noise Analyzer. The carrier frequencies are 178.5 MHz, 357 MHz, 714 MHz, 254.434 MHz and 508.868 MHz.

<figure><img src="image_3.png" /><figcaption>Figure 2: The PN-PSD for the KEK ATF Low-Level RF system case: (brown) is the 125 MHz, (blue) is the 178.5 MHz, (red) is the 357 MHz, (black) is the 714 MHz.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3: The PN-PSD for the SuperKEKB Low-Level RF case: (purple) is the 254.434 MHz, (orange) is the 508.868 MHz.</figcaption></figure>

In addition, the IDROGEN clock at 125 MHz PN-PSD was measured in order to evaluate the board oscillator quality. The first 5 signals (see Fig. 2) are related to the KEK Accelerator Test Facility LLRF system signals, while the rest (see Fig. 3) are related to the SuperKEKB.

The PN8000 measurement offset frequency is from 1 Hz to 10 MHz. The PN-PSD is integrated from 10 Hz to 1 MHz

to calculate timing and phase jitter (RMS). The RMS phase jitter is equal to 1.5-1.6 ps for every signal mentioned above. Some spurs in the 125 MHz and 178.5 MHz data contribute significantly to the phase noise, but these remain under investigation. As a consequence, the picosecond synchronization long-term stability level can be achieved. Except in the 178.5 MHz data, the spurs are not significantly affecting this value, but remain under a detailed investigation.

# THE LOW-LEVEL RF SIGNALS LONG-TERM SYNCHRONIZATION STABILITY MEASUREMENT RESULTS

The ADCs digitize the I and Q signals with 125 MSa/s sampling rate. In total, the waveform contains 500 digital points, which were utilized to calculate the Low-Level RF Test System phase&amplitude long-term stability [8] (see Fig. 1). It was measured during 8 hours. This time span is the duration of 1 shift at the KEK Accelerator Test Facility.

Figures 4(a) and 4(b) depict the amplitude and phase stability of 357 MHz CW signal, respectively. The amplitude settings were set to partial saturation to achieve the best stability of the first-trial measurements.

As can be seen from the Figures 4(a) and 4(b), the electronics warm-up and temperature stabilization take 2 hours. Then, the phase and amplitude suddenly shift at 1 ° and 1 %. Currently, there are no firm explanations for this sudden shift, although measurements were performed in a regular office during a winter period. It was possible that the lab room experienced some environmental changes such that door or window was opened.

Afterwards, the setup synchronization remain stable from 4.5 hour to 8 hour. The drift and/or fluctuation are not detected during last 3.5 hours. The data allocated at the last 3.5 hour is utilized to plot histograms and calculate RMS noise. The static phase and amplitude drifts from Set Points at the last hours are 0.63 ° (see Fig. 5) and 0.24 % (see Fig. 6), while the RMS noises are 0.12 ° and 0.08 %, respectively. Furthermore, the phase noise (RMS) in the time domain is 960 fs at 357 MHz. It is sub-picosecond long-term synchronization level.

# CONCLUSION

The IDROGEN board with SkyWorks Si5362-A-EVB clock generator has sub-picosecond synchronization level, which is applicable for large-scale accelerator Low-Level RF system (example: KEK Accelerator Test Facility, KEK Superconducting Test Facility, International Linear Collider project etc) including laser synchronization for Compton polarimeter at SuperKEKB [2] and in any future colliders. Moreover, the IDROGEN board in combination with SkyWorks Si5362-A-EVB can be utilized as Local Oscillator to distribute low-noise RF signal over fiber for Low-Level RF front-end electronics for several applications: frequency down- and up-conversion, calibration tone for cavity Beam Position Monitor.
<figure><img src="image_5.png" /><figcaption>Figure 4: The Low-Level RF Test System 357 MHz signal stability stripchart: (a) is the amplitude drift from Set Point vs time, (b) is the phase drift from Set Point vs time.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5: The Low-Level RF Test System 357 MHz signal phase drift histogram.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 6: The Low-Level RF Test System 357 MHz signal amplitude drift histogram.</figcaption></figure>

## OUTLOOK

The Low-Level RF system architecture synchronization stability will be tested in the KEK Accelerator Test Facility with upgraded version of IDROGEN board in June 2025 operation. The dataset associated with the laser pulse to beam arrival synchronization stability will be acquired. The results will be reported in the future publications.

## ACKNOWLEDGEMENTS

This work was supported by [MEXT Development of key element technologies to improve the performance

of future accelerators Program] Japan Grant Number JP-MXP1423812204. Also, the research leading to these results has received funding from the European Union's Horizon Europe MSCA under grant agreement no. 101086276. The KEK researchers visit to the France, Paris-Saclay University, IJCLab was supported by Nagoya University Kobayashi-Maskawa Institute (KMI) oversea study grant. The authors are also grateful for the support from the TYL-FJPPN.
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