Precision Optical Timing Distribution and Electron Arrival-time

Measurement at XFEL/SPring-8 Yuji OtakeA), Takashi OhshimaA), Hirokazu MaesakaA), Naoyasu HosodaA),

Shinichi Mastubara A) Mituru MusyaB), Kenji Tamasaku A)

A) RIKEN, XFEL Joint Project /SPring-8B) Institute of Laser Science, Univ. of Electro-communications

RIKEN + JASRI

contents

1. Optical time and RF reference system for XFEL/SPring-8.

2. Direct 770nm pump laser activation by a 1540 nm comb laser pulse for the accelerator timing to realize a time jitter with several femto-seconds.

XFEL 8Gev, 0.3 nc, 0.1 nm light（2010, construction finish）

SCSS Test Accelerator250 MeV, 0. 3 nc, 60 nm light

Japan

Korea

Electron Gun Bunch Compressor C‐band Accelerator Undulator

Cavity ΔV/V(%rms) Δφ (deg. rms) Δt (ps rms)238 MHz SHB ± 0.01 ± 0.01 ± 0.12476 MHz Booster ± 0.01 ± 0.02 ± 0.12L-band Cor. Cavity ± 0.03 ± 0.06 ± 0.12L-band APS Cavity ± 0.01 ± 0.06 ± 0.12C-band Cor. Cavity ± 0.1 ± 0.06 ± 0.049S-band Accelerator ± 0.01 ± 0.1 ± 0.09724 C-band Accelerator (up-stream) ± 0.01 ± 0.2 ± 0.097104 C-band Accelerator (Down-stream) ± 0.01 ± 0.5 ± 0.24

rf

BeamThe Crest acceleration Part

OSC e.g. Klystrons

Off-crest acceleration Part

Max.

Tolerance of the Phase & Amplitude of RF Cavities at XFEL/SPring-8

Find perturbations, such as a temperature change, vibration, noise etc.

Reduce/eliminate these perturbations as small as possible.

Design criterion of the timing system for realizing stable SASE generation.

Optical Time Reference System

Carrier Signal Spectrum

Install

Not InstallDeveloped

Within 0.2 K

Optical Time Reference Signal Transmission System

The optical system dose not increase the master oscillator’s noise.

Increase7 fs

Carrier Frequency

Pink: Master Oscillator NoiseBrown : After 500 m Optical Transmission

SSB Noise Spectrum ofEO & OE transmission

Temperature variation around the fiber

Optical Fiber

Glass Wool

Cooling Water

0.5K

T fiber1

T fiber2

T fiber3

Fiber Temperature Control by Water

TrigDelay

IQ Mod.

T fiber1 T fiber2

T fiber3

SCSS test accelerator

C-band klystronMaster OSC

WDM (Wave length Division Multiplex) optical transmission system 

Test of Optical Transmission Line using SCSS Test Accelerator

2 weeks

Coaxial Cable Case

Energy fluctuation measured by the rf zero phasing method at the end of the C-band cavity

Case DE DtCoaxial Cable 0.49MeV 138fs (rms)Optical Fiber 0.50MeV 140fs (rms)

Stability of SASE intensityCase mean stdCoaxial Cable 6.17 0.5Optical Fiber 6.20 0.66

(Arbitrary Unit)

Stability of SASE & Beam EnergyOptical Fiber Drive Case

2 hours

1

at E = ~ 120 MeV

When the RF signal was driven by the optical fiber system.

Both of the optical and coaxial cables have the same performance of rf phase and amplitude stability at the SCSS test accelerator.

Including instability at an rf zero phase point, therefore the changes were enhanced.

Optical Fiber Case

Use the 4760 MHz BPM intensity detection cavity

5236MHz

4760MHz

Cavity BPM

Synchronized with acceleration RF

476MHz

TM010 Reference cavity

TM110 Position detection cavity

0o

90o

IQ mixer

0o

90o

I

IQ

Q

BeamThis cavity is placed just before the IDs.

This frequency to prevent the dark current generated by the 5712 MHz accelerator guides.

Performance of Timing System at SCSS Test Accelerator

Beam Time jitter is 46 fs0.54 mV Standard Deviation.Horizontal Scale. 4 ns/div.10 pps Beam Reptation.

12 hours

1 %δE/E

Now, the beam energy stability was improve less than 0.1 % (rms) in the long term (more than half day) by the rf phase feedback control of the cavities (238, 476, 2856 MHz).

R56, 20 mm (BC)Beam energy jitter valueabout 0.06 % (rms)

Results of Beam Arrival Time Jitter Measurements

770 nm Pump Laser Directly Activated by a 1540 nm Comb Laser Pulse

Design criterion to realize a low time jitter fora pump-prove experiment

1.Eliminate electrical synchronization circuits for a pump laser.

2. We only use optical components to pump a 770 nm TiSapphire Laser from a 1540 nm optical master oscillator for the accelerator.

Mode-Lock Laser(Ti Sapphire)

OpticalFiber Line

79.5 MHz Time reference line

RF system: 5712 MHz Klystrons etc

Master Oscillator(Sinusoidal Wave) E/O O/E

Sample

Pump Laser

Our devised & modified part

79.5 MHz rf Signal

XFEL

Present Plan for Pump-Probe Experiments for XFEL/SPring-8

For about 1 km

Devised 770 nm Laser Driven by 1540 nm Comb Laser

Optical comb pulse & its spectrum

5712 MHz Repetition,1ps pulse width

Optical Comb Generator

The laser source (1538 nm) using DFB laser diode in which the light is locked to acetylene absorption spectrum. The frequency stability is almost 10-11 in a frequency region below 10 Hz.

In the optical comb generator, an electro-optic (E/O, LN) crystal is inserted into an optical Fabriot-Periot cavity.The output rf signal of the master oscillator is added into the E/O crystal to generate the comb pulse having a period of the rf signal.

Method to Cull 1540 nm laser pulse train using LN Modulator

1540 nm

Method to Cull 770 nm Comb Laser Pulses using Pockel Cell

60 Hz Signal

Optical Output

LN Modulator

Pockel Cell

Mirror

Mirror

Lens

SHG Crystal

A 60 Hz laser comb pulse train is picked up from 5712MHz comb pulse train

Pulse Culling System

Avalanche Pulser

Experimental Set up for 770 nm Laser System Driven by 1540 nm Laser

Spectrum of the 770 nm laser converted by a PPLN (Periodically Poled Lithium Niobate) crystal. The CW 1500 nm laser light is injected in to the PPLN crystal.

Second Harmonic (770 nm) Generation by PPLN Crystal

5712MHz RF

1540nm Pulse

770nm Pulse

The pulse response of this photo diode is slow, therefore, the pulse width of the 770 nm laser pulse dose not show a true pulse width.

Second Harmonic Generation & Culling 10 Hz Laser Pulse form 5712MHz Comb Laser

We achieved non­linear amplification of a 50 nm VUV laser with 10 % fluctuation (saturation) at the SCSS test accelerator by using our timing system. This fact shows great possibility to realize XFEL/SPring­8.The master oscillator has a sufficient SSB noise level to realize a beam energy stability of 10­4. The beam energy variation, and the beam time jitter to the acceleration rf, are less than 0.06% (rms) at 120 MeV and 46 fs at the test accelerator. 

The 1540 nm & 770 nm laser pulse culling system works well, and the wavelength conversion system from 1540 nm to 770 nm also works well. We will try to drive a TiSapphire regenerative amplifier by the developed system. 

Summary

Calculated data

Frequency dividing.Very low-noise power supply.PLL connection between 10 MHz and 2856 MHz sources.

Circuit Diagram & Calculated Noise of Master Oscillator

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Frequency (Hz)

SSB noise Power (dBc)

5712 MHz

5236 MHz

2856 MHz

2380 MHz

476 MHz

238 MHz

Decreases of noise levels at the individual frequencies are proportional to the dividing ratio.

These noise correspond to about 30 fs time jitter with rf pulse modulation (2ms)

SSB Noise Spectra of Master Oscillator

Prototype of Optical Fiber Length Controller

Phase-stabilized optical fiber has a thermal optical length coefficient of 2 ppm/K. The optical length of the fiber moves 1.6 mm/K for 800m.This values corresponds to a phase shift of 11 deg./K and 5.4 ps/K at 5712 MHz, and is not acceptable to employ this method for the X-FEL. Therefore, we developed this fiber length control system.

ExperimentalApparatus

Satisfy the original system performance developed for ALMA. （Controlled it optical length within 3 μm for 25 km.)

Use the 1 km phase stabilized optical fiber cable settled along the circumference of the SPring‐8 ring

1 km FiberError Length Spectrum of Control

Experiment of Fiber Length Control

Timing Control System

IQ modulator rf phase and amplitude control.IQ demodulator detects rf phase and amplitude.

Baseband waveforms are processed by VME high‐speed D/A and A/D converter boards.

Sampling rate: 238 MSPSResolution: 14 bits (D/A) and 12 bits (A/D)

All modules are enclosed in a water‐cooled 19‐inch rack. Regulated within 0.4 K.

To reduce the thermal  drift.

DC power is distributed from a low‐noise power supply.

Clean and stable powerSmall heat load for the 19­inch rack

26

To check the performance of the optical transmission line,

the stability of SASE intensity andthe energy change cursed by the acceleration field strength  change of the C­band accelerator guides by using the zero­phase method.

RF BPF

PDundulator

e-beam

Experimental Set-up for Test

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05

Free All V/sqrtHz

Lock Hi All

After Replacing New Fiber Stretcher with Fast Response