Generation of THz CSR withlaser-bunch slicing in UVSOR-II
electron storage ring
Serge Bielawski5, Clement Evain5, T. Hara3, M. Hosaka2
M. Katoh1, S. Kimura1, A. Mochihashi1*, M. Shimada1,Christphe Szwaj5, Y. Takashima2, T. Takahashi4
1 UVSOR facility, institute for molecular science
2 Nagoya university, 3 RIKEN/SPring-8

4 Kyoto University, 5U. Sci. Tech de Lille*Present affiliation… JASRI/SPring-8

Contents• Bunch-slicing experiment in UVSOR-II– Setup– Experimental results• Bunch slicing with amplitude-modulated laser– Principle– Experimental results• SummaryUVSOR-II Electron Storage Ring
Electron Energy
Circumference
Natural EmittanceNatural Energy Spread
Natural Bunch Length
RF Frequency
Revolution Frequency
Synchrotron Frequency
Damping time
Undulator Length

600 MeV53.2 m
17.4 nm-rad
3.4 x 10-4
3.1 cm (~100psec)90.1 MHz
5.6 MHz
14.4 kHz19 msec
2.31 mThe laser system issettled outside the ringnow!

Setup for Laser-bunch slicing
in UVSOR-II(3) THz CSR
corresponds tothe dip structure
can be generated.
(1) Electrons in the ring interactwith the laser in the undulator
section and are partially energymodulated.
(2) The energy-modulated
electrons go away from theircentral orbit and make a dip on
the electron bunch.

Generation of a Dip on a Bunch
Short laser pulse
duration

Passing througha section with 2
bendingmagnets…

∆E ( z )z0 → z1 + laser 0 R56
ELong laser pulse
duration

Examples of dips byshort/long laser
pulsesForm Factor

F (k ) = ∫ ρ (z )e dz
ikz

2Form factor
from shorter
pulse has higher
frequency
components.
CSR spectrumcan change

Ti:Sa Laser and Timing System
Pulse Energy
2.5mJ/pulse
Pulse Width
130fs ~ 2ps
Repetiton Rate
1 kHz
(synchronized with RF signal of 90.1MHz)
Wavelength
790 ~ 810nm

Peak power ~18GW@130fsMomentum
acceptance ~ 0.01
Natural energy
spread ~ 3.4E-4

Detection System
at BL6B, UVSOR-II
Detector TypeHot electron (InSb)

Time Resolution

1.6µsec

Spectral Range

3 ~ 50cm-1

Sensitivity

5×105 V/W
(including beam-line optics)

Temperture

4K (cooled by Liq. He)

Bending #6
Magic mirror
To BL6U
Diamond/z-cut
QuartzwindowBruker
IFS66v/S

JASCO THz microscope
FARIS-1

Martine-Puplettinterferometer
Resolution<0.5cm-1

Refectionabsorption
spectroscopy
0500 1000station

Experimantal Results(Temporal structure)
Temporal relation between the laser
trigger and THz signal observed atBL6B.
The intensity of the THz pulse is104 - 105 larger than that of thenormal THz radiation.

•Pulse width of the single pulse ~ 1.6µs… that almost corresponds to theresponse time of the detector.
•Revolution period of the beam = 0.18µs… detectors that have faster responsetime are needed !

Experimental Results(Ib dependence, Spectrum)
Cut-off fromoptics

Absorption bywater•Squared-dependence of the intensity on the peak current … CSR•The CSR spectra depend on the pulse duration of the laser …
longer laser pulse duration makes longer dip, so that longer
wavelength components become coherent radiation.

CSR generation with AMlaser pulse
Power spectrum of CSR:

P (k ) = N [1 + ( N − 1) F (k )] p (k )2F (k ) = ∫ ρ ( z )e dz
ρ (z ) : Longitudinal electronic distribution
ikz•Wide spectrum

Single pulse …

•Small spectral peak

Modulated pulse …
( with M-peaks)•Narrow spectrum ∝ 1

M•Large spectral peak ∝ M 2

Electronicdistribution

THz CSR spectrum(excluding whole bunchcomponent)

'Chirped pulse beating'
Weling, A.S & Auston, D.H, J. Opt. Soc. Am. B 13, 2783-2791 (1996)•CCR, RRF : retroreflector•BS : beam splitter•G1, G2 : gratings

1. Making chirp on the laser pulse with a pair of gratings

E (t ) ∝ exp(−iωt − at 2 )2. Leading the chirped pulses to the Michelsoninterfelometer, and adjusting optical delay.
Generation of the beat structure that corresponds tothe frequency difference between two chirped pulses

I =| E (t + τ / 2) + E (t − τ / 2) | 2= C1 + C2 cos[2 Im(a )tτ + ωτ ]Modulated laser pulse andrelated CSR spectrum
Measurement of themodulated laser pulse withauto correlator•Narrower spectral width with themodulated laser pulse•The spectral peak frequency is tunableby changing the beat structure of thelaser.
Band width ~ 4cm-1@ pulse duration = 1 ~ 2 ps

Output from auto-correlator

CSR spectrum
Band width ~ 1cm-1

Laser pulse duration
60ps(much beating)Band width ~ 4cm-1
Laser pulse duration
1~2 ps(less beating)Changing CSR frequency
We changed the beating
frequency and estimated the laser
modulation frequency from theauto-correlator measurement

Clear correlation between thelaser frequency and the CSR peakspectrum

Summary• We have observed intense THz synchrotron
radiation by laser-bunch slicing method inUVSOR-II.
– squared dependence of the peak intensity on thepeak current … CSR– band width depends on the pulse duration of thelaser• THz CSR generation with AM laser pulse– narrow band THz CSR– peak frequency depends on the AM (tunable)