Feedback R&D for Optical Cavity

Ryuta TANAKA (Hiroshima univ.)

19th Feb 2013SAPPHiRE DAY

collaborators

Feedback R&D for Optical Cavity

Hiroshima universityR. Tanaka, T. Akagi, T. Takahashi, H. Yoshitama

KEKS. Araki, Y. Funahashi, Y. Honda, T. Okugi,T. Omori, H. Shimizu, N. Terunuma, J. Urakawa

Waseda universityK. Sakaue, M. Washio

19th Feb 2013

Special thanks for French Team

contents

•Positron source using laser Compton

•Cavity control

•Gamma yield

•issue

Feedback R&D for Optical Cavity 19th Feb 2013

IntroductionPolarized e+ by laser Compton Scheme

e-

laser

Gamma ray

Toward the positron sources -> increase intensity of gamma rays

proof of principle experimentsM. Fukuda et al., Physical Review Letters 91, 164801(2003)T. Omori et al., Physical Review Letters 96, 114801(2006)

Feedback R&D for Optical Cavity 19th Feb 2013

Increase laser intensityusing laser cavity

For increase gamma-ray yield=Increse Laser intensity at interaction point with electron

laser cavity

Laser pulseGamma-ray

•High efficiency collision by timing synchronizationelectron and laser pulse•Increase of laser intensity by resonance of laser cavity•Condensed laser at IP

Electron beam

Feedback R&D for Optical Cavity 19th Feb 2013

Experiments at the KEK ATF

detectorγ

Laser cavity

ATF parameter1.3GeV1×1010 electron/bunchUp to 10 bunch/train2.16×106 turn/s

Feedback R&D for Optical Cavity 19th Feb 2013

laser cavity

IP

Plane mirror

Concave mirror

Plane mirror

Concave mirror

Feedback R&D for Optical Cavity 19th Feb 2013

Polarization property

Cavity length

L-pol R-pol

geometric phase due to twisted pass

– cavity only resonates with circular polarization– Different resonance point in left and right polarization

Feedback R&D for Optical Cavity 19th Feb 2013

Control method using Polarization property

Different slope in left and right pol.

Feedback R&D for Optical Cavity 19th Feb 2013

Advantage in the control method

exchange polarity feedback loop is exchange stack laser pol. ->e+ pol. can be controlled by the polarity control loop

Transmitted power

R-pol power L-pol power

PD

PBS

QWPPD

PD

Feedback R&D for Optical Cavity 19th Feb 2013

Cavity controlS

tack

po

we

r

control

stack power [W]

L-pol R-pol

1.4% fluctuation->4pm

110pm

Laser power = 2.6kWTiming jitter = 8ps

Feedback R&D for Optical Cavity 19th Feb 2013

Laser size at IP

Electron beam(σe=10um)

laser

Vertical position [um]

Ga

mm

a y

ield

[A.U

]

σ= 17um ->σl=13um

Feedback R&D for Optical Cavity 19th Feb 2013

Gamma-ray yield

5bunch/train(7.7mA)

Gamma energy [GeV]

Co

unt

~2.6×108/sec

2970±20 MeV ⇒ ~120s/train

Feedback R&D for Optical Cavity 19th Feb 2013

Gamma-ray yield each bunch

time [ns]

Gam

ma

yiel

d [A

.U.]

time [ns]

Bun

ch c

urre

nt [

A.U

.]

no bunch dependence ( yield is proportional to e- current)

5bunches/train

e-

laser

5.6ns

Feedback R&D for Optical Cavity 19th Feb 2013

Thermal effect

Target: more intensity stacking

Transmitted power

Reflected power

Feedback on

20s

Thermal deformation of the mirror ->Change of transmitted profile =Incident efficiency is reduced->make stacking power limit

Feedback R&D for Optical Cavity 19th Feb 2013

When low power

summary

◇R&D e+ source using laser Compton

◇gamma-ray yield = 2.6×108/sec

◇cavity have stack power limit by thermal effect

Feedback R&D for Optical Cavity 19th Feb 2013