1. Recent developments of optical cavity systems for advanced photon sources based on Compton backscattering
A. Martens, F. Zomer, P. Favier, K. Cassou, R. Chiche, V. Soskov
Laboratoire de l’Accélérateur Linéaire, Orsay, France
Y. Honda, K. Sakaue, N. Terunuma, J. Urakawa
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan

2. Origin of the collaboration
Production of polarized positrons for the ILC
Needs for gamma-rays
High flux
Circularly polarized
Small bandwidth
Production by Compton backscattering
Electron-photon interaction
Photon energy boosted e-
γ-rays e+
Laser photon
(~ 1 eV)
e- (~ GeV)
γ-ray (~ 30 MeV)
10/05/2017
P. Favier - LAL, Orsay - FJPPL Workshop 2017

3. e- ring + optical resonator
High flux  High repetition rate
Storage ring
Optical resonator
Joined efforts to store high average power
Goals turned to societal applications  X-rays
MIGHTYLASER experiment
ATF ring
Optical cavity
e- ring IP
Scientific Reports 6, article number: 36569 (2016)
Stored power ~ 30 kW
Flux: 3 x 108 photons/s
Energy: 30 MeV
Flux α stored power
10/05/2017
P. Favier - LAL, Orsay - FJPPL Workshop 2017

4. Today’s machines: cERL
Stored power: 10.4 kW
Flux: 2.6 x 107 photons/s
X-rays energy: 7 keV
10/05/2017
P. Favier - LAL, Orsay - FJPPL Workshop 2017

5. Today’s machines: ThomX
ThomX Technical Design Report,2014
10/05/2017
P. Favier - LAL, Orsay - FJPPL Workshop 2017

6. ThomX Fabry-Perot cavity
10/05/2017
P. Favier - LAL, Orsay - FJPPL Workshop 2017

7. ThomX laser oscillator
Oscillator choice is crucial
Frequency noise must be < cavity linewidth
R&D on numeric feedback to lock
oscillator on FP cavity

8. ThomX oscillator OneFive MENLO Laser oscillator EOM PDH Transmission
Finesse ~
Linewidth ~ 10 kHz
PDH
MENLO
OneFive
Blue : PDH error signal
« Visual measure » of the phase noise
Yellow/purple: cavity transmission
Green: laser PZT
10/05/2017

9. ThomX laser amplifier Fiber amplifier made by CELIA (Bordeaux, FRANCE)
Optical cavity
e- ring
Laser oscillator IP
<P> ~ 10 mW
<P> ~ 150 W
<P> ~ 500 kW
Stretcher
Compressor
CVBG
CVBG
5 mW
Amp. #1
150 W
Amp. #2
Amp. #3
6 x 976nm
COMB
DC YDF 40/200 PZ

10. ThomX cavity mirrors Absorption < 1 ppm Transmission ~ 2 ppm
Thermal effects in the substrates
Severe coupling loss
Mightylaser experiment
ThomX: Three ULE mirrors (Ultra Low Expansion glass)
Deformation : 55x less than Silica
Already demonstrated
One Sapphire mirror
LMA Coating (Lyon, France)
Absorption < 1 ppm
Transmission ~ 2 ppm
Very high Finesse
H. Carstens et al., ASSL JTh5A (2013) 3

11. Future: burst mode cavities
Non superconducting linacs
100’s of electron bunches/beam
Reduce cost and size
10/05/2017
P. Favier - LAL, Orsay - FJPPL Workshop 2017

12. Optimal working point Ratio of circulating energy over input energy
Experiment
Enhancement factor
Simulation
Number of electron bunches
Higher efficiency than most proposed
optical systems
Same flux with less input laser energy
Higher flux with same input laser energy
Submitted to Optics Express
10/05/2017
P. Favier - LAL, Orsay - FJPPL Workshop 2017

13. P. Favier - LAL, Orsay - FJPPL Workshop 2017
Conclusion
Mightylaser: joined experiment for ILC
Parallel R&D on high gain Fabry-Perot cavities
ERL (cERL) & storage ring (ThomX)
Limitations due to thermal effects
Limited spatial coupling
Development on burst-mode cavities
F. Zomer visited LUCX KEK last year for that purpose
Fruitful discussion with Sakaue-san. Visited LAL last year too
Possible joined experiment using the LUCX system to test our optimization and improve the machine efficiency
10/05/2017
P. Favier - LAL, Orsay - FJPPL Workshop 2017

14. Recent developments of optical cavity systems for advanced photon sources based on Compton backscattering
A. Martens, F. Zomer, P. Favier, K. Cassou, R. Chiche, V. Soskov
Laboratoire de l’Accélérateur Linéaire, Orsay, France
Y. Honda, K. Sakaue, N. Terunuma, J. Urakawa
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan