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

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

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

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

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

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

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

ThomX oscillator OneFive MENLO Laser oscillator EOM PDH Transmission Finesse ~ 25 000 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

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 55W @ 976nm COMB DC YDF 40/200 PZ

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

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

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

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 installation @ 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

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