Status Report on the B820 CO 2 Laser R&D Program

1. Status Report on the B820 CO2 Laser R&D Program Mikhail Polyanskiy

2. Where we stand 2 TW !

3. Outline • Introduction • Picosecond long-wave infrared (LWIR) laser • ATF’s know-hows (CPA, mixed-isotope CO2) • Steady progress since last ATF user’s meeting • Pulse duration reduced (a lot) • Puzzle of pulse extension at high energy • Mystery solved, path for further improvement • Pleasant surprise: high gain of rare-isotope CO2 molecules • Technical details • New optical layout of TW room • New re-incarnation of CO2 freeze-out system • Reflection isolator “Plasma shutter” • Vacuum beam transport • Copper apodizing mirrors • Non-linear pulse compressor: simplified design for sub-scale test • Summary

4. Introduction

5. ATF’s Long Wave Infra Red laser 102-103× Solid state (OPA) 104× J++ 10-6 J СЩ2 CO2 CO2 10-2 J Seed laser Produces a picosecond seed pulse Final amplifier Main source of energy Regenerative amplifier Provides most of amplification

6. ATF’s know-how Chirped Pulse Amplification CO2 CO2 COMPRESSOR STRETCHER Mixed-isotopeactivemedium CO2 CO2

7. Why mixed-isotope active medium? “1 ps” 0.15 µm 1 bar 10 bar 10 bar, isotopes C16O2 : C16O18O : C18O2 25% : 50% : 25% 2 ps @ 9.2 µm Simulations: co2amp

8. Transition to full-scale CPA and isotopic active medium 2014: isotopes & CPA in regen only COMPRESSOR STRETCHER • Polyanskiy, Pogorelsky, Yakimenko, “Picosecond pulse amplification in isotopic CO2active medium,” Opt. Express 19, 7717-7725, 2011 • Polyanskiy, Babzien, Pogorelsky, "Chirped-pulse amplification in a CO2 laser," Optica 2, 675-681, 2015 2018: full-scale isotopes & CPA CO2 CO2 CO2 CO2 COMPRESSOR STRETCHER

9. Pulse duration improvement over last year

10. Pulse duration Nov 2017 (20th ATF Users’ meeting) optimization Dec 2017 10.3 μm30% of energy in post-pulses mixed-isotope amplifiers Aug 2018 (AAC) optimization Nov 2018 (Now) new (denser) gratings [Mid-2019] AGSe 9.2 μmsingle pulse

11. Our understanding of CPA as of 2017 75 lines/mm (present) 100 lines/mm(new gratings) configuration pulse 10.3 μm 4 ps 0.11 THz 20 ps 2× 9.2 μm 2 ps 0.22 THz (mixed-isotope CO2) 2× 40 ps 80 ps * calculated for 10-J pulse

12. Why 10% difference in wavelength is non-negligible? 9.2 μm 10.3 μm Grating pair Group delay dispersion (ps/THz) in dispersive elements of CPA ~2× ≠

13. Accurate model 75 lines/mm (present) 100 lines/mm(new gratings) configuration pulse 10.3 μm 4 ps 0.11 THz 21 ps 1.4× 9.2 μm 2 ps 0.22 THz (mixed-isotope CO2) 2.1× 29 ps 62 ps 40 ps 80 ps * calculated for 20-J pulse

14. What stretching do we need? Old system (10.3 μm) Mixed-isotope CO2 (9.2 μm) 75 -> 100 lines/mm gratings 0.1 J/cm2 0.5 J/cm2 1.0 J/cm2 1.5 J/cm2 2.0 J/cm2 2.5 J/cm2 0.22 THz 0.22 THz 0.11 THz 2018 2017 2019 2 ps 2 ps 4 ps

15. Peak power [5 TW]need compressor efficiency ~70%(now 50%) Next milestone: 4 TW Now: 2 TW

16. Gain of isotopic CO2 active medium revisited 43% Oxygen-18: 54% 46% 50% Experiment Simulations Gain: 2.1 % / cm New constants, 43% 18O New constants:A(C16O2) same A(C16O18O) 1.5x A(C18O2) 2.25x Old constants, 54% 18O HITRAN 2008 –> HITRAN 2016 A(C16O2) ~same A(C16O18O) ~same A(C18O2) 1.9x 1.4 % / cm Simulations: co2amp

17. What else?

18. Compressor re-configured, re-located to TW room 3 m Spectrometer gratings Amplifier Autocorrelator

19. CO2 recovery (enabling technology!) CO2 N2 He~1:1:18 N2 He v. 2018final amplifier ++2-3 hours v. 2008regen~8 hours LN2 v. 2015regen2-3 hours

20. Plasma shutter Nd:YAG Switching Pulse T0Shutter Open CO2 Pulse Retroreflected From Experiment CO2 Pulse From Amplifier To Experiment T0+ 20-25 ns Shutter Closed Amplifier Protected

21. Vacuum beam transport FEL Room Ion chamber, existing experimental chamber Laser shutter, 1x Plasma shutter Mirror box, 10x Compressed Laser Pulse Gate valve, 4x ExperimentalHall BL1 experimental chambers (3x)

22. Laser-machined apodizing Cu mirrors Collaboration with Euclid Techlabs 0 ≤ α≤ 1 αR -R R 0 sin2

23. Non-linear pulse compressor: design of sub-scale proof-of-principle setup NaCl 25 mm 12-m vacuum enclosure Brewster ZnSe 12 mm 50 mm 5 mm dia.aperture NaCl 60 mm 4.5 m 7.5 m ~0.5 TW ~1.5 TW • Input NaCl (vacuum window) minimum impact on the pulse • ZnSe (nonlinear component) self-phase modulation + self-focusing • Aperture (spatial filter) performance stabilization • Output NaCl (dispersive compressor) pulse compression via linear dispersion E = 1 J E = 0.35 J E = 0.31 J E = 0.92 J 160 fs FWHM = 2 ps ω = 15 mm

24. Conclusion

25. Summary • 2.5 TW achieved (reliable operation @ 2 TW) • Full-scale mixed-isotope, CPA operation • 9.2 μm (!), 2 ps, 2 TW • Better understanding of CPA, path to higher peak power (4-5 TW) • Progress on spectroscopy/simulations front • All-new beam delivery infrastructure (reflection isolation, vacuum) • Simplified design of NLPC [5 TW]need compressor efficiency ~70%(now 50%) Next milestone: 4 TW Now: 2 TW ===== THE END =====