“KANAL-2” INSTALLATION AND INTERACTION OF LASER RADIATION WITH TARGETS

1. “KANAL-2” INSTALLATION AND INTERACTION OF LASER RADIATION WITH TARGETS Alexander N. Starodub Deputy DirectorN.G.Basov Institute of Quantum Radiophysicsof P.N.Lebedev Physical Institute of the RAS Leninsky prospect 53, 119991, Moscow, Russia LNF Seminar’ 2009 Frascati, Italy

2. The experiments have been performed with a Nd-glass laser facility KANAL-2with controllable coherence of radiation with the following parameters • laser pulse duration 2.5 ns • pulse energy upto300 J • output aperture60mm • degree of spatial coherence ~ 0.05 – 0.015 • degree of temporal coherence ~ 5х10-4–5х10-3 • degree of radiation polarization ~ 0.5 • pulse radiation contrast > 106 LNF Seminar’ 2009 Frascati, Italy

3. Hybrid Fusion-Fission Reactor Fast Blanket Thermal Blanket Fusion Camera LNF Seminar’ 2009 Frascati, Italy

4. Hybrid Fusion-Fission Power Station the only real way to new energetic 200 kJ • New generation of power station with very high level of safety (Subcritical nuclear reactor, external neutron source) • Two beam (or one beam) irradiation • Very simple targets weak compressed (or even non-compressed) • Neutron yieldup to 5x1016 • Gain 3000-5000 • Usual nuclear technologies • Etc. LNF Seminar’ 2009 Frascati, Italy

5. New Approach for Laser Driver and High Power Laser Systems We suggested to use • Laser with Controllable Coherence of Radiation • To use physical properties of laser light, first of all its coherence, to exclude many difficulties which arising with growth of laser system energy LNF Seminar’ 2009 Frascati, Italy

6. Advantages Suppression of speckle structures More easy to suppress self-focusing No phase plates, adaptive optics, spatial filters Control of flux distribution I(x,y) Increasing of energy density in rods and increasing of output energy Higher laser efficiency More compact laser installation Lower requirements for cleaning Lower requirements for optics quality More simple and cheap Laser with Controllable Coherence of Radiation LNF Seminar’ 2009 Frascati, Italy

7. Self-Focusing Process • The coherency degree influences very strongly on self-focusing process, especially on small-scale self-focusing. • In the case of low coherence radiation the picture of interaction is non-constant both in time and in space and the interference picture is formed onlywithin the coherence dimension. • The decrease of coherence level lead to the increase of the threshold of small-scaled self-focusing, and due to this to its suppression without using of spatial filtration. LNF Seminar’ 2009 Frascati, Italy

8. Laser with Controllable Coherence of Radiation (CCR Laser) • Optical scheme of the master oscillator (A), of the system of formation of spatial-temporal characteristics of laser radiation (B), and of the amplify system (C): • 1 - spherical mirror, • 2 - Kerr shutter, • 3 - forming diaphragm, • 4 - lens, • 5 - active element, • 6 - output diaphragm, • 7 - output mirror of the resonator, • 8 - rotatable mirror, • 9 – disk active element, • 10 – Faraday rotator. LNF Seminar’ 2009 Frascati, Italy

9. CCR Laser Radiation LNF Seminar’ 2009 Frascati, Italy

10. Goals: • Study into the processes of propagation and amplification of partially coherent laser radiation in a nonlinear medium at heightening the efficiency of energy yield in an active element of a high-power facility • Optical scheme optimization for the “Kanal-2” high-power laser facility to carry out the regime of target irradiation by a small number of beams as applied to the problem of a hybrid fission-fusion reactor • Switching of the “Kanal-2” laser facility to a two-beam operation regime LNF Seminar’ 2009 Frascati, Italy

11. Experimental Measurement of Gain Coefficient Energy at two-passage amplifier input – 1-14J 1 – amplifier, 2 – laser beam, 3 – calorimeters, 4 – beam-splitters, 5 – 100%mirror. LNF Seminar’ 2009 Frascati, Italy

12. Experimental results LNF Seminar’ 2009 Frascati, Italy

13. Experimentally justified that Laser with Controllable Coherence of Radiation may be considered as basis for creation of high power laser systems and Laser Driver • Suppression of speckle structures and self-focusing • Control of flux distribution I(x,y) and spatial-angular characteristics • Increasing of energy density in rods and increasing of output energy • Higher laser efficiency • No phase plates, adaptive optics, spatial filters • More compact laser installation • Lower requirements for cleaning • Lower requirements for optics quality LNF Seminar’ 2009 Frascati, Italy

14. Conclusions I • It is suggested the method for Nd-glass laser radiation amplification based on splitting of initial laser pulse into several pulses, each of which is amplified in the same active element by a multipassage scheme. The method makes it possible not only to increase the efficiency of the active element energy gain, but to carry out a compact scheme of a multi beam target irradiation. • It is shown that energy yield is heightened by 1,5 –2 times in a two-passage scheme, provided that amplified pulses are not time overlapped. LNF Seminar’ 2009 Frascati, Italy

15. Experimental setup 1 – interaction chamber 2 – target 3 –focusing lens 4 – objective 5 – prism 6 – lens 7 – spectrogragh 8 – digital camera 9 – objective of microscope 10 – polarized wedge 11 – system of interference filters 12 – mirror 13 – prism spectrogragh 14 – 90° prism 15 – electron-optical camera Big interaction chamber I – heating radiation II, III, IV – diagnostic channels LNF Seminar’ 2009 Frascati, Italy

16. mm LNF Seminar’ 2009 Frascati, Italy

17. The experiments have been performed with an Nd-glass laser facility KANAL-2 with the following parameters • laser pulse duration 2.5 ns • pulse energy 50 – 150 J • output aperture 45 mm • degree of spatial coherence ~ 0.05 – 0.015 • degree of temporal coherence ~ 5х10-4–5х10-3 • degree of radiation polarization ~ 0.5 • pulse radiation contrast > 106 LNF Seminar’ 2009 Frascati, Italy

18. Motivation • Volume-structured materials (foils) are considered as different functional elements in the ICF targets. • First of all, smoothing of a heating inhomogeneity and production of a steady compression of ICF targets is one of the methods for smoothing of a laser radiation inhomogeneity. • Investigation in the specific features of laser radiation absorption and scattering by such media, as well as the energy transfer, plasma formation and plasma dynamics, is a topical problem LNF Seminar’ 2009 Frascati, Italy

19. Foam Targets for «KANAL-2» LNF Seminar’ 2009 Frascati, Italy

20. SEM Image • As the targets, the of triacetate cellulose aerogels of the density of 9.1 mg/cm3, 4.5 mg/cm3and 2.25 mg/cm3 have been used. In several experiments up to 10 weight percent of coppernanoparticles of average diameter of 40 nm have been introduced into the polymer. The electron concentration and average density did not change. • 3D polymer nets do not change their structure under the change of density from 50 mg/cm3to 1 mg/cm3 . • The distance between the filaments is 0.6 to 1.7 mcm, and the filament diameter is 70 to 40 nm. • The aerogel density fluctuations at volume averaging 0.3х0.3х0.3 mm3do not exceed 0.5% at the aerogel average density higher than 4 mg/cm3, and grow up to 3% for the aerogel density of 1 mg/cm3 LNF Seminar’ 2009 Frascati, Italy

21. Image of plasma on frequencies 2ωо, 3/2ωо, 5/2ωоandon the fundamental frequency Target TAC 9 mg/cm3 400 μm Einc=17.8 J Ebs=30 mJ A – 1.06 μm; B – 0.53 μm; C – 0.71 μm; D – 0.42 μm. Spatial distribution of plasma radiation intensity on frequencies 2ωо, 3/2ωо, 5/2ωоand on fundamental frequency Size of focal spot 350 μm. Energy concentrated in spotis~ 2·10-5J for λ=1,06 μmin spatial angle 1.6·10-4 steradian, and energyis ~ 2·10-9J for λ=0,53 μm. LNF Seminar’ 2009 Frascati, Italy

22. Diagrams of plasma backscattering on fundamental frequency • Diagrams of scattering for the microstructured triacetate cellulose targets with the density 2.25; 4.5; 9; 10 mg/cm3 are presented. The diagrams of scattering for solid-state Fe and Be targets are presented also. • With an increase in the linear mass of TAC targets the plasma angle of divergence increases. Diagrams for TAC targets 4.5/400 and 10/200 are similar. These diagrams seem also to correspond to the diagrams for solid-state targets on fundamental frequency as well. LNF Seminar’ 2009 Frascati, Italy

23. Diagrams of forward scattering radiation Forward scattering radiation on fundamental frequency corresponds to the angle of incident radiation, and forward scattering on the second harmonic frequency occurs diffusely in space. LNF Seminar’ 2009 Frascati, Italy

24. Incident radiation spectrum • Forward scattering radiation spectrum LNF Seminar’ 2009 Frascati, Italy

25. Incident radiation spectrum • Forward scattering 2ωradiation spectrum LNF Seminar’ 2009 Frascati, Italy

26. Incident radiation spectrum • Forward scattering 2ωradiation spectrum LNF Seminar’ 2009 Frascati, Italy

27. Laser Radiation Backscattering • Energy of the radiation scattered into the focusing lens aperture is less than 1%. • The nonlinear scattering processes (SMBS, SRS) are non-essential from the energy viewpoint, and may affect the scattered radiation linewidth only. • The nonlinear processes of forward scattering are not energy essential as well. LNF Seminar’ 2009 Frascati, Italy

28. The Main Measurement Results on the Radiation Passed Through the Aerogel • Energy of the passed radiation makes a part comparable to the incident radiation energy • The passed radiation energy is 11.2 J at incident laser energy of 16.4 J, the TAC target thickness being 100 mcm and target density 4.5 mg/сm3. • The energy passed through the aerogel decreases • with increase in the aerogel thickness • with increase in the target density • with increase in the target linear mass • Duration of the aerogel-passed pulse is comparable with pulse duration of the incident radiation. • The passed radiation linewidth increases and may reach 200 Å. LNF Seminar’ 2009 Frascati, Italy

29. Temporal Evolution of Pulse Passing • The energy comparable with the target incident energy passes through a target at the pulse beginning when the size of plasma is not large. • As the plasma develops the absorption grows, and the pulse passing ceases. As seen from X-ray measurements the X-ray radiation intensity grows, which corresponds to a going-on plasma heating. • Pulse duration of a passed energy is comparable to the duration of the incident laser pulse. Target TAC 9 mg/cm3 500 μm Einc=66,7 J Efs=0,4 J LNF Seminar’ 2009 Frascati, Italy

30. Interpretation • The interpretations of the target geometrical transparency(Guskov, Rozanov (1997);Bugrov et al. (1999))are not capable to explain the observed duration of a passed pulse energy, and value of that energy. • It is possible that conditions for a nonlinear transparency of plasma produced under aerogel irradiation are realized. • Such a transparency arises due to the plasma density modulation in a laser field(Mironov (1971);Vladimirsky, Silin, Starodub (1977);Gorbunov, Zauer (1977)). As a result, there may arise even a full transparency of a layer under certain relationship between laser intensity, plasma layer size, and the radiation wavelength. • The second harmonic registration shows that such a density modulation actually takes place, including the conditions when the target density is above critical. LNF Seminar’ 2009 Frascati, Italy

31. Summary • Recent results on the interaction of laser radiation with plastic aerogel target, which is a submicron three-dimension polymer network of triacetate cellulose (TAC), are presented and discussed. • The energy balance measurements have shown that energy transmitted through the target is comparable with energy of the incident radiation. Part of it scattered into aperture of a focusing lens does not exceed 1%. Value of energy passed through the TAC target depends on an aerogel density and thickness, and may achieve 70% even when the density proves to be higher than critical. • It is found that spectrum of the transmitted radiation is considerably broadened up to 200 Å. The second-harmonic generation and other nonlinear effects were experimentally registered under the TAC target irradiation. • It is suggested that the TAC target could be used for the laser radiation conversion to optimize the light absorption and to obtain a broad linewidth of incident radiation. As a result, the efficiency of energy yield from active elements may be higher and the laser efficiency increased. • Laser radiation transmission through aerogel is considered as nonlinear transparency of a plasma layer arising from laser interaction with the low-density target. LNF Seminar’ 2009 Frascati, Italy