STUDIES OF INTERACTION OF PARTIALLY COHERENT LASER RADIATION WITH PLASMA

Transcription

1 STUDIES OF INTERACTION OF PARTIALLY COHERENT LASER RADIATION WITH PLASMA Alexander N. Starodub Deputy Director N.G.Basov Institute of Quantum Radiophysics of P.N.Lebedev Physical Institute of the RAS Leninsky prospect 53, , Moscow, Russia FPPT-3, Bangkok,

2 Research Team Sergey I. Fedotov Anastasiya A. Fronya Boris V. Kruglov Svetlana V. Mal kova Mikhail V. Osipov Victor N. Puzyrev Artyom T. Sahakyan Boris L. Vasin Oleg F. Yakushev FPPT-3, Bangkok,

3 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 yield up to 5x10 16 Gain Usual nuclear technologies Etc. FPPT-3, Bangkok,

4 Hybrid Fusion-Fission Reactor Fast Blanket Thermal Blanket Q Fusion Camera D = 0,7 (1 k )(1 k ) 1 2 M = 0,2Q (1 k )(1 k ) 1 2 FPPT-3, Bangkok,

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 FPPT-3, Bangkok,

6 Laser with Controllable Coherence of Radiation 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 FPPT-3, Bangkok,

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 nonconstant both in time and in space and the interference picture is formed only within 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. FPPT-3, Bangkok,

8 Small-Scale Self-Focusing Interference scheme for radiation of elongated and point sources L L b a L=10 3 cm a=1 cm L=λL 2 /16a 2 =12 cm ϑ β Active element of amplifier A picture of interaction of two waves is non-constant both in time and in space. An interference picture is formed only within the coherence dimension of the strong wave. B-integral is defined by value L FPPT-3, Bangkok,

9 Speckle Formation The problem of the speckle formation, unremovable by conventional methods of linear optics, might be solved easy with the help of lasers with a controllable coherence. The coherence control gives a possibility to smooth a speckle structure of the field at a target, because the lesser the time of a stationary interference pattern, the lesser the probability of development of all kinds of instabilities leading either to a speckle formation, or to breaking a stability of the target compression. It is possible to have very high level of homogeneity of irradiation of a target without using traditional correcting systems (such as phase plates, adaptive optics and etc.). FPPT-3, Bangkok,

10 Master Oscillator The new type of cavity was used in the master oscillator, which allowed operation within a wide range of the divergence angles of the modulated radiation. The original scheme of formation of a master oscillator pulse and its successive amplification in the serial cascade amplifiers permits one to make a required spatial and temporal coherence of the radiation pulse. Design of the master oscillator permits one to change divergence of the output radiation within by means of changing the value of the forming diaphragm. Changing the transmittance shape and profile of the forming diaphragm one can control the angular distribution of the output radiation. FPPT-3, Bangkok,

11 Kanal-2 first module Master oscillator Pulse duration 40 ns Radiation energy 0,7 J Divergence 1, rad Aperture 6 mm Amplify system Pulse duration 2 ns Radiation energy up to 300 J Divergence 1, rad Output aperture 60 mm Energy contrast >10 6 Flux density on target up to W/cm 2 FPPT-3, Bangkok,

12 CCR Laser Radiation a) b) c) a) pulse shape, b) radiation near field intensity distribution, c) radiation far field intensity distribution FPPT-3, Bangkok,

13 Spatial-Angular Characteristics of CCR Laser Radiation Laser radiation distribution Top single-mode YAG laser Bottom CCR laser Really there are no diffraction and interference phenomena Intensity distribution of the CCR laser radiation is very smooth FPPT-3, Bangkok,

14 Spatial-Angular Characteristics of CCR Laser Radiation Changing of profiling diaphragm gives possibility to form required intensity distribution on target Top Intensity distribution of laser radiation along caustics of lens Bottom Left: Intensity distribution of laser radiation in focal plane for different profiling diaphragms Right: Output aperture of oscillator FPPT-3, Bangkok,

15 What about efficiency of the CCR laser interaction with matter?? FPPT-3, Bangkok,

16 Nonlinear Conversion of CCR Laser Pulse into Second Harmonic The experiments have been performed with an Ndlaser facility KANAL-2 with the following parameters laser pulse duration 2.5 ns pulse energy J output aperture 45 mm degree of spatial coherence ~ degree of temporal coherence ~ 5х10-4 5х10-3 degree of radiation polarization ~ 0.5 pulse radiation contrast > 10 6 FPPT-3, Bangkok,

17 Nonlinear Conversion of CCR Laser Pulse by KDP Crystal Second harmonic radiation intensity distribution Left far-field Right grid aperture Uniform distribution of intensity of CCR laser radiation second harmonic No speckles FPPT-3, Bangkok,

18 Nonlinear Conversion of CCR Laser Pulse by KDP Crystal Top Second harmonic pulse Pulse duration 2,5 ns Second harmonic intensity versus divergence angle Bottom Angular distribution of second harmonic intensity Divergence angle 1, rad Coherence length 200 mcm Coherence time 0, s Ряд θ, 10-3 rad FPPT-3, Bangkok,

19 Intensity Distribution in Near-Field Region KDP Crystal, oee-type Left CCR laser radiation Right second harmonic Supergaussian distribution of radiation Smallscale perturbation are absent No diffraction Distribution of power density of radiation is practically constant FPPT-3, Bangkok,

20 Intensity Distribution in Far-Field Region KDP Crystal, oee-type Degree of spatial coherence 0,018 Divergence angle of CCR laser radiation 1,4x10-3 rad Left CCR laser radiation Right second harmonic Angular width of directional diagram of second harmonic radiation is more than twice times narrow in comparison with one for pump directional diagram FPPT-3, Bangkok,

21 Intensity Distribution in Far-Field Region KDP Crystal, ooe-type Left CCR laser radiation Right second harmonic Angular half-width of second harmonic 10-3 High efficiency of radiation conversion within an interval of angle width of synchronism FPPT-3, Bangkok,

22 Intensity Distribution of Second Harmonic and Conversion Efficiency CCR Laser radiation Second harmonic 25 Near Field efficiency, % Row1 Row2 Row3 Row4 5 1 cm power density, GW/cm 2 oee-type, KDP crystal length 40 mm Far Field Row 1 Ø = 3 mm λ 3 Å Row 2 Ø= 5 mm λ 3 Å Row 3 Ø = 8 mm λ 3 Å Row 4 λ 30 Å Spectral width 3 Å 10 Å 30 Å Degree of temporal 5x х10-4 5x10-4 coherence FPPT-3, Bangkok,

23 Nonlinear Conversion of CCR Laser Pulse into Second Harmonic by KDP Crystal Efficiency of conversion versus CCR laser load ooe-type, KDP crystal Row 1 crystal length 15 mm, no polarized radiation Row 2 crystal length 19 mm, no polarized radiation Row 3 crystal length 19 mm, polarized radiation efficiency, % power density, GW/cm 2 Row1 Row2 Row3 Increasing of polarization degree lead to increasing of second harmonic generation efficiency FPPT-3, Bangkok,

24 Scattering of CCR Laser Radiation 4 I IIa III Fe, 1,06 mkm Be, 1,06 mkm Be, 0,53 mkm 1,06 mcm fundamental frequency, 0,53 mcm second harmonic frequency IIb I experiment scheme for investigation of radiation scattered by plasma (1 focusing lens; 2 target; 3 fibers; 4 registration system); II photos of fiber exiting faces and it densitogram; III the directivity diagram of scattering radiation. IIc Reflected energy 0,4 0,35 Cu 0,3 Al 0,25 Ti 0,2 Be 0,15 Fe 0,1 Ta 0,05 Mg Incident energy, J Reflection is less then 1 % FPPT-3, Bangkok,

25 X-ray Radiation under CCR Laser Interaction with Target Fe target Mg target X-ray pinhole camera image of laser plasma radiation. Aperture 14 µm. X-ray pulse X-ray spectrum of laser plasma radiation. Intensity of heating laser radiation 2x10 14 W/cm 2. Bulk temperature Te= 300 ev. Cu target. FPPT-3, Bangkok,

26 Harmonics Generation under CCR Laser Target Interaction Left - Second harmonic radiation space distribution Bottom right - Spectrograms of harmonics radiation Red ω 0 Green 2ω 0 harmonic Blue (5/2)ω 0 harmonic Flux density W/cm 2 Smooth spatial distribution of second harmonic smooth (without speckles) spatial distribution of CCR laser radiation near critical surface FPPT-3, Bangkok,

27 Spectra of Plasma Emission Blue shift of second harmonic acceleration of plasma during laser pulse FPPT-3, Bangkok,

28 Investigation of Ultimate Characteristics of CCR Laser preliminary experiments Surface damage of active rod Input energy density - 4 J/cm 2 Gain of signal K=[q(L)/q(0)]=1+E storage /E input = = 6-8 Energy density in active rod up to 20 J/cm 2 FPPT-3, Bangkok,

29 Outlook 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 Up to 20 J/cm 2 at least Higher laser efficiency No phase plates, adaptive optics, spatial filters More compact laser installation Lower requirements for cleaning Lower requirements for optics quality Effective laser-target interaction Absorption, heating and X-ray generation Harmonics generation FPPT-3, Bangkok,

30 Small-Scale Self-Focusing 1 1 ϑ M 1b 1а Fig.1. Scheme of development of small-scale perturbations of a coherent beam: 1) spectrum of the strong wave, 2) noise spectrum at the amplifier input, 3) spectrum of the amplified noise. Difference: a strong wave represents some curve I(θ), which defines angular spectrum of an amplified radiation, whose width may be compatible with the scattered radiation spectrum 10-3 ϑ Fig. 2. Scheme of development of smallscale perturbations in a beam with the controllable degree of coherence: 1) spectrum of the strong wave I(θ) (curves 1a and b correspond to different profiles of a forming diaphragm), 2) noise spectrum at the amplifier input, 3) spectrum of the amplified noise ϑ FPPT-3, Bangkok,

31 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 A B J C 8 0.2J 4J 8J 46J 150J FPPT-3, Bangkok,