HONG JIN KONG Department of Physics, KAIST, Gusong-dong, Yusong-gu, Daejon Korea

Transcription

1 Beam combined high energy/high power laser system operating at a repetition rate over 10Hz for laser fusion driver using stimulated Brillouin scattering phase conjugate mirrors HONG JIN KONG Department of Physics, KAIST, Gusong-dong, Yusong-gu, Daejon Korea

2 Daeduk Valley(Science Town) Daejon 11/28/2007 2

3 KAIST-from 1972 Korean governmental supported academic institute Full national scholarship/military-duty-exempted/all-dormitory students 600 under-graduate student/year =2, graduate students/year=3,600 ~6,000 students total 400 professors 11/28/2007 3

4 Contents Laser Fusion Energy What problems in the laser fusion driver? What is SBS PCM? How to control the phase of SBS wave? How to preserve the pulse shape of SBS wave? Proposal of 4 beam combination and laser fusion driver. 4

5 Dream Energy Fusion energy Infinite amount No pollution Non-explosive-safe MFE Chamber material problem LFE Laser driver problem 5

6 Motivation LFE needs a high energy laser(~1mj@2ns) with high repetition over 10Hz. LFE s bottle neck is the laser fusion driver with high rep. rate over 10 Hz. (NIF operates only at several shots per day) Fast Ignition reduced the required laser energy 1/10 and 500kJ@10Hz is required. Requires ~200 beam lines of 2.5KJ@10Hz for uniform illumination on the target. We need to develop the 2.5KJ@10Hz laser system for 1 beam line. Problem : Thermal problem of the laser medium Solutions : KrF Laser : e-beam foil lifetime is too short. (material problem) LD pumping ; not enough High thermal conductivity laser media (Ceramic YAG) ; not enough Beam combination of small lasers ; enough ; demonstrated only with 2 x 100mJ level in this work The current technology is available for 100J@10Hz, and 25 beam combination is OK. 11/28/2007 6

7 Why bottleneck in lasers? Cooling Solid amp High thermal load Cooling 7

8 Beam combination and its problems Phase distortion Break up Easy cooling Piston error 8

9 Proposed laser fusion driver with SBS beam combination Wave-front dividing method Proposed in XXVI ECLIM 1996 in Madrid by H.J.Kong 11/28/2007 9

10 How to solve these problems? Phase distortions By phase conjugation mirror such as SBS-PCM Piston errors of SBS-PCM By phase locking (previous works : hard to apply to many beams over ~10) or controlling the phase (this work : easy to apply to many beams unlimited) 10

11 Comparison of PCM to conventional mirror 11

12 Proposed laser fusion driver with SBS beam combination Wave-front dividing method Proposed in XXVI ECLIM 1996 in Madrid by H.J.Kong 11/28/

13 SBS-PCM (stimulated Brillouin scattering-phase conjugation mirror) Most simple optics among many PCM 13

14 Principles of SBS Stimulated Brillouin scattering (SBS) Advantages High reflectivity 50% for 10mJ pumping 90 % for 100mJ pumping Phase conjugated wave ; high fidelity over 90% Low frequency shift ; ~ 1GHz for liquid Very easy and simple installation Problems of SBS-phase conjugate wave-historically Random phase of SBS wave (solved by Kong) Pulse shape distortion (solved by Kong) 14

15 Reflectivity and breakdown probability depending on the laser mode of SBS-PCM with FC-75 Seong Ku Lee, et. al, JKPS 46, pp.443~447, Reflectivity(%) Laser input energy(mj) Reflectivity(%) / Breakdown(%) Reflectivity Breakdown Laser Input Energy(mJ)) Single mode pumping ( ν~120mhz, Γ=350MHz) (No break down) Multi mode pumping ( ν~30ghz, Γ=350MHz) (Break down occurs) 11/28/

16 Previous works for phase locking of SBS-PCM 1. Overlapping the SBS focal points locks the phases of the beams. 2. Phase locking by back seeding the Stokes shifted beam, which locks the phase of the PC wave. Lens SBS Cell Seeding Beam Polarizer Faraday Rotator SBS Cell SBS Cell Beam Splitter Mirror a) Overlap of two focal points D.A.Rockwell and C.R.Giuliano, Opt. Lett. 11, 147 (1986) Impractical for many beams > 10 11/28/2007 b) Back-seeding of Stokes wave T.R.Loree, et. al., Opt. Lett. 12, 178 (1987) PC can be broken by back seeding beam No PC anymore 16

17 4-wave mixing-laser system M.Bowers and R.Boyd, Phase locking via Brillouin-enhanced four-wave mixing phase conjugation, IEEE QE-34, 634(1998) Good for controlling phase Very much complicated optics 17

18 Proposed Self-phase control Feed back mirror > Counter propagating beams > standing wave > density modulation Standing density modulation locks the ignition position of the moving phonon This phonon locks the phase of the SBS wave. Phase controlling is possible by positioning the feed back mirror. (a)concentric type (b) Confocal type (most simple ever and forever) 18

19 Self-density modulation phase control Standing wave can ignite the moving grating to stimulate process of SBS 19

20 Experimental setup for self-phase locking of two beams H. J. Kong, S. K. Lee and D. W. Lee, Laser and Particle Beams 23, 107 (2005). H.J. Kong, et al, Laser and Particle Beams, 23, 55-59, 2005 H.J.Kong, et al, Appl. Phys. Lett. 86, , S. K. Lee, H. J. Kong and M. Nakatsuka, Appl. Phys. Lett. 87, (2005). 11/28/2007 M1&M2, mirror; W1&W2, wedge; L1&L2, cylindrical lens: L3&L4, focusing lens, CM1&CM2, concave mirror; HWP1&HWP2, half wave-plate; P1, polarizer; PBS, polarization beam splitter 20

21 Normal case: No locking 180 Average fluctuation ~ 0.3 λ Phase (degree) Number of shot (A.U.) Intensity profile of interference Intensity profile of interference for 160 shots

22 Confocal case for wavefront dividing Phase (degree) Average fluctuation ~ λ Number of shot (A.U) 228/238 : success (96%) Intensity profile of interference for 238 shots Fluctuation is due to the beam energy balancing for the beam pointing fluctuates. 11/28/

23 Experimental setup for amplitude dividing Average fluctuation ~0.08 λ 180 Sd= mJ and mj Pump energy:10.56 mj and mj CCD Phase (degree) SBS- PCM BS2 BS Number of shot BS3 M /256 : success (100%) 8 10 Intensity profile of interference for 256 shots Long term fluctuation is problem!! 11/28/

24 Thermal fluctuation and its stabilization 11/28/

25 Two beam combination with amplifiers Experimental setup for measuring the amplifier one pass gain 11/28/

26 Two beam combination with amplifiers Experimental Setup for phase control with Amplifiers Feedback Computer

27 Two beam combination with amplifiers Experimental Result Input energy (E in ): 20 mj ±0.50% (sub-beam: 10 mj) Output energy (E out ): mj ±2.76% net gain: 2.21 Without PZT control With PZT control Output Energy [A.U] Phase Difference [Degree] Output Energy [A.U] Controlled Energy: 4.92%fluctuation Controlled Phase: λ fluctuation Phase Difference [Degree] Count of shot Count of shot 11/28/

28 Two beam combination with amplifiers Experimental Result Input energy (E in ): 60 mj ±0.50% (sub-beam: 30 mj) Output energy (E out ): mj ±1.16% net gain: 2.25 Without PZT control With PZT control Output Energy [AU] Phase Difference[Degree] Output Energy [AU] Controlled Energy: 5.98%fluctuation Controlled Phase: λ fluctuation Phase Difference[Degree] Count of shot Count of shot 11/28/

29 Two beam combination with amplifiers Experimental Result Input energy (E in ): 100 mj ±0.50% (sub-beam: 50 mj) Output energy (E out ): mj ±1.09% net gain: 2.04 Without PZT control Abrupt jump With PZT control Controlled Energy: 5.55%fluctuation Output Energy [AU] Phase Difference[Degree] Output Energy [AU] Controlled Phase: λ fluctuation Phase Difference[Degree] Count of shot Count of shot 11/28/

30 Two pulse monitoring system for removing the abrupt jump of phase Monitoring the fluctuation just before flash lamp pumping 11/28/

31 Pre-pulse technique for preserving the pulseform Motivation Backward SBS wave has a steep rising edge. This can generate undesirable effects such as optical breakdown, and gives poor fidelity when entering the next SBS cell. Purpose Preserving a waveform of the backward SBS wave using a pre-pulse. Idea Some critical energy is necessary to generate the Brillouin grating, This corresponds to the threshold energy for the SBS process. Pre-pulse will produce the grating so that the main pulse need not to consume its energy to generate the grating any more. 31

32 Transmitted and spent for generating an acoustic wave 11/28/2007 Pulse shape of the pump and the SBS wave 32

33 Experimental setup 11/28/

34 E(pre)=3mJ, 4mJ, and 5mJ at 5ns delay (a) Input beam (b) 3mJ (c) 4mJ (d) 5mJ 11/28/

35 E(pre)=5mJ at delay time=3ns, 5ns, 7ns, 10ns (a) 3ns (b) 5ns (c) 7ns (d)10ns 11/28/

36 Final setup for pulse-form preservation H. J. Kong, et al., Opt. Lett. 30, 3401 (2005) 11/28/

37 Proposed 4 beams combination laser with pre-pulse

38 Proposed laser fusion driver with SBS beam combination Unlimited output Energy with high reprate over 10Hz Alignment free-symmetric cross type of SBS-PCM (mechanical alignment is enough) Moduled amplifiers make the maintenance easy No spatial filter small volume Low cost optical components Applicable to any type of pulse laser (fs~100ns) 11/28/

39 Publications of this work H. J. Kong, J. Y. Lee, Y. S. Shin, J. O. Byun, H. S. Park and H. Kim, Opt. Rev. 4, 277 (1997). H. J. Kong, Y. S. Shin, and H. Kim, Fusion Engineering and Design 44, 407 (1999). S.K.Lee, D.W. Lee, and H.J.Kong, JKPS 46, (2005) H. J. Kong, S. K. Lee, D. W. Lee, and H. Guo, Appl. Phys. Lett. 86, (2005). S. K. Lee, H. J. Kong, and M. Nakatsuka, Appl. Phys. Lett, 87, (2005). H.J.Kong, S.K.Lee, and D.W.Lee, Laser & Particle beams 23, (2005) H.J.Kong, S.K.Lee, and D.W.Lee, Laser & Particle beams 23, (2005) H. J. Kong, S. K. Lee, J. W. Yoon, and D. H. Beak, Opt. Rev. 13, 119 (2006). H.J.Kong, J.W.Yoon, J.S.Shin, D.H.Beak, and B.J.Lee, Laser & Particle beams 24, (2006) H.J.Kong, J.W.Yoon, D.H.Beak, J.S.Shin, S.K.Lee and D.W.Lee, Laser fusion driver usin g stimulated Brillouin scattering phase conjugate mirrors by a self-density modulation, to be published in Laser & Particle beams (25, #2, 2007) 39

40 IX. Future work More work to stabilize the energy fluctuation removing the unknown abrupt spiking by two pulse monitoring technique being done. If we have a stable beam pointing oscillator, wavefront-dividing may be applied. Demonstrate 4 x 100mJ/10Hz system Stage I : Demonstrate 4 X 100J@10Hz. Stage II : Demonstrate 16 X 100J@10Hz Stage III : Demonstrate 1-beam line: 256 X 100J@10Hz =25kJ@10Hz Stage IV : Construct a full laser fusion driver, 200 beam lines ~ 5MJ@10Hz Develop beam combination for the Fast Ignitor (ps 10Hz operation; possible Beam combined fiber laser possible for many applications 40

41 VIII. Conclusions We have proposed and demonstrated the most simple and accurate way for phase controlling of the SBS wave, the self-density modulation. We have proposed and demonstrated the pulse preserving technique of SBS wave, the pre-pulse technique. These 2 key techniques can be applied to build a many beams combination laser system using SBS-PCM for high energy/power with high repetition rate. This new technique of beam combination has basically no limitation of the number beams combined to realize the laser fusion driver soon. This beam combination technique can be applied to any kind of pulsed lasers. Also this technology will open a new laser machining technique using a holographic beam forming unit without scanning the laser beam. Static laser machining possible 41

42 Thank you for your attention! Waiting for the laser fusion energy~ 42

43 Theoretical model of the phase controlling and pre-pulse Some critical energy is necessary to generate the SBS process. The standing wave can ignite the Brillouin grating (moving grating), even if their two wavelengths are slightly different from each other. The phases of the 3 waves, pumping, SBS, and acoustic grating have the relation; φa = φp φb (a:acoustic wave, p:pumping wave, B:Brillouin wave) Pre-pulse with the energy of the threshold energy can preserve the pulse-form of the main SBS pulse. 11/28/