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					 DPSSL Driver: Smoothing, Zooming and
          Chamber Interface




                                      Mercury


         Lawrence Livermore National Laboratory
  Ray Beach, John Perkins, Wayne Meier, Chris Ebbers,
Jeff Latkowski, Ken Manes, Richard Town, Camille Bibeau

                           Presented at

     High Average Power Laser Program Workshop
         Princeton Plasma Physics Laboratory
                                             This work was performed under the
                 Princeton, New Jersey       auspices of the U. S. Department of
                                             Energy by the University of California,
                                             Lawrence Livermore National
                October 27 and 28, 2004      Laboratory under Contract No. W-
                                             7405-Eng-48.
         We are investigating options, issues, and trades
         for DPSSL driver and direct-drive targets

    •    Motivation: We are working towards an updated laser design as basis for
         systems model and future integrated power plant study

    •    We are considering trades (e.g., target performance and driver efficiency
         for 1, 2, 3 and 4 options

    •    We are developing design concepts for beam delivery:
             - final optics, phase plate, turning/focusing mirrors
             - beam segmentation, number of beams
         that meet target requirements:
             - energy, pulse shape, illumination uniformity,
             - beam smoothing and zooming

    •    Next step will be to update laser architecture, and cost scaling


RJB/VG
         Mercury is a demonstration of a 1 laser engine
         that could drive 2, 3 or 4 beam lines
                                                  2                3 or 4
                                               conversion          conversion
               1 laser engine                  module              module




                                 Mercury
                                 LLNL




          Mercury is a sub aperture demonstration (100 J1) of a DPSSL beam line
               architecture that scales in pulse energy to the multi-kJ level


RJB/VG
         Envisioned IFE laser driver design builds on NIF
         architecture
         • NIF consists of 192 beams that will generate 1.8 MJ3
         • Beam line integration architecture is defined by “bays,” “clusters”,
           “bundles”, and “quads”




RJB/VG
         IFE baseline DPSSL aperture size is a
         compromise between high efficiency laser and
         high pulse energy aperture

                                                                                   Optical-Optical Efficiency vs B-integral
    • Optimized aperture is                                             0.35




                                                                                                          Bmax
     13 cm x 20 cm yielding
                                                                        0.30                                         10 x 15 cm2, 1.7 kJ




                                         Diode light - 1  efficiency
     an output energy of ~3.1 kJ1                                                                                   13 x 20 cm2, 3.1 kJ
                                                                        0.25                                         20 x 30 cm2, 4.2 kJ
    • 96 chamber ports for laser entry                                                                               30 x 45 cm2, 8.3 kJ
                                                                        0.20
    • ~4.5 MJ1 total 4 pulse energy
                                                                        0.15
      or ~47 kJ per chamber port
                                                                        0.10
    • 4.5 MJ1 is built up with
     1,536 total beam lines                                             0.05
      (or 8 x NIF)
                                                                        0.00
                                                                               0         1        2        3         4        5
                                                                                             B-integral (Radians)




                 IFE DPSSL will have 16 individual beams per port (4 x NIF) –
         the 16 beams (“IFE port bundle”) are the building blocks of a DPSSL driver

RJB/VG
         We are investigating the construction of a drive
         pulse using “IFE port bundles” consistent with
         target and chamber requirements




                                                                                    Proposed 2
                                                                                    drive pulse




     • Target considerations:
           • Beam smoothing through speckle averaging is required to eliminate laser imprint
           • Dynamic zooming of laser spot is required for efficient utilization of laser energy

     • Chamber considerations:
          • Minimization of solid angle dedicated to laser ports is desired to minimize neutron leakage
            and achieve adequate tritium breeding
RJB/VG
         Smoothing requirements impose a limit on
         product of target illumination solid angle and
         laser bandwidth
         “… low spatial frequency speckle relevant to direct drive is fundamentally determined by the
             product of the optical bandwidth and the illumination solid angle,” – Josh Rothenberg
                                                                                     1
                                                     tot 
                                                       2
                                                                                                               2
                                                                                                             
                                                                  tot  dtarg /                 mode   / 2 
                                                                                                        max 

                           Standoff for 0.48 cm dia. target implosion                                          Time resolved maximum lmode
                                                                                                  1000
                           0.08                                                                                                                              7x10
                                                                                                                                                                 14




                           0.07                                                                                lmode spectrum averaged                       6x10
                                                                                                                                                                 14


                                                                                                               over 1 nsec target
                           0.06
                                                                                                  100          response time – maximum                       5x10
                                                                                                                                                                 14
           Standoff (cm)




                                                                                     Max l-mode
                                                                                                               lmode is 267 at t=0




                                                                                                                                                                      Power (W)
                           0.05
                                                                                                                                                                 14
                                                                                                                                                             4x10
                           0.04
                                                                                                                                                                 14
                                                                                                                                                             3x10
                           0.03                                                                    10
                                                                                                                                                                 14
                                                                                                                                                             2x10
                           0.02
                                                                                                                                                                 14
                                                                                                                                                             1x10
                           0.01

                                                                                                    1                                                        0
                           0.00                                                                                         -9        -8          -8        -8
                                             -9        -8         -8        -8
                                                                                                         0.0       5.0x10    1.0x10      1.5x10    2.0x10
                                  0.0   5.0x10    1.0x10     1.5x10    2.0x10
                                                                                                                                sec
                                                      nsec

         kimprint.dstandoff> 2 or lmode>2rcritical/dstandoff – condition for coronal thermal smoothing
         of laser imprint where dstandoff is distance between ablation front and critical radius
RJB/VG
                                            Fraction of solid angle needed for a 300 GHz2 pulse to
                                            achieve 0.5% rms on target between l=0 and the
                                            maximum l-mode that impacts target stability
                                                                                           1
                                                   tot                                                      2
                                                                        2                                  
                                                                   tot         d targ /       mode   / 2 
                                                                        0     max                   max                                    • Target integration
                                            0.01
                                                                                                                                                   time, , is 1 nsec
                                                                                                                        7x10
                                                                                                                            14                   • dtarg ~ 4.8 mm
                                                                                                         M
                                                                                                                                                 •  = 523.5 nm
    Fraction of solid angle for rms=0.5%




                                                                                                                                                 • 2 = 300 GHz
                                                                                                                            14
                                                                                                                        6x10
                                            1E-3

                                                                                                                        5x10
                                                                                                                            14                   • tot = 0.5%




                                                                                                                                 Power (W)
                                                                                                                            14
                                                                                                                        4x10
                                            1E-4

                                                                                                                            14
                                                                                                                        3x10

                                                                                                                            14
                                            1E-5                                                                        2x10
                                                                                                                                             The earliest portion of the pulse,
                                                    P                                  R                                    14               the picket, requires the largest
                                                                                                                        1x10
                                                                                                                                             solid angle to achieve smoothing
                                                                        F
                                                                                                                        0
                                                                                                                                             (~1% of 4)
                                            1E-6
                                                                   -9             -8                -8             -8
                                                   0.0        5.0x10        1.0x10             1.5x10        2.0x10
                                                                                 sec
RJB/VG
         Because the target is most susceptible to laser imprint and
         subsequent instabilities early in the pulse, the picket
         requires the largest area in each port bundle for smoothing

             Single aperture at 20 m from target

                                               M   • Each of the 16 aperture sources ~2.6 kJ2
                                                   • Picket aperture is 72 cm x 72 cm
                                               M
                                                         /4 = 0.99%
                                               M   • All other apertures are 10.3 cm x 10.3 cm
                                                         /4 = 0.3%
                          P                    M   • Total solid angle dedicated to ports is
                                                     1.3%:
                                               M
                                                              total N ports Aport
                                                                     
                                               M               4        4 r 2
                                                                          96  72cm  10.3cm 
                                                                                                 2

                                               M                      
                                                                             4  2000cm 
                                                                                             2


         F     R    R     M    M     M    M    M                       0.013 (1.3%)

         • 10.3 cm beam aperture is consistent with 20 m standoff of final optic, 5 TDL
           beam, and ~4.8 mm target size
                                                             F         20 x103 mm  0.5235m
         • Spot size at target with 5 TDL beam: dt arg et        TDL                        5  0.51mm
                                                            dbeam            10.3x104 m
RJB/VG
         Pulse construction from port bundle using only
         rectangular in time pulses

         • Picket pulse overlaid with pulse constructed from 16 independently zoomed
           and overlapped beams (1 Picket,1 Foot, 2 Ramp, and 12 Main)
                        14
                     7 10
                        14                 16 Beam Pulse                   Picket Pulse
                     6 10
                        14
                     5 10
             W




                        14
                     4 10
             Power




                        14
                     3 10
                        14
                     2 10
                        14
                     1 10
                            0
                                       -9           -8      -8      -8
                                0   5 10        1 10   1.5 10    2 10
                                                  nsec
            • All 12 beams in Main are 4.3 nsec or longer in duration
              with optical-optical conversion efficiency ~ 0.28 (diode pump light to 1)
            • Harmonic generation issues are mitigated with rectangular in time pulses
RJB/VG
                                     Beam line point design assumes a 13 cm x 20 cm Yb:S-
                                     FAP crystal aperture and B-integral limited extraction
                                              Extraction Efficiency vs B-Integral
                              0.30



                              0.25
                                                                                                                       Diode light
                                         Foot (6.7 nsec)
                                                                                               Component   Duration       to 1
 Optical-Optical Efficiency




                              0.20
                                                                                                                       efficiency


                                                                                    B limit
                              0.15
                                         Ramp and Main (3.4 nsec)                                Picket    0.67 nsec      0.11

                              0.10
                                                                                                 Foot      6.7 nsec       0.29
                                                    Picket (0.67 nsec)                           Ramp      3.4 nsec       0.26
                              0.05
                                                                                                 Main      3.4 nsec       0.26
                              0.00
                                          1                     2                     3
                                                      B-Integral (Radians)



                                              • Due to B-integral limited extraction, shorter duration
                                                components are necessarily generated at lower efficiency
                                              • Pulse-averaged optical-optical efficiency (1) = 0.25
                                              • Each beam is assumed to have 150 GHz of bandwidth @ 1 


RJB/VG
         Harmonic conversion efficiency
         • 1 irradiance is 3.3 GW/cm2
         • 2 irradiance is 1.7 GW/cm2
         • 3 irradiance is 1.1 GW/cm2
         • 4 irradiance is 0.8 GW/cm2
                   Process                  1       Efficiency   diodefinal
                                         Bandwidth                 Efficiency


                   1  2                150 GHz       0.9          0.23



                   1  3                100 GHz       0.8          0.20



                   1  4                75 GHz        0.8          0.20



RJB/VG
         Independent zooming enables 88% of total pulse energy
         delivered to chamber to intercept target within critical
         radius
                        14
                     7´10
                        14
                     6´10
    L
    H
                                                                M

                                                                M                          • Energy
                        14
                     5´10                                       M
                                                                                             delivered to
                                                P               M
             W




                        14                                                                   chamber
                     4´10
                                                                M

                                                                M
                                                                                           • Energy
             Power




                        14                                      M
                     3´10           F   R   R   M   M   M   M   M                            delivered
                        14                                                                   within target
                     2´10
                                                                                             critical radius
                        14
                     1´10
                            0
                                0


         • Total Pulse Energy = 4.02 MJ
                                        5´10
                                                -9

                                                                HL
                                                                1´10
                                                                    -8

                                                                 nsec
                                                                              -8
                                                                         1.5´10
                                                                                      -8
                                                                                   2´10



         • With zooming as shown, 3.55 MJ falls within time resolved critical radius
            - 88% of pulse energy
         • Without zooming, only 2.52 MJ falls within time resolved critical radius
           - 63% of pulse energy
RJB/VG
                          Final optics scheme for port bundle – louvered GIM
                          design
                                                                                     135
                                                                                           cm

                                                                                                10.3
                                                                                                       cm




                Mirrors for
                72 cm x 72 cm
                picket pulse                                                                                GIMs for 15 pulse
                                                                                                            components that are
                                                                                                            after the picket

                            Al Reflectivity
                  1
                                     S-pol                 • Small GIMs for 10.3 cm x 10.3 cm beams need to be 135 cm
               0.95                                          in length (inc=85.6o) to limit absorbed fluence to 10 mJ/cm2
Refelctivity




                                                             - for S-polarized green light
                0.9
                                              P-pol        • Single GIM for the large area picket portion of the pulse
               0.85
                                                             (72 cm x 72 cm) would require a 9.45 m long optic
                0.8
                                                           • Two mirrors as small as 1.02 m long can be used to replace
                                                             the single large picket GIM (as shown above)
                      0      20     40       60       80
                                      Deg

  RJB/VG
         Summary


         • Based on a “NIF-like” architecture, we are proposing “chamber
           bundles” as the basic building block of an IFE DPSSL driver

         • Chamber bundles permit both zooming and smoothing to meet
           target and chamber requirements

         • Concept is applicable to 1 laser engine with 2, 3 or 4
           harmonic conversion option

         • We have developed a final optics concept that uses meter size
           GIMs

         • The next step will be to update laser architecture, and cost
           scaling models



RJB/VG

				
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posted:6/4/2013
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