Simulations and ongoing measurements indicate that SSD results in small degradation to the near field beam quality. The measured effect of SSD bandwidth on conversion to the third harmonic and smoothing of the target illumination will also be described

Morgan, B.

Rothenberg, J.E.

Wegner, P.

Weiland, T.W.

UNT Digital Library

UCRL-JC-128870PREPRINTPerformance of Smoothing by Spectral Dispersion (SSD) withFrequency Conversion on the Beamlet Laser for theNational Ignition FacilityJ. E. Rothenberg, B. Moran, P. Wegner, T. WeilandThis paper was prepared for submittal to theConference on Lasers and Electro-Optics ’98San Francisco, CAMay 5-7,1998November 4,1997DISCLAIMERThis document was prepared as an account of work sponsored by an agency ofthe United States Government.  Neither the United States Government nor theUniversity of California nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California.  The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernment or the University of California, and shall not be used for advertisingor product endorsement purposes..,”Performance of Smoothing by Spectral Dispersion (SSD) with FrequencyConversion on the Beamlet Laser for the National Ignition FacilityJoshua E. Rothenberg, Bryan Moran, Paul Wegner, and Tim WeilandLawrence Livermore National Laboratory, L-439P. O. Box 808, Livermore, CA 94551Phone: (510) 423-8613, FAX: (510) 422-5537Email: JR1 @?LLNL.GOVAbstract: Simulations and ongoing measurements indicate that SSD results insmall degradation to the near field beam quality. The measured effect of SSDbandwidth on conversion to the third harmonic and smoothing of the targetillumination will also be described.This work was performed under the auspices of the U. S. Department of Energyby Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.1673.ps — J. E Rothenberg et al, ‘Performance of Smoot.himgby Spectral Dispersion...”Performance of Smoothing by Spectral Dispersion (SSD) with FrequencyConversion on the Beamlet Laser for the National Ignition FacilityJoshua E. Rothen&’rg, Bryan Moran, Paul Wegner, and Tim Weiland,Lawrence Livermore National Laboratory, L-439P. O. Box 808, Livermore, CA 94551Phone: (510) 423-8613, FAX (510) 422-5537Email: JR1 @ LLNL.GOVInertial confinement fusion (ICF) utilizing direct or indirect laser driverequires the target illumination to be uniform over a wide range of spatialfrequencies. A number of approaches have been suggested to achieve thedesired level of illumination uniformity.1-4 Angular dispersion of phasemodulated (FM) light (termed smoothing by spectral dispersion - SSD)4 isattractive for ICF using glass lasers, since pure phase modulation preserves theuniform intensity profiles necessary for high power laser amplification. ID SSDhas been demonstrated on the NOVA laser,5 however the National IgnitionFacility (Nil?) will require much more efficient and reliable operation. Therefore,it is of interest to investigate the performance of ID SSD on the Beamlet laser,which is a NIF prototypical multipass laser system.Numerical simulations of the Beamlet laser using PROP92 have beenperformed for 1 ns pulses with &200 krad main cavity spatial filter pinholes.These simulations show that the critical parameter for the laser performance isthe amount of additional divergence imposed on the beam by SSD in comparisonto the size of the spatial filter pinholes. Figure 1 shows the results of the PROP92calculations of the contrast of the near field intensity at 10 as a function of pulse2,... 1673.ps —energy andJ. E. Rothenberg et al, ‘Performance of Smoothing by Spectral Dispersion...”SSD divergence. One sees that the degradation of the beam isenhanced slight with increasing amounts of SSD. For example, with SSDdivergence of 25 wad, the contrast calculated is the same as a beam without SSD,but with pulse energy less by about 5%.Figure 2 shows measurements of the 10 Schlieren far field taken on Beamletfor a 3.5 KJ, 1 ns pulse with (left) and without (right) SSD present. In thismeasurement the divergence of SSD was -25 ~rad. One sees that frequencycomponents generated by Gibbsphenomena near the pinhole edge (indicated bythe arcle) are not significantly enhanced with or without SSD present. Near fieldmeasurements at this pulse energy also show that SSD does not significantlyimpact the beam contrast, as predicted in Fig. 1. The only effect of SSD which isclearly apparent in Fig. 2 is that the speckle structure in the far field is smoothedin the vertical SSD direction, which is the desired effect on the illumination in thetarget plane. Measurement of the 10 laser performance at higher energies andthe -20 ns pulse lengths required for ignition will also be described.The 1.053 pm beam is converted to the thrid harmonic by a 11/9 mm pair oftype I KDP / type II KD*P crystals. It is expected that the harmonic conversionefficiency will be reduced by the addition of the large bandwdith associated withSSD. Figure 3 shows the calculation of the peak conversion efficiency in thepresence of SSD bandwidth for a fundamental input intensity of 3.0 GW/cm2.Measurements of the conversion efficiency, bandwidth at the third harmonic,and the effect of SSD on the Beamlet focal distribution (smoothing of speckle andbroadening of the focal envelope) will also be discussed.3..1673.ps — J. E. Rothenberg et al, ‘Performance of Smoothing by Spectral Dispersion...”This work was performed under the auspices of the U. S. Department ofEnergy by Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.References1. R. H. Lehmberg and S. l’. obenschain, Optics Comm. 46,27 (1983) and R.H. Lehmberg and J. Goldhar, Fusion Technology 11,532 (1987).2. Y. Kato et al, Phys. Rev. Lett. 53,1057 (1984).3. D. V&on et al, Optics Comm. 65,42 (1988).4. S. Skupsky et al, J. APP1. Phys. 66,3456 (1989).5. D. M. Pennington et al, Proc. Sot. Photo-Opt. Instrum. Eng. 1870, 175(1993).6. D. Eimerl, J. M. Auerbach, and P. W. Milloni, J. Mod. Opt. 42,1037 (1995).4. ,. “. 1673.ps — J. E. Rothenberg et al, “Performance of Smoothing by Spectral Dispersion...”Figure CaptionsFigure 1: PROP92 calculations showing the contrast (RMS variation as afraction of the average) of the near field intensity just after thetransport spatial filter, at the end of a 1 ns pulse of varying energy,and with the indicated amount of SSD divergence. The cavityspatial filter is assumed to “use~00 prad pinholes. The effect ofSSD is to reduce the effective pulse energy where breakup occurs --by -5% for 25 ~rad of SSD and by -10% for 50 prad of SSD.Figure 2: Measured Schlieren far field images of lo Beamlet beam for a 3.5 KJ,Ins pulse with (left) and without (right) SSD present. The ring showsthe location of the cavity spatial filter pinhole edge. No additionalnonlinear enhancement of angular components near the pinhole edgeis apparent with SSD implemented. However, one sees the effect ofsmoothing of the speckle structure in the vertical SSD direction.Figure 3: Calculation of the conversion efficiency from 1.053 ym to 351 nm at3.o GW/cm2 as a function of the bandwidth at 1.053 Pml using a 11mm type I KDP doubler and 9.0 mm type II KD*P tripler.5.*.,. 1673.ps — J. E. Rothenberg et al, ‘Performance of Smoothing by Spectral Dispersion...”20181614nE12108642.1“ /’o2 3 4 5 6Pulse Energy (KJ)Figure 1: PROP92 calculations showing the contrast (RMS variation as afraction of the average) of the near field intensity just after thetransport spatial filter, at the end of a 1 ns pulse ~ tand with the indicated amount of SSD divergence. The cavityspatial filter is assumed to use HOO prad pinholes. The effect ofSSD is to reduce the effective pulse energy where breakup occurs --by -5% for 25 prad of SSD and by -10% for 50 pad of SSD.6.. .1673.ps — J. E.Rothenberget al, ‘Terfonnanceof Smoothhg by Spectral Dispersion..:’WithSSD WithoutSSDFigure 2: Measured Schlieren far field images of 10I Beamlet beam for a 3.5 KJ,Ins pulse with (left) and without (right) SSD present. The ring showsthe location of the cavity spatial filter pinhole edge. No additionalnonlinear enhancement of angular components near the pinhole edgeis apparent with SSD implemented. However, one sees the effect ofsmoothing of the speckle structure in the vertical SSD direction.7,,- *.. 1673.ps — J. E. Rothenberg et al, ‘Terfornmnce of Smootiing by Spectral Dispersion...”I III I I t I 1. IoFigure 3:0.5 1 1.5 2 2.5 3 3.5 4 4.5 5IR Bandwidth (A)A&Calculation of the,,conversion efficiency from 1.053 pm to 351 nm at ~-~~ lkk’-23.o GW/cm2 as a function of ~ bandwidth at 1“053 Yml using a 11mm type I KDP doubler and 9.0 mm type 11KD*P tripler.8Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

Performance of smoothing by spectral dispersion (SSD) with frequency conversion on the Beamlet Laser for the National Ignition Facility

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Performance of smoothing by spectral dispersion (SSD) with frequency conversion on the Beamlet Laser for the National Ignition Facility

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