A preliminary error budget for the third harmonic converter for the National Ignition Facility (NIF) laser driver has been developed using a root-sum-square-accumulation of error sources. Such a budget sets an upper bound on the allowable magnitude of the various effects that reduce conversion efficiency. Development efforts on crystal mounting technology and crystal quality studies are discussed

Adams, C.H.

Auerbach, J.M.

Barker, C.E.

UNT Digital Library

December 9, 1996This is a preprint of a paper intended for publication in a journal or proceedings.  Sincechanges may be made before publication, this preprint is made available with theunderstanding that it will not be cited or reproduced without the permission of theauthor.LawrenceLivermoreNationalLaboratoryUCRL-JC-124856PREPRINTThis paper was prepared for submittal to the2nd Annual International Conference on Solid-StateLasers for Application to Inertial Confinement FusionParis, FranceOctober 22-25, 1996National Ignition Facility FrequencyConverter DevelopmentC. E. Barker, J. M. Auerbach, C. H. Adams,S. E. Bumpas, R. L. Hibbard, C. L. Lee,D. H. Roberts, J. H. Campbell, P. J. Wegner,B. M. Van Wonterghem, and J. A. CairdDISCLAIMERThis 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.National Ignition Facility frequency converter developmentC. E. Barker, J. M. Auerbach, C. H. Adams, S. E. Bumpas, R. L. Hibbard, C. L. Lee, D. H. Roberts,J. H. Campbell, P. J. Wegner, B. M. Van Wonterghem, and J. A. CairdUniversity of CaliforniaLawrence Livermore National LaboratoryP. O. Box 5508, L-487Livermore, CA  94551-9900, USA(510) 423-3282 / FAX (510) 423-9242AbstractA preliminary error budget for the third harmonic converter for the National Ignition Facility (NIF) laser driver has beendeveloped using a root-sum-square-accumulation of error sources.  Such a budget sets an upper bound on the allowablemagnitude of the various effects that reduce conversion efficiency.  Development efforts on crystal mounting technology andcrystal quality studies are discussed.Keywords: harmonic generation, frequency conversion, nonlinear optics, KDP1. IntroductionThird harmonic generation experiments on the Beamlet laser1 have demonstrated whole-beam energy third harmonic energyconversion efficiencies of up to 80.6%, as indicated by the plot in Figure 1.2  Third harmonic conversion efficiencies of up to85% at the peak irradiance of the input laser pulse are necessary to meet current NIF power and energy requirements.  At aninput drive irradiance of 3.3 GW/cm2 incident upon an 11 mm type I KDP second harmonic generation crystal followed by atype II KD*P mixing crystal, the ideal plane wave third harmonic conversion efficiency, including absorption in the crystals,is 90.8%.  This sets a total peak conversion efficiency error budget of 5.8%.1.00.80.60.40.20.0Third Harmonic Conversion Efficiency543210Average 1ω Input Irradiance (GW/cm2 ) Beamlet data - 32 cm xtals(energy conversion efficiency) ideal plane wave full modelThird Harmonic Generation with 32 cm XtalsFigure 1. Third harmonic conversion efficiency versus input 1ω driveirradiance.  These data were recorded with a 29.6 cm aperture beamincident upon 32 cm square KDP. crystals.2. Preliminary Error BudgetThe frequency converter error budget must account for many effects.  The effects that act to reduce harmonic generationefficiency can be grouped into three sets of terms: loss terms, dynamic range terms, and angular sensitivity terms.  An upperbound on the allowable magnitude of the various error budget terms can be determined from a root-sum-square accumulationof errors.  The allocation of the 5.8% error budget in a root-sum-square accumulation of the three groups of error terms listedabove is given in Table 1.A number of effects, including the presence of phase-modulated bandwidth and depolarization as well as surfacereflectivities, can be treated as losses.  These terms, which lead to a net reduction of 2.8% in conversion efficiency, aresummarized in Table 2.  The dynamic range terms are listed in Table 3.  Since we are concerned only with peak irradiance, thespatial and temporal edges of the pulse are not included in the error budget.Because mix ratio control in the type I / type II design is accomplished by angular detuning of the doubler, this design isvery sensitive to angular errors.3  From the plot given in Figure 2, we find that the angular sensitivity of third harmonicgeneration is dominated by detuning in the doubler crystal.  An angular error of ±40 µrad internal to the doubling crystalresults in a third harmonic conversion efficiency reduction of 5%.  Consequently, the preliminary angular error budget will beset at ±40 µrad, and apportioned entirely to the doubler.  The terms that contribute to the angular sensitivity are given inTable 4.  Of particular concern are long-scale-length surface figure and phasematching angle uniformity of the crystals.Table 1. Frequency converter error budgetSource of efficiency reduction MagnitudeLoss terms 2.8%Dynamic range terms 1%Angular sensitivity terms 5%Total root-sum-square accumulation 5.8%Table 2. Loss termsEffect Budget EfficiencyreductionCommentsbandwidth 30 GHz 1.4% required for SBS suppressioncoating reflectivity SHG xtalR = 0.1% (both faces)0.2% SHG xtal: 1ω / 2ω compromise silicone/sol-gelcoatingsTHG xtalR = 0.9% (input)R = 0.1% (output)1.0% THG xtal: 1ω / 2ω compromise sol-gel coating oninput; 3ω optimized sol-gel coating on outputdepolarization ≤ ±2.5˚ rotation ≤0.2% type I/II insensitive to depolarizationabsorption in bulkcrystal5.8%/cm @ 1ω(o-wave in SHG)– – already included in 90.8% efficiency value from planewave calculationTable 3. Dynamic range termsEffect Budget EfficiencyreductionCommentsamplitude modulation 1.2:1 uniform distribution 1% Irradiance distribution will be gaussian-likespatial and temporaledgesN/A 0 Spatial and temporal edges of the pulse affect wholebeam energy conversion efficiency, not peak irradianceefficiency3. Crystal Deformation and MountingDeformation of the conversion crystal surfaces leads to wavefront deformation as the laser beam enters the crystal.  Thesewavefront aberrations cause local variations in phasematching conditions that reduce conversion efficiency.  Suchdeformations of the crystal surfaces can be caused by crystal cutting operations, the crystal mount, and gravitational sag.When supported only at its corners, as is currently the case for Nova and Beamlet conversion crystals, the deflection of a 41cm square crystal under its own weight can result in a conversion efficiency reduction of 2.5%.  Shown in Figures 3 and 4,respectively, are contour plots of displacement calculated for a 41 cm square by 1.1 cm thick type I KDP doubler and a 41 cm1.00.80.60.40.20.0Third Harmonic Conversion Efficiency5004003002001000Internal Angle from Phasematched Direction (µrad)±40 µrad±120 µrad Input irradiance 3.3 GW/cm2 11 mm type I SHG  9 mm type II THG   doubler detuning    tripler detuning  Figure 2. Third harmonic conversion versus internal detuning anglefrom phasematched direction for an 11 mm type I doubler and a 9 mmtype II tripler at an input drive irradiance of 3.3 GW/cm2.  A detuningof ±40 µrad internal to the doubler reduces third harmonic generationefficiency from 90.8% to 85%.  The tripler is three times lesssensitive to internal detuning.Table 4.  Angular sensitivity termsEffect Internal angleallocation (µrad)Commentsalignment ±26 root-sum-square accumulation of COMS, reference surface, and beamalignmentstemperature ±5 doubler thermally tunes at 50 µrad/˚C; tripler at 200 µrad/˚C→  ∆T ≤ ±0.1˚Cbeam divergence ±13 divergence with phase plate ≤35 µrad (external); 20 µrad (external) allocated tobeamphasematchinguniformity±15 CCI and COMS data indicate that 10 µrad is achievable with high qualitycrystalssurface figure (longscale)±30 45 µrad (external) is allocated among mounting effects, surface figure ofcrystal, and gravitational sagTotal (root-sum-square)±40 allocation based on sensitivity of tripling efficiency to doubler detuningsquare by 0.9 cm thick KDP tripler.  The displacements are given in waves at 1 µm, and are calculated for boundaryconditions of simple support in the four corners with gravity acting parallel to the plate normal direction.  Under theseconditions, the maximum deflection of the doubler is 28 µm and the maximum deflection of the tripler is 40 µm.The gravitational deflection of the crystals can be reduced significantly by supporting them completely around theirperimeter.  With simple support on all four edges, the doubler sag is calculated to be 4 µm and tripler sag is reduced to lessthan 7 µm, resulting in a conversion efficiency reduction of only 0.1%.  If clamped boundary conditions are applied along allfour edges, doubler sag is calculated to be less than 1.5 µm and tripler sag is found to be less than 2 µm.  In this case there isno discernible loss in conversion efficiency.  The crystal deflections and the corresponding conversion efficiency reductions forthese various sets of boundary conditions are summarized in Table 5.  We anticipate that the boundary conditions for realmounts will lie somewhere between the idealized cases of simple support and clamped support.The design of the Final Optics Cell (FOC) for the National Ignition Facility laser provides full edge support for both thedoubler and tripler crystals, as illustrated in Figure 5.  This monolithic cell also contains the final focus lens, and an optionaldiffractive optic plate.  A two-thirds scale, multi-piece prototype FOC has been fabricated by single-point diamond turning.The profile of the crystal mounting surface has been measured by phase-shift interferometry, as shown in Figure 6.  Thissurface has been found to be flat to approximately 1 µm, with a maximum slope of about 8 µrad.Figure 3. Contour plot of the calculated gravitationaldeflection of a 41 cm square by 1.1 cm type I KDPdoubler simply supported at four corners.  Gravity actsparallel to the plate normal direction.Figure 4. Contour plot of the calculated gravitationaldeflection of a 41 cm square by 0.9 cm type II KDPtripler simply supported at four corners.  Gravity actsparallel to the plate normal direction.Table 5. Calculated crystal deflections for various boundary conditionsBoundary condition Doubler sag- 11mm type I(waves @ 1µm)†Tripler sag- 9mm type II(waves @ 1µm)†Efficiency losssimple support all four corners 28 40 2.5%simple support all four edges 4 <7 0.1%clamped support all four edges <1.5 <2 0%† gravity acts parallel to the plate normal direction4. Crystal QualityHighly efficient frequency conversion requires very good crystal quality.  Third harmonic generation experiments onBeamlet using the 37 cm frequency conversion crystals produced lower whole beam energy conversion efficiency values thanwere obtained in experiments using the 32 cm crystals.2  Although whole beam energy conversion efficiency was only 72 %,an examination of near field images taken during recent Beamlet experiments indicates that some regions of the aperture wereconverting in excess of 80% while other regions showed much lower efficiency.4  The phase maps of the 37 cm doubling andtripling crystals, which are presented in Figures 7 and 8, show significant structure.  Furthermore, phasematching anglemeasurements made during crystal cutting and finishing indicate an internal phasematching angle variation of at least 63 µradin the 37 cm doubler, whereas the 32 cm doubler showed a variation of only 5 µrad.  On the other hand, the 32 cm and 37 cmFigure 7. Phase map of the 37 cm doubler, obtained byphase-shift interferometry.Figure 6. Measured surface profile of the 2/3-scale prototypeFinal Optics Cell.  This measurement was performed with aphase-shift interferometer operating at 632.8 nm.  The surface isflat to approximately 1 µm.Figure 8. Phase map of the 37 cm tripler, obtained byphase-shift interferometry.Figure 5. Illustration of the National IgnitionFacility Final Optics Cell (FOC).  In this figure,the beam enters from the top, passing through thedoubling crystal, the tripling crystal, the final focuslens, and an optional diffractive optic plate.  Thecrystals will be held against the precision mountingsurfaces by compliant pads.tripler crystals displayed similar levels of phasematching angle variation: 19 µrad and 16 µrad, respectively.  Consequently webelieve that the differences in conversion efficiencies that we have measured with the 32 cm and 37 cm crystals is due to thepoorer quality of the 37 cm crystals, and we are pursuing further off-line studies to better characterize these crystals.5. SummaryWe have developed a preliminary error budget for the third harmonic generator for the NIF laser driver.  This budget isbased on a root-sum-square accumulation of error from all of the various effects that can reduce conversion efficiency.  Becausesome, but not all, of these effects can be treated as statistically independent, this error budget sets an upper bound on theallowable magnitude of these error sources.  Lower bounds can be established by direct addition of the effects of all of theseerror terms.  It is obvious that the allowable magnitudes of these error terms will have to be significantly reduced to meet thistighter budget.  Consequently, a much more careful apportionment of the error budget will be required.  The most realisticbudget will lie between these two sets of bounds, but extensive modeling will be needed to establish the interactions of thevarious error terms.  Such a budget will be developed early in the Title II engineering phase of NIF.We have modeled the gravitational sag of the frequency conversion crystals under worst-case conditions, i.e. gravity actingnormal to the conversion crystal plates, and found that full edge support of the crystals will nearly eliminate efficiencyreductions due to that effect.  Measurements of our first prototype Final Optics Cell show that it achieved a very flatmounting surface.An analysis of Beamlet results and limited crystal characterization data indicate that crystal quality is the primary cause ofthe reduced conversion efficiency that has been observed with the 37 cm conversion crystals during Beamlet experiments.6. AcknowledgmentsWe acknowledge the LLNL engineers, technicians, scientists, and support personnel who make Beamlet’s daily operationpossible.  This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore NationalLaboratory under Contract No. W-7405-Eng-48.7. References1. B. M. Van Wonterghem, J. R. Murray, J. H. Campbell, D. R. Speck, C. E. Barker, I. C. Smith, D. F. Browning, andW. C. Behrendt, “System Description and Initial Performance Results for Beamlet,” ICF Quarterly Report, Vol. 5, No. 1, pp1-17, Lawrence Livermore National Laboratory, Livermore, CA, UCRL-LR-105821-95-1, Oct. - Dec. 1994.2. C. E. Barker, B. M. Van Wonterghem, J. M. Auerbach, R. J. Foley, J. R. Murray, J. H. Campbell, J. A. Caird, D. R.Speck, and B. Woods, “Design and performance of the Beamlet laser third harmonic frequency converter,” in First AnnualInternational Conference on Solid-State Lasers for Application to Inertial Confinement Fusion, Michel Andre, Howard T.Powell, Editors, Proc. SPIE 2633, pp. 398-404 (1995).3. R. S. Craxton, “High Efficiency Tripling Schemes for High-Power Nd:Glass Lasers,” IEEE J. Quantum Electron., QE-17, No. 9, pp. 1771-1782, Sept. 1981.4. P. J. Wegner, B. M. Van Wonterghem, C. E. Barker, J. A. Caird, S. N. Dixit, M. A. Henesian, J. T. Hunt, L. G.Seppala, and C. E. Thompson, "Third-Harmonic Performance of the Beamlet Prototype Laser," presented at the SecondAnnual International Conference on Solid-State Lasers for Application to Inertial Confinement Fusion, (paper 10-1), Paris,France, October 22 - 25, 1996.Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

National Ignition Facility frequency converter development

https://digital.library.unt.edu/ark:/67531/metadc680367/m2/1/high_res_d/462928.pdf

National Ignition Facility frequency converter development

Abstract

Similar works

Full text

Available Versions

UNT Digital Library