Beam control and diagnostic systems are required to align the National Ignition Facility (NIF) laser prior to a shot as well as to provide diagnostics on 192 beam lines at shot time. A design that allows each beam`s large spatial filter lenses to also serve as objective lenses for beam control and diagnostic sensor packages helps to accomplish the task at a reasonable cost. However, this approach also causes a high concentration of small optics near the pinhole plane of the transport spatial filter (TSF) at the output of each beam. This paper describes the optomechanical design in and near the central vacuum vessel of the TSF

Ables, E.

Bliss, E.S.

Holdener, F.R.

UNT Digital Library

UCRteJC-124S57PREPRINTBeam Control and Diagnostic Functionsin the NIF Transport Spatial FilterF. R. Holdener, E. Ables, E. S. Bliss,S. J. Boege, R D. Boyd, C. J. Chocol,D. T. Davis, R. D. Demaret, R E. English,C W. Laumarm, J. L. Miller, and S. W. ThomasThis paper was prepared for submittal to the2nd Annual International Conference onSolid-State Lasers for Application to ICFParis, FranceOctober 22-25,1996October 22,1996DISCLAIMERThis 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.*..UCRL-JC-124857Beam control and diagnostic functions in the NIF transport spatial filterFAR. Holdener, Elden Ables, Erlan S. Bliss, Steve J. Binge, Robeti D. Boyd,Clifford J. Chocol, Donald T. Davis, Robert D. Demaret, R. Ed English,Curt W. Laumann, John L. Miller and Stanley W. ThomasLawrence Livermore National Laboratory, Livermore, CA 94550ABSTRACTBeam control and diagnostic systems are required to align the National Ignition Facility (NIF) laser prior to a shot aswell as to provide diagnostics on 192 beam lines at shot time. A design that allows each beam’s large spatial filter lenses toalso serve as objective lenses fw beam control and diagnostic sensor packages helps to accomplish the task at a reasonablecost. However, this approach also causes a high concentration of small optics near the pinhole plane of the transport spatialfilter (TSF) at the output of each beam. This paper describes the optomechanieal design in and near the central vacuumvessel of the TSF.(Keywords: control, diagnostic, laser, pinhole, spatial filter, retitles, wavefron~ eentroids, target)2. INTRODUCTIONThe NIF will be capable of delivering 1.8MJ of energy to target with a peak power of 500TW at a wavelength d0.3 51pm (30). Diagnostic instruments must measure beam energy, power versus time, wavetint quality, and transverseintensity profile while alignment systems must assure that the beam passes through the system correctly and is deliveredaccurately to the target Many of these fimetions are implemented in or near the TSF, which must also accommdtecomponents associated with propagation of the main beam through the system. Figure 1 shows the NIF laser facility andthe Ioeation of the TSF.Fig. 1. NIF laser facility showing looation of TSFFigure 2 is an isometric simplified view of the beam path. A 45 mm square beam fomned in the prc-amplifier module(PAM) pmpagstes via the preamplifier beam transport system (PABTS) to the TSF injection mimer. The injected beammust match the f/80.7 of the TSF as well as project an image of the input apodiir (relay plane) 16.8 m past SF3.Congestion in the vicinity of the TSF pinholes strongly influences the design of the injection optics. A compact telephotodesign for the injection telescope, using spherical fused-silica elemenb, provides an effecdvc fd length of 3.6 m, with aback fecal length of 1.2 m. Wavefront performance is dit%action limited over a field of view of * 1 mad.After injection threugb pinhole #1, the beam passes through the boester amplifier, is reflected down by LM3, andretleztd off of the pekwizcr. It passes through the cavity spatial filter (CSF) pinhole #1 and through the main 1l-slabilaahlamp pumped amplifier. Atlcr mflecdon off the deformable mirrer (LMl) it passes back through the main amplifier,through pinhole #2, and is switched to vertical pehization as it passes through the activated Pockel.s cell. The beam cannow pass through the polarizer and is reflected tlum mirror LM2. As it returns through the pelarizer and the Peckels cell,which is still active, the beam’s polarization is rotated again. It passes thmugb pinhole #3, makes a third paas through themain amplifier, reflect.va secmrd time off of LM 1, and makes a final pass through the main amplifier and through pinhole #tin the CSF. It then proceeds through a now deactivated Peckels cell with horizontal polarization and is deflected out of thecavity by the pelarizer. Finally it reflects off of LM3 and makes a second paas through the booster amplifier. The pulse exitsthe laser at full enemv through TSF ~inhole #4 and the spatial filter output lens (SF4). pmp~ation to the @et chamber is“aa series of tramp% mirro% (LM4-8).11 slab mainamplifier%Cavity spatial filter//Ml *7” ;“3 3-5 slab booster45mm x45mm windowln@ed baam— I ;::(rrf..e MZN ------ .“aamp!-adbeam fmm fmrrtsurfaea of focusinglens at targetchamberFig.2. Isometric of beam patha through NfF laser (note fidl size beams of 372 MM x 372 nun mm square megreatly reduced to clcady show beam pati beams actually start messing -2 m frem the TSF pinhole plane andtily ovedap in the main and beeatar amplifiers)....3. BEAM ALIGNMENT AND DIAGNOSTIC DESCRIPTIONTo measure the properties of the beam, beam samples are taken tlom low reflectivity surt%es already used in the system.Figure 3 illustrates how these optical samples are handled in the central region of the TSF. The 1ce output beam issampled at the exit surface of the TSF output lens (SF4). ‘l%issurface is flat when the TSF is at vacuum, so the reflectionfbcuses at the pinhole plane. The SF4 lens has a sol-gel AR coating with a nominal reflectivity of 0.l% and is tilted toseparate the diagnostic reflection from the output beam on pass ##l in the TSF as illustrated on the right side of Figure 3. Amirror located in front of the focus picks off the beam sample. This beam sampling fiction determines the shape of SF4 butdoesn’t affkct its clear apertnm.TSF tower #1 - Alignment towerEncoded pinholbeamv-lsample -wheel withinsertable baffles(not shown)Beamdumppass #2& #3aa H \Insertable 10 A ~light source ● ‘>/“ Illuminator‘Dp’~ “ignm~t ‘mic’e 3co target chamber /(Schematic tODView)alignment light sourceTSF Tower #2 - Diagnostic tower -3e) KDPInput tosampledTo target ~beam focus lens reflectiondump(Schematictop view) . = steppingmotorFig. 3. Multiple fimctions are implemented in the region of the TSF spatial filter pinholes‘fhe first surface of the target chamber fdcus lens is also flat end coated with sol-gel. It provides a 3m beam sample thatreturns to the TSF by reflecting off of the tmmmort mimxs. l%e maximum transmission throwh this set of o~tics is about10VO(the product of the 3ol re%ctivities). 71; target chamber lens is also tilted to ofiket the-mfkcted diagnostic beam inthe TSF. This decenters the 3eI beam at SF4, and transporting the full 30 diagnostic beam requires increasing the clearaperture of SF4 and some of the transpcxt mirrors. The 3(i) fm of SF4 is about 1.7 m in front of the 10) focus. Aremotely insertable pickoff mirror intercepts this 361 sample and sends it to the output sensor during a shot. However,during alignmen~ the lco reflection from the target chamber lens provides a way to set the angle of the KDP frequencyconversion crystals given that they are mounted referenced to one another in the final optics assembly. During KDP angleadjuslmen~ the 3@pickoff mirror moves out of the beam.me diagnostic samples described above and an alignment sample from the alignment tower am transported from the TSFtowers to the output sensor below the TSF as shown in Figure 4. ‘I%epath lengths to the sensors axe 8 tolO meters. Eachoutput sensor is shared between two beams using beamsplitters and shutters in the transport paths. One must choose whichof the two beams to record for some diagnostics measurements on a shoq but alignment and wavefmnt functions meperformed on both beams by temporal multiplexing during alignment.Output sensorsFig.4. Isometric sketch of central vessel of the transport spatial filter with locations shown foraligmnent snd diagnostic towers consisting of four levels each. On each level two bans ampicked off in the towers and relayed down to the output sensors.‘Ibe alignment path tim the spatial filter to the output sensor delivers light for multiple functions. F@re. 5 illushateshow a moving pickoff in the spatisl falter determines which of five slignment functions am active (l&2) nti-field viewingof beams frompasses 1 and 4 to image centering sources in the laser cavity and at the tinel optics; (3) fm.field viewing of thepass #4 beam to monitor its output Pointins (4) fzwtield viewing of Iiiht scatted from- a re.tici.sat Dass #I to-set theinjection pointin~ and finally (5) fro-field viewing of the 1co = mflw-ted from the final optics sssem-bly bsck to a TSFreticle to set the KDP angle. The required field of view for the faw%ld functions is + 250 pxad, set by the angnkw extent ofthe spot reflected from the final optics and the KDP tuning rsnge. In the near-field, the &smsport optics relay a 20-mm-bigbimage of LM1 to the output sensor.(1) OS NFview (2) OS NF view-Ln*I‘n’+(3) OS FF viewfilm;‘lightsource1i-‘? k 14I Fig. 5. The three positiom and five functions of the ahgument splitter cube on tower #1The 1co diagnostic path to the OS must have high @ansmission for wavefiont sensor calibration, but for shots most ofthe energy must bc dumped. l%erefore the pick-off mirror is a maximum reflector and a low-transmission, partial rcfhztor isinserted as a shot attenuator. The reflection is used for the 1ro energy diagnostic, which is an integrating sphere with a time-intc.grated photodiode. This is lncated before the merge as shown achematicslly in figure 6, so 1m energica are mcamn-cdforall hcams. There are two afocal relays in each path to the output sensor. They provide a 20 mm wide mar-field image at theoutput sensor. The design input angular range is +400 wnd.beaml beam2+ i10 sampled beamIntegratingsphereswith photcdiode + F! \o - m - ● Motorized Stage-A/OutpuisensorFig. 6. Schematic of insertable Inr energy diagnostic in relay optics from TSF to OSFor the 3co diagnostic path high transmission is not needed. So the pick-off is an uncoated bed silica surface formaximum damage threshold. Most of the energy is dumped into m absorber hchind the pick off. Two sfocal relays put animage Of the final optics at the OS. The design angulsr range is also +40Q wad,Six output diagnostic functions arc carried out at the TSF. As described,(1) the lnr energy is mea.xucdin the trmaportpath, the other functions are in the OS: (2&3) Beam samples for 10Iand 3rJItcmpcmd puke shape meaaurementa are coupledinto fiber bundles for tmnaport to the power sensor packaged. (4&5) 1or and 3to nezwfield images we obtained using CCDcrunersa. (6) The 1w wavefront (phsae distribution) is measured using a Hartmannsensor comprising a lens array and CCDCalncra4. DESCRIPTION OF HARDWARE NEAR THE CENTEROF THE. TRANSPORT SPATIAL FILTERh the central region of the tmmspnrt spatial filter there area pinhole holder/changer, 3 beam pickoffs, 2 reference retitles, 2insertable fiber light sources, an incohmnt light source, the main beam injection mirmr/optics, and a beam dump and baffles.Uae of the fiber light sarrces and raiclea fw law and target al@nent functions is deacrii in a companion ~. Thebeam dumps am for psasea 2 and 3 to intercept Pockels cell Icakage or hack reflections fmm the target. Thcae components meillustrated achenmtically in figure 3 with tower locations shnwn in figure 4. The pinhole holder/changer shown in I@ne 7allows up to 10 replaceable shot pinholes, 3 alignment retitles, and an 80mm cle+waperture for optics inspection.‘ Thomas et. al., Temporal multiplexing for economicalmesawement of power versus tie cmNfF, paper P27, thiiCOnfemme.zBocge et. aI., NfF pointing end centering systems snd target aligmncnt usinga351 mn Iascr source, paper 6-5, thiicQntkmce.Pinhole assemblyx..——Reticle illuminatorwith baffles KBeam dumps k&AlignmentreticleNote: TowerhWbstructureNOT SHOWN.3 viewing~u. oDticsFig. 7. Ismnetric drawing of the tep sssembly of the TSF alignment tower #l contatimg pinholeibaffleassembly, reticle viewing optics with translator, beam dumps for passes 2 snd 3 snd an iIlmninator forreticle viewing.Fieure 8 is a more detailed view of the Dinholdbatlle sssembly. On the pinhole wheel are mounted alignment mticlcs–=. .(pinhole #1 reticle and a focus-lens reflection-reticle), so opening fo; optics in+tion (gO mm diameter) as w;I1 ss Iecationsfor 10 sets of shot pinholes. A stepping motor rmd metsl belt drives the wheel, end a direckmupled rotary encoder monitorsits pesition. To achieve the required eccmacy of 5 ym repeatability, the wheel’s tie rests on two preeiQon kinematic V-blocks. A sepamtc katlle ss.sembly, also stepper meter driven, swings to three differmt locations for the four reqdredoperating modes as shown in l@re 8. For mede (l), the pass #1 reference reticle is Iecated on sxis f& pointing tieinjcctcd beam. A 1re light source on the W assembly is positioned bcbiid pinhole lkt t?xothexaligmnent functions. Inmede (2), the alignment retitles remainin place for alignment, but the haflle sssernbly moves the light source out ef the wayss psrt of the alignment sequerrcc as described by Bocge2. In mode (3), the large gOmm holes in both the pinhole wheel andthe baffle sswrnbly = eligned with the nomml Psss #4 beam path. TMs aUows a clear line-cf-sight through the pinholeassembly for scattcrcd light that must be transmitted during laser optics inspection. In mode (4) the bdlle and one of the tcnavsihdde sets of shot pinholes arc in plsce for a laser shot.Figure 9 shows the complexity of the TSF diagnostic tower showing injection optics, 1ro eod 3re pickof%, end the 3rotarget cbsmber fiber light sew-e. All eight beamlines src monitered on thii tower where the ssmple hems arc picked elTsndmflded to tbe central pertion of the tower whwc they we relayed tn the output sensor ss illushated. The 3re tergets@ment fiber light source is mounted on a motnrized x-y stage fnr insertion end withdmwal fmm the pass #4 beam psth.lle 30J light is launched 1.7 m frem the focal plsne of the pinhole so that it is on fccu.s at target chsnrber center. It is usedas psrt of the alignment sequeru fer aligning beams on target with the target alignment sensor described in a compsrdonpaper’.2Beege et. al., NIF peinting snd centering systems snd target aligmncnt using a 351 mu laser source, psper 6-5, thisconfemm‘lg.,8. Close-up ~f,pinhole wheel with fdiient retitles, 80 mm insps~ion open pass, and 10 sets of shot pirdIIJIeSm“ple,with a three postron baftle assembly: (1) 10Ifiber source insertable behind purhole M (711mrepeatable p+x.itionsccuracy]2) open pesition for observingretitles without 101light sotuw (3) pn WsitiOnfOrOptiCSirrswtiOn (4) b~e iIIPl~e fO]II /Injeotion\ ‘:’::::’”ODtiCS ,=R optlcslomponentsJI30 target zchamher fiberlight sourceh-YFOA focue lens3m refleoted beam 1--, -’ig. 9. nF diWOStiC tOwer shOw~g ~jectiOn OPtiCS~d 10I ~d 3UIpickOffs ~d 30 -et ~-ent fiber fight sO~e5. SUMMARY‘Hds paper has describd a mst effective design for performing alignment and diagnostic functions witbii the TSF of theNational Ignition Fecility. Two sampliig smt&cs akady psrt of the main leser, i.e. the exit surfsce of the TSF relay lensand the frnnt surfece of the finsl focus lens at the tsrget chamber prnvide the lro nnd 301 sample beems. These sample hamsme collected in the centml portion of the TSF where small optics em sutlicient for implementing alkgnment and d@mosticfimctions. Thk concentration of components puts significant constraints nn both the mechanical nnd optical desigm, but wehave found the suitable preliiinsry design solutions.5. ACKNOWLEDGMENTSThe authors wish to especially thank the design team of Jorge Victoria, Jee Lown, Psul Gschweng and Msrek Kuta htheir efforts in the design of these components.llk work was performed under the auspices of the U.S. Deparbnent of Energy by Lawrence Lkermore Natinnal Laboratory underContract No. W-7405-Eng-48.Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

Beam control and diagnostic functions in the NIF transport spatial filter

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Beam control and diagnostic functions in the NIF transport spatial filter

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