All experiments at the National Ignition Facility (NIF) will produce debris and shrapnel from vaporized, melted, or fragmented target/diagnostics components. For some experiments mitigation is needed to reduce the impact of debris and shrapnel on optics and diagnostics. The final optics, e.g., wedge focus lens, are protected by two layers of debris shields. There are 192 relatively thin (1-3 mm) disposable debris shields (DDS's) located in front of an equal number of thicker (10 mm) main debris shields (MDS's). The rate of deposition of debris on DDS's affects their replacement rate and hence has an impact on operations. Shrapnel (molten and solid) can have an impact on both types of debris shields. There is a benefit to better understanding these impacts and appropriate mitigation. Our experiments on the Omega laser showed that shrapnel from Ta pinhole foils could be redirected by tilting the foils. Other mitigation steps include changing location or material of the component identified as the shrapnel source. Decisions on the best method to reduce the impact of debris and shrapnel are based on results from a number of advanced simulation codes. These codes are validated by a series of dedicated experiments. One of the 3D codes, NIF's ALE-AMR, is being developed with the primary focus being a predictive capability for debris/shrapnel generation. Target experiments are planned next year on NIF using 96 beams. Evaluations of debris and shrapnel for hohlraum and capsule campaigns are presented

Eder, D.

Fisher, A.

Jones, O.

Koniges, A.

Landen, O.

Masters, N.

Suratwala, T.

Suter, L.

UNT Digital Library

UCRL-CONF-234490Debris and Shrapnel MitigationProcedure for NIF ExperimentsD. Eder, A. Koniges, O. Landen, N. Masters, A. Fisher,O. Jones, T. Suratwala, L. SuterSeptember 10, 2007IFSA ConferenceKobe, JapanSeptember 9, 2007 through September 14, 2007Disclaimer   This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.  Debris and Shrapnel Mitigation Procedure for NIFExperimentsD Eder, A Koniges, O Landen, N Masters, A Fisher, O Jones, T Suratwala, and LSuterLawrence Livermore National Laboratory, Livermore, CA, U.S.A.E-mail: deder@llnl.govAbstract. All experiments at the National Ignition Facility (NIF) will produce debris andshrapnel from vaporized, melted, or fragmented target/diagnostics components. For someexperiments mitigation is needed to reduce the impact of debris and shrapnel on optics anddiagnostics. The final optics, e.g., wedge focus lens, are protected by two layers of debrisshields. There are 192 relatively thin (1-3 mm) disposable debris shields (DDS’s) located infront of an equal number of thicker (10 mm) main debris shields (MDS’s). The rate ofdeposition of debris on DDS’s affects their replacement rate and hence has an impact onoperations. Shrapnel (molten and solid) can have an impact on both types of debris shields.There is a benefit to better understanding these impacts and appropriate mitigation. Ourexperiments on the Omega laser showed that shrapnel from Ta pinhole foils could beredirected by tilting the foils. Other mitigation steps include changing location or material ofthe component identified as the shrapnel source. Decisions on the best method to reduce theimpact of debris and shrapnel are based on results from a number of advanced simulationcodes. These codes are validated by a series of dedicated experiments. One of the 3D codes,NIF’s ALE-AMR, is being developed with the primary focus being a predictive capability fordebris/shrapnel generation. Target experiments are planned next year on NIF using 96 beams.Evaluations of debris and shrapnel for hohlraum and capsule campaigns are presented.1. IntroductionThe National Ignition Facility (NIF) in the United States and the Laser MegaJoule (LMJ) in Francewill operate with a large number of optics and many diagnostics exposed to debris and shrapnel fromtargets and other chamber components. For cost effective operation, it is critical to establish a debrisand shrapnel mitigation procedure. There are two main aspects of this mitigation procedure. First is todesign the optical assemblies and diagnostics to withstand the impacts of debris and shrapnel in a costeffective manner and second is to modify targets and associated components, as necessary, to reducedebris and shrapnel loading. The plan for both large laser facilities is to use a layered approach toprotect the more expensive optics, e.g., wedge focus lens on NIF. The first layer, closest to targetchamber center (TCC), are relatively thin (1-3 mm) disposable debris shields (DDS) with the secondlayer being thicker (10 mm) main debris shields (MDS). The lifetime of the DDS’s is expected to bebetween 1 and 20 shots, depending on target and laser parameters. The lifetime of the MDS’s isexpected to be between 100 and 200 shots. Diagnostics will use a range of filters and collimators toprotect cameras and other vulnerable components to ensure that data is obtained. The focus of thispaper is on the second aspect of the mitigation procedure: evaluation of the sources of the debris andshrapnel and the associated impacts on optics and diagnostics.Our understanding of the generation of debris and shrapnel and the impact on optics and diagnostics isbased on past experiments, e.g., Omega and 4-beam NIF shots, dedicated fragmentation experimentson HELEN, JANUS, LIL, etc. laser facilities, and on detailed numerical simulations. For a review ofexperiments and corresponding simulations see ref. [1]. For information on aspects of a newsimulation code, NIF’s ALE-AMR, used in the evaluations discussed in this paper see refs. [2-3].There are three major differences between simulations of debris/shrapnel generation and simulationsof ICF capsules/targets: 1) longer time-scale (µs vs. ns), 2) larger spatial scale (µm-m vs. µm-cm), and3) different physics (ablation and fragmentation vs. LPI, mix, and burn) drives the simulation. Earlywork focused on the first difference, where it was shown that the majority of the vaporized hohlraummass is directed towards the waist/equator of the chamber and not towards the optics.[4] To tackle thesecond difference and the desire to model a “complete target,” hohlraum, flanges, shields, coolingrings, and support structure, we use a new code that combines standard ALE hydrodynamics withadaptive mesh refinement (AMR). The initial mesh can be concentrated in structures (hohlraum wall,shields, etc.) with coarser mesh in the vacuum between components. During the simulation the meshcan refine to better resolve shocks or model generation of voids and subsequent fragmentation. TheAMR capability also allows different material models to be used at different levels of refinement. Thecode retains the benefits of ALE codes that allow the mesh to move with the rapidly expandingmaterial with remeshing as needed.In section 2, we review the constraints placed on debris and shrapnel generation. In section 3, we showsome results of our evaluations for the NIF 96-beam energetics campaign, planned for the summer of2008, and a 96-beam capsule campaign using a keyhole target. We conclude in section 4.2. Constraints on debris and shrapnel generationThe constraints on debris are primarily associated with loss of transmission through the DDS’s andcorresponding reduction in their lifetime. The constraints on shrapnel are primarily associated withDDS penetration/breakage, MDS damage/penetration, and diagnostic filter penetration. The DDS’swill be held in cassettes with up to 10 DDS’s to reduce the replacement time. In contrast, personnelmust vent and open a final optics assembly to replace a MDS leading to a more rigid constraint onshrapnel generation associated with MDS lifetimes. The shrapnel constraint associated with DDSbreakage places a limit on the mass and velocity of molten material that can strike DDS’s. Finally,shrapnel constraints associated with diagnostic filter penetrations depend on the configurations of thediagnostics on a given shot.There are three main sources of debris in NIF experiments: 1) vaporization of targets componentsclose to TCC, 2) ablation of material by unconverted laser light, 3) ablation of material from target x-rays. (One of the major differences in the design of NIF and LMJ is that unconverted light does notenter the LMJ chamber so LMJ experiments do not have the second source of debris.) For NIF, thefirst and second sources are often comparable. The third source is only important for very high energyor yield shots where the x-ray fluence on the first-wall if of order 1 J/cm2 or greater. Ablation from thefirst-wall, by unconverted light and x rays, is reduced by approximately a factor of 10 by the use ofstainless-steel louvers. The allowed mass depends on the level of DDS transmission loss that isallotted to debris.On early NIF shots we observed a type of DDS transmission loss that is not associated with debris.This loss is associated with reactions in the DDS borofloat glass when exposed to a combination of 1ωand 3ω laser light. After approximately 25 shots with 3ω energies of order 2 kJ/beam or less, weobserved a drop in transmission from this effect to be of order 2%. We have a goal of ~10 shots priorto DDS replacement with a transmission loss of order 3%. We assign ~1% of this to reactions in theDDS from 1ω + 3ω and 2% to the effects of debris.  To relate this transmission loss to mass of debris,we use Nova measurements that showed a debris mass density of 1 µg/cm2 gave a 6% drop intransmission out of the beam. Inside the beam, a transmission loss of 2% was measured for the samedebris loading. If we assume similar beam cleaning for NIF, the allowed mass density is 1 µg/cm2.Given that the DDS’s are ~7 m from TCC, this corresponds to a total mass of 6 g. If beam-dumpablation contributes ~1/3 of the total mass, the allowed vaporized target mass per shot is 400 mgassuming 10 shots per DDS.One shrapnel constraint is to avoid penetration of DDS’s. This assures that shrapnel does not impactMDS’s. We have developed simple models of DDS penetration based on the formation of Hertziancracks going through the thickness of the DDS. In Fig. 1 we show the calculated velocity that a steelshrapnel fragment would need to cause a penetration. We give results for 1 and 3 mm thick DDS’s as afunction of the fragment diameter. For our default thickness of 1 mm, a shrapnel fragment with adiameter of 200 µm would have to impact with a velocity of slightly more than 2000 m/s.  Anothershrapnel constraint, which has a greater impact than the penetration of a DDS, is to avoid breakage ofa DDS by a molten shrapnel spray. We observed that molten shrapnel can break DDS glass during anOmega shot to test redirection of shrapnel by tilting Ta foils.[5] If a spray having the same kineticenergy as a 200 um diameter fragment moving at 2000 m/s impacts a DDS, our simple model predictsbreakage. Curving sources of molten spray can cause divergence of the spray and reduce the massstriking a particular DDS.  We don’t have a constraint on small solid fragments that produce a largenumber of small damage sites on DDS’s. While these sites can grow in size, for the relatively fewnumber of shots (of order 10) that DDS’s are in place we don’t expect the growth to be significant andthe scatter from these sites should be significantly less than 1%.3. Evaluations for the NIF 96-beam campaignsThe 96-beam energetics campaign will use scale 0.7 cryogenic hohlraums with the radiationtemperature measured by the Dante diagnostic. Other primary diagnostics measure properties of thebackscatter light. None of these diagnostics place tighter constraints than those associated with theoptics. The vaporized target mass is expected to be 340 mg, which is below the 400 mg limit discussedabove. The expected mass ablated by unconverted light and x rays that enters the chamber is less thanthe 200 mg limit. It is necessary to confirm that the hohlraum is vaporized for the lowest energy beingconsidered for this campaign. This energy of 33 kJ is associated with truncating the pulse after 6 nscorresponding to the 3rd shock. The current cryogenic hohlraum design has an outer layer of Al that is150 µm thick. We did a 1D simulation, with a model for leakage out of the laser entrance holes, tocalculate the temperature of the coolest Al zone as a function of time. The coolest zone is above thevaporization temperature (~2800oK) for over 500 ns allowing the hohlraum to be vaporized. The Sicooling rings are not expected to be completely vaporized for any of the shots and must be evaluatedfor shrapnel generation. The rings are forced to rapidly expand because of the expanding vaporizedhohlraum pushing the rings outward. We estimate that the rings expand at ~1/10 the velocity of anexpanding hohlraum or ~1000 m/s. We model the expansion of Si rings using a brittle failure modeland also model Al rings using a ductile failure model. We calculate that Si breaks into smallerfragments than Al as expected. CEA is considering Al rings for LMJ targets and Al is interesting as abounding case for Si, which can become more ductile when heated by neutrons. (During these 96-beam experiments, neutrons will not be generated.) In Fig. 2 we show results of a simulation usingNIF’s ALE-AMR code for a scale 0.7 Al cooling ring at a time of 2 µs. The color bar denotes howclose a zone is to failure with red denoting complete failure. The initial ring is 500-µm thick and 2000-µm wide in the radial direction. At 2 µs, we see that the inner portion of the ring is shooting out thesides in very small fragments/droplets and the ring is in the process of failing. This process continuesand we don’t calculate any large (>100 µm) fragments that would penetrate a 1-mm thick DDS basedon the curve given in Fig. 1.Fig.1 Velocity to penetrate 1 and 3 mmDDS’s as a function of shrapnel diameter.Fig. 2 Simulation of the fragmentation of a rapidlyexpanding Al cooling ring. Image is at a time of 2 µs.The 96-beam shock timing campaign will use keyhole targets that contain a cone filled with liquid D2,which penetrates the capsule and extends outside the hohlraum. The portion of the cone inside thehohlraum is Au and the portion outside is Al. We determined that the VISAR diagnostic is adequatelyprotected by its blast window. We are evaluating how many spare windows will be required based onthe number and sizes of damage sites caused by shrapnel impacts. Another component for this target isa very large outer cone to block unconverted laser light from striking the quartz window holding in theD2.  The current design for this cone is ~50 µm-thick CH with a thin Al coating. Initial evaluationshows that this cone meets debris and shrapnel constraints. The relatively large amount of CH requiresan evaluation of the impact of associated non-volatile residue (NVR) on the sol-gel antireflectioncoating on the DDS’s. This was determined not to be a problem given the relatively short time DDS’sare exposed to the chamber environment.4. ConclusionWe have a debris and shrapnel mitigation procedure in place and have applied it to 96-beamcampaigns. The goal of the procedure is to manage generation of debris and shrapnel so as to reducethe probability of data loss and allow optimization of optics use. We have defined a set of constraintsfor debris and shrapnel and have shown that the 96-beam campaigns meet these constraints. Ourevaluations use a range of simulation tools including a new code, NIF’s ALE-AMR, that has uniquecapabilities relevant to debris and shrapnel modeling. We will apply the mitigation procedure to thewide range of planned experiments on NIF to achieve ignition and study high-energy density physics.5. References[1] Koniges A, et al., this conference[2] Masters N, et al., this conference.[3] Fisher F, et al., this conference.[4] Eder D, Koniges A, Jones O, Marinak M, Tobin M and MacGowan B 2004 Nuclear Fusion 44,709[5] Blue B E, Hansen J F, Tobin M T, Eder D C and Robey H E 2004 Rev. of Sci. Inst. 75, 4775Work by LLNL was performed under the auspices of the U.S. Department of Energy by the Universityof California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.

Debris and Shrapnel Mitigation Procedure for NIF Experiments

https://digital.library.unt.edu/ark:/67531/metadc897817/m2/1/high_res_d/922113.pdf

Debris and Shrapnel Mitigation Procedure for NIF Experiments

Abstract

Similar works

Full text

Available Versions

UNT Digital Library