Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session NO4: Compression and Burn II |
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Chair: Mordecai Rosen, Lawrence Livermore National Laboratory Room: Salon E |
Wednesday, October 29, 2014 9:30AM - 9:42AM |
NO4.00001: Plans and status of the Beryllium ablator campaign on NIF J.L. Kline, S.A. Yi, A.N. Simakov, D.C. Wilson, R.E. Olson, N.S. Krasheninnikova, G.A. Kyrala, T.S. Perry, S.H. Batha, E.L. Dewald, M.J. Edwards, A.J. MacKinnon, N.B. Meezan Beryllium has long been known to have excellent properties for indirectly driven ICF implosions including enhanced ablation pressure, implosion velocity, and mass ablation rate. The high ablation velocity leads to stabilization of ablative hydrodynamic instabilities and higher ablation pressures. Recent ``high foot'' experiments have shown ablative Rayleigh-Taylor to be a leading cause of degraded performance for ICF implosions. While Beryllium ablators have these advantages, there are also risks associated with Beryllium target designs. A campaign is underway to design and to test these advantages for comparison with other ablator options and determine which provides the best path forward for ICF. Experiments using Beryllium ablators are expected to start in the late summer of 2014. This presentation will discuss the status of the experiments and layout the plans/goals for the campaign. [Preview Abstract] |
Wednesday, October 29, 2014 9:42AM - 9:54AM |
NO4.00002: The First Indirect Drive, High-Foot Beryllium Campaign on the National Ignition Facility A.N. Simakov, D.C. Wilson, S.A. Yi, J.L. Kline, R.E. Olson, N.S. Krasheninnikova, G.A. Kyrala, T.S. Perry, S.H. Batha, D.S. Clark, B.A. Hammel, J.L. Milovich, J.D. Salmonson For indirect drive ICF, beryllium (Be) ablators offer a number of important advantages over carbon-based ablators, which can be used to significantly improve the target ignition margin. Recently we designed a number of modern NIF Be high-foot targets optimized for hydrodynamic stability. They employ the standard 5.75 mm gold hohlraum and allow for a range of adiabats, laser drive powers/energies, and fuel ice thicknesses. Here, we will outline the first NIF Be experimental campaign that began in August of 2014. It is based upon a low-yield (high 10$^{14}$ neutrons) but very hydrodynamically robust high-foot target driven by a 350 TW/1.4 MJ pulse and using a 130 $\mu $m DT ice layer. The goal is to obtain a near-1D implosion while quantifying Be target performance uncertainties, cross-comparing with other ablators to elucidate main limitations of our predictive capabilities, and testing superior Be ablator properties near high-foot plastic performance cliffs. [Preview Abstract] |
Wednesday, October 29, 2014 9:54AM - 10:06AM |
NO4.00003: Performance scalings for indirect drive high-foot NIF beryllium targets S.A. Yi, A.N. Simakov, D.C. Wilson, J.L. Kline, R.E. Olson, N.S. Krasheninnikova, G.A. Kyrala, T.S. Perry, S.H. Batha, D.S. Clark, B.A. Hammel, J.L. Milovich, J.D. Salmonson Beryllium (Be) ablators offer an attractive path to ignition on the National Ignition Facility (NIF). We have designed a 1.4 MJ, 350 TW cryogenic target for the first NIF Be experiments, utilizing a 3-shock high-foot pulse shape. This initial target is designed to perform close to 1D predictions at the expense of absolute yield ($\sim 10^{15}$ neutrons). Two target parameters that can be used to scale to higher yields are the DT fuel layer thickness and the power in the initial portion of the laser pulse (i.e., the laser ``foot''). Designs with thicker fuel layers and higher feet are more hydrodynamically stable, but at the expense of implosion velocity and compression. Targets with thinner fuel layers and lower foot drives achieve higher velocity, but are more susceptible to instabilities. Thus, different trade-offs are possible between 1D yield and 2D hydrodynamic stability. We present a range of NIF Be targets and quantify these trade-offs as we scale to higher performance designs. [Preview Abstract] |
Wednesday, October 29, 2014 10:06AM - 10:18AM |
NO4.00004: Expectations and results from the first NIF beryllium shock timing experiment D.C. Wilson, S.A. Yi, A.N. Simakov, H.F. Robey, D.E. Hinkel, J.E. Ralph, D.J. Strozzi, J.L. Milovich, J.L. Kline, R.E. Olson, N.S. Krasheninnikova, L. Berzak Hopkins, T.S. Perry, G.A. Kyrala, S.H. Batha The first NIF beryllium experiments are based on the highly successful hi-foot implosions fielded using plastic capsules. A VISAR diagnosed shock velocities and timing in a liquid DD filled Be capsule in both polar and equatorial directions. The laser pulse contains a 28 TW picket, a low power trough, a 2$^{nd}$ 45 TW pulse, and a 3$^{rd}$ high power pulse. To avoid laser damage from SBS backscatter, the 3$^{rd}$ pulse has 250TW inner beams and 350TW outer beams, The total laser energy is only 0.58 MJ. First shock breakout times at the pole and equator determine inner cone vs outer cone power fractions in the picket. Comparing measured and calculated shock velocities gives picket and 2$^{nd}$ pulse drive multipliers. The picket laser power will adjust the first shock to 28 $\mu $m/ns. Cone fraction changes to the 2$^{nd}$ pulse make the 1$^{st}$ and 2$^{nd}$ shocks merge simultaneously at pole and equator. The timing the 2$^{nd}$ pulse adjusts merger depth. The second Be experiment, a symmetry capsule with more energy, will incorporate these changes. [Preview Abstract] |
Wednesday, October 29, 2014 10:18AM - 10:30AM |
NO4.00005: High-density carbon (HDC) capsule designs for $\alpha $-heating and for ignition D. Ho, A. Amendt, D. Clark, S. Haan, J. Milovich, J. Salmonson, G. Zimmerman, L. Berzak Hopkins, J. Biener, N. Meezan, C. Thomas, L. Benedict, S. Le Pape, A. Mackinnon, S. Ross We show capsule designs that have HDC ablators, using 2, 3 and 4 shocks. Their advantages and disadvantages will be discussed. Two-shock designs have the shortest pulse length but have the worst 1-D ignition margin because of the high fuel adiabat. Four-shock designs have the highest 1-D ignition margin with the lowest adiabat, but have higher RT ablation front growth. This disadvantage can be overcome by using a picket to generate the 1$^{\mathrm{st}}$ shock. The picket reduces the RT growth factor while the decaying 1$^{\mathrm{st}}$ shock lowers the fuel adiabat further. The picket has the additional advantage of shortening the pulse length. Dopant requirements for different hohlraums will be discussed. A 3-shock design for achieving alpha heating is described, which can use either high-gas-fill (1.6 mg/cc) or near-vacuum hohlraums. A rugby-shaped hohlraum with low gas-fill (0.5 mg/cc) has high laser coupling efficiency and provides good symmetry for a 4-shock design. Comparison of simulations for selected recent HDC shots with experimental data will be presented. [Preview Abstract] |
Wednesday, October 29, 2014 10:30AM - 10:42AM |
NO4.00006: Proposed NIF Experiments to Explore Convergence Ratio and Robustness of Hot Spot Formation in DT Liquid Layer HDC Capsules R. Olson, R. Leeper, G. Grim, J. Kline, R. Peterson, L. Berzak Hopkins, A. Hamza, D. Ho, O. Jones, S. LePape, A. MacKinnon, N. Meezan, H. Robey DT Liquid Layer ICF capsules allow for flexibility in hot spot convergence ratio via the adjustment of the initial cryogenic capsule temperature and, hence, DT vapor density.\footnote{R.E. Olson and R.J. Leeper, Phys. Plasmas \underline {20}, 092705 (2013).} High Density Carbon (HDC) is a leading candidate as an ablator material for ICF capsules,\footnote{A. J. MacKinnon et al., Phys. Plasmas \underline {21}, 056318 (2014).} and a technique has been developed for lining the inner surface of a HDC shell with an ultra-low-density hydrocarbon foam that will survive wetting with liquid hydrogen.\footnote{J. Biener et al., Nucl. Fusion \underline {52}, 062001 (2012).} In this presentation, we propose a series of NIF experiments using liquid DT layer (wetted foam) HDC capsules to test the hypothesis that our predictive capability of hot spot formation is robust for a relatively low convergence ratio hot spot, but will become more difficult as vapor pressure is reduced and hot spot convergence ratio is increased. The proposed liquid DT layer HDC capsule ``sub-scale'' experiments utilize near-vacuum hohlraums with NIF laser pulse energies of about 1 MJ, but larger scale experiments are also considered. [Preview Abstract] |
Wednesday, October 29, 2014 10:42AM - 10:54AM |
NO4.00007: Development of a Two-shock, Vacuum Hohlraum, Plastic Capsule Implosion Experimental Platform on NIF Jay Salmonson, Stephen Maclaren, Thomas Dittrich, Tammy Ma, Jesse Pino, Robert Tipton, Richard Olson A new experimental platform has been developed to study a variety of indirect drive capsule implosion characteristics. A relatively small, $\sim$ 1700 micron outer diameter, and thick, $\sim$ 200 microns, uniformly Silicon doped, gas-filled plastic capsule is driven inside a standard size 5750 micron diameter ignition hohlraum. The hohlraum fill is near vacuum to reduce back-scatter and improve laser/drive coupling. A two-shock pulse of about $\sim$ 1 MJ of laser energy drives the capsule. The thick capsule prevents ablation front feed-through to the imploded core. Compared to an NIF ignition experiment, this relatively simple, low laser energy platform will allow detailed studies, via sequences of shots, scanning implosion symmetry, capsule gas-fill and convergence, roughness and mix, as well as optimizing stagnation pressure. Recent experimental results toward commissioning this platform will be discussed. [Preview Abstract] |
Wednesday, October 29, 2014 10:54AM - 11:06AM |
NO4.00008: NIF Sub-scale Platform Development R.P.J. Town, F. Albert, L.R. Benedetti, D.K. Bradley, P.M. Celliers, E.L. Dewald, L. Divol, D.E. Eder, G.N. Hall, O.S. Jones, S. Le Pape, B.J. MacGowan, J.L. Milovich, J.D. Moody, A. Pak, J. Ralph, H.F. Robey, J.R. Rygg, M.B. Schneider, D.J. Strozzi In order to increase the shot rate on the National Ignition Facility (NIF) a smaller, lower-energy, room-temperature experimental capability has been designed. The goal of the sub-scale design was to reduce the energy requirement to 900kJ. The starting point for the sub-scale design was a layered plastic capsule in a full scale (575) gold hohlraum that was driven by a four shock, low adiabat, 1.8MJ, 420TW, 21-ns long laser pulse. Simple scaling arguments showed that scaling the capsule and hohlraum dimensions to 80{\%} of full scale should meet the energy requirements. The capability includes sub-scale versions of the ignition-scale re-emit,\footnote{E. L. Dewald, et al, Phys. Rev. Lett. 111, 235001 (2013).} keyhole,\footnote{H. F. Robey, et al, Phys. Rev. Lett. 108, 215004 (2012).} symmetry,\footnote{G. A. Kyrala, et al, Phys. Plasmas 18, 056307 (2011).} backlit,\footnote{J. R. Rygg, et al, Phys. Rev. Lett. 112, 195001 (2014).} and hydro-growth radiography\footnote{K. S. Raman, et al, submitted to Phys. Plasmas (2014).} platforms. An experimental campaign to commission these platforms was performed. This talk will review the design and results of these commissioning experiments. [Preview Abstract] |
Wednesday, October 29, 2014 11:06AM - 11:18AM |
NO4.00009: High Foot Target Design Without Cross-Beam Energy Transfer In a Cylindrical Hohlraum D.E. Hinkel, D.A. Callahan, O.A. Hurricane, P.A. Michel, W.L. Kruer Recent High Foot implosions at the National Ignition Facility (NIF), where the laser power is high early in time, during the ``foot,'' have resulted in record neutron yields [1]. To obtain near-spherical, low-mode implosion symmetry, these targets rely on cross-beam energy transfer (CBET), where outer beam power is transferred to the inner beams [2]. CBET has a temporal dependence, as large amounts of transfer occur early in the laser pulse, when the electron temperature is low, and at peak power, when the laser intensity is at its highest. Furthermore, there is also spatial non-uniformity across laser spots after transfer. We have designed a cylindrical High Foot target without CBET to mitigate these effects. Such a target is feasible because: (i) thinner ablator High Foot targets perform well at relatively low powers ($\sim$ 390 TW) and (ii) post-shot modeling of High Foot shots indicates that CBET is shutting off midway through peak power, and thus the average peak power cone fraction is typically less than 40{\%}. Such a target design tests this hypothesis. We report here on the primary features of this design, comparing it with an analogous NIF shot where cross-beam energy transfer is used to achieve the desired peak power cone fraction. \\[4pt] [1] Hurricane \textit{et al}., \textit{Nature} \textbf{506}, 343-348.\\[0pt] [2] P. Michel \textit{et al}., Phys. Plasmas \textbf{17}, 056305 (2010). [Preview Abstract] |
Wednesday, October 29, 2014 11:18AM - 11:30AM |
NO4.00010: Optimizing the hohlraum gas density for better symmetry control of indirect drive implosion experiments Nobuhiko Izumi, G.N. Hall, S.R. Nagel, S. Khan, R.R. Rygg, A.J. Mackinnon, D.D. Ho, L. Berzak Hopkins, O.S. Jones, R.P.J. Town, D.K. Bradley To achieve a spherically symmetric implosion, control of drive uniformity is essential. Both the ablation pressure and the mass ablation rate on the capsule surface should be made as uniform as possible for the duration of the drive. For an indirect drive implosion, the drive uniformity changes during the pulse because of: (1) the dynamic movement of the laser spots due to blow-off of the hohlraum wall, and (2) cross-beam energy transfer caused by laser-plasma interaction in the hohlraum. To tamp the wall blow-off, we use gas filled hohlraums. The cross-beam energy transfer can be controlled by applying a wave length separation between the cones of the laser beams. However, both of those dynamic effects are sensitive to the initial density of the hohlraum gas fill. To assess this, we performed implosion experiments with different hohlraum gas densities and tested the effect on drive asymmetry. The uniformity of the acceleration was measured by in-flight x-ray backlit imaging of the capsule. The uniformity of the core assembly was observed by imaging the self emission x-ray from the core. We will report on the experimental results and compare them to hydrodynamic simulations. [Preview Abstract] |
Wednesday, October 29, 2014 11:30AM - 11:42AM |
NO4.00011: Backward crossed-beam energy transfer in indirect-drive ignition hohlraums David Turnbull, Pierre Michel, Joseph Ralph, Laurent Divol, Andrea Kritcher, John Moody NIF has recently fielded near-vacuum hohlraums (NVHs) with lower gas fill density (.03mg/cc) than the earlier point design (.96mg/cc). Improved early time beam propagation can allow laser ``glint'' from inner cone beams to exit the opposing laser entrance hole. This light appears on the backscatter diagnostics with numerous features enabling its discrimination from typical backward Stimulated Brillouin Scattering (SBS). Near time zero, we infer signal levels and durations consistent with previous work. The presence of transmitted light also raises the possibility of inter-hemisphere seeding of scattered light, which we refer to as backward Crossed-Beam Energy Transfer (bCBET). Previously, there was no evidence of this process on indirect-drive targets, although it is understood to have a major impact on direct-drive targets. However, the NVHs produce relatively large SBS signals that appear on the inner cone backscatter diagnostics at peak power and carry signatures indicative of bCBET. Upcoming experiments will attempt to clarify whether it is seeded scattering as well as which beams are providing the energy in these signals. [Preview Abstract] |
Wednesday, October 29, 2014 11:42AM - 11:54AM |
NO4.00012: Inline Modeling of Cross-Beam Energy Transfer and Backscatter in Hohlraums D.J. Strozzi, S.M. Sepke, G.D. Kerbel, P. Michel, M.M. Marinak, O.S. Jones NIF Ignition experiments with gas-filled hohlraums use significant cross-beam energy transfer (CBET) to control implosion symmetry. They also display substantial stimulated Raman backscatter (SRS) from inner laser beams, and associated ``hot'' electrons. The radiation-hydrodynamics code HYDRA has been extended to include inline models for CBET and SRS. Coupled-mode equations in the strong damping limit (with linear, kinetic gain rates) are solved along the entire path of incident laser rays. Driven ion-acoustic and Langmuir waves, and inverse-bremsstrahlung absorption, are treated. The inline model includes heating by CBET-driven ion waves, which reduces subsequent CBET.\footnote{P. Michel et al., Phys. Rev. Lett. 109, 195004 (2012)} SRS developing inside the target leads to more heating of the underdense fill - and more depletion of the inner beams reaching the hohlraum wall - than removing the escaping SRS light from the incident laser. Thus, SRS also modifies the plasma conditions so as to limit CBET. We compare inline results with post-processing CBET calculations on plasma conditions from simulations that do not include CBET or SRS. [Preview Abstract] |
Wednesday, October 29, 2014 11:54AM - 12:06PM |
NO4.00013: A Pathway to Ignition-Hydrodynamic-Equivalent Implosions in OMEGA Direct Drive Through the Reduction of Cross-Beam Energy Transfer D.H. Froula, G. Fiksel, V.N. Goncharov, S.X. Hu, H. Huang, I.V. Igumenshchev, T.J. Kessler, D.D. Meyerhofer, D.T. Michel, T.C. Sangster, A. Shvydky, J.D. Zuegel Cross-beam energy transfer (CBET) in OMEGA cryogenic ignition-hydrodynamic-equivalent designs reduces the ablation pressure from 230 Mbar to 140 Mbar. To maintain an ignition-relevant velocity of $3.7 \times 10^{7}$ cm/s, areal density of 300 mg/cm$^2$, and hot-spot pressure greater than 100 Gbar on OMEGA, this reduction in ablation pressure requires that the mass of the shell and the adiabat be reduced by 75{\%} and 50{\%}, respectively. Measurements indicate these implosions are hydrodynamically unstable. To improve the stability, the thickness of the shell (target mass) and the adiabat can be increased while maintaining relevant conditions when reducing CBET. To mitigate CBET, several methods to reduce the diameter of the laser beams while maintaining acceptable drive uniformity are being investigated for OMEGA: (1) direct reduction of the laser spots over the entire laser pulse and (2) reduction of the diameter of the laser spots after a sufficient conduction zone has been generated. This two-state zooming is predicted to maintain low-mode uniformity while mitigating CBET. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Wednesday, October 29, 2014 12:06PM - 12:18PM |
NO4.00014: Cross-Beam Energy Transfer Mitigation Strategy for Polar Drive at the National Ignition Facility J.A. Marozas, T.J.B. Collins, P.W. McKenty, J.D. Zuegel, P.B. Radha, F.J. Marshall, W. Seka, D.T. Michel, M. Hohenberger Cross-beam energy transfer (CBET) causes two-beam energy exchange via stimulated Brillouin scattering,\footnote{C. J. Randall, J. R. Albritton, and J. J. Thomson, Phys. Fluids \textbf{24}, 1474 (1981).} which reduces absorbed light and implosion velocity, alters time-resolved scattered-light spectra, and redistributes absorbed light. These effects reduce target performance in symmetric direct-drive and polar-drive (PD) experiments on the OMEGA Laser System and the National Ignition Facility (NIF). The CBET package (\textit{Adaawam}) incorporated into the 2-D hydrodynamics code \textit{DRACO} is an integral part of the 3-D ray-trace package (\textit{Mazinisin}). The CBET exchange occurs primarily over the equatorial region in PD, where successful mitigation strategies concentrate. Detuning the initial laser wavelength (d$\lambda_{0}$) reduces the CBET interaction volume, which can be combined with spot-shape alterations. Employing opposed $\pm$ d$\lambda_{0}$ in each hemisphere offers the best single CBET mitigation option. The current NIF layout can be used to test detuning by altering the NIF PD repointing strategy while maintaining adequate symmetry. Simulations (2-D \textit{DRACO)} predict measurable results: shell trajectory and shape and scattered-light spectrum and distribution. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Wednesday, October 29, 2014 12:18PM - 12:30PM |
NO4.00015: Evaluation of Wavelength Detuning to Mitigate Cross-Beam Energy Transfer Using the Nike Laser P.W. McKenty, J.A. Delettrez, J.A. Marozas, J. Weaver, S. Obenschain, A. Schmitt Cross-beam energy transfer (CBET) has become a serious threat to the overall success of polar-drive--ignition experiments. CBET redirects incident laser light before it can be absorbed into the target, thereby degrading overall target performance. CBET is particularly effective over the equator of the target, which is hydrodynamically very sensitive to such losses. A promising solution uses laser wavelength detuning between beams to break the resonance between them and reduce energy transfer. Testing this process for direct drive has been limited because of the lack of sufficient detuning capabilities. However, the Naval Research Laboratory's Nike laser has the capability of providing a wide range of detuning between its main drive and backlighter beams. This paper explores the design of an experimental platform on Nike to directly evaluate the benefit of frequency detuning in mitigating CBET. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
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