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 UO4: Hydro Instabilities II |
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Chair: Mario Manuel, University of Michigan Room: Salon E |
Thursday, October 30, 2014 2:00PM - 2:12PM |
UO4.00001: Reduced indirect drive RT instability growth using a decaying first shock at the National Ignition Facility Andrew MacPhee, Kevin Baker, Daniel Casey, Peter Celliers, Daniel Clark, Emilio Giraldez, Alex Hamza, Kenneth Jancaitis, Ogden Jones, Jeremy Kroll, Kai LaFortune, Brian MacGowan, Jose Milovich, Abbas Nikroo, Luc Peterson, Kumar Raman, Harry Robey, Vladimir Smalyuk, Christopher Weber, Clifford Widmayer, Steven Haan Hydrodynamic instabilities and poor fuel compression are major factors for capsule performance degradation in ignition experiments on the NIF. We are developing modified drives to reduce instability growth compared to previous ignition pulses whilst maintaining the low fuel adiabat needed for increased compression. Laser drive profiles with a decaying first shock (``adiabat shaping'') were developed that alter the Richtmyer-Meshkov growth phase and reduce the subsequent inflight Rayleigh-Taylor growth. Using in-flight x-ray radiography of 1D ripples at various wavelengths, the first growth measurements using these new shaped drives will be presented for 3-shock and 4-shock designs. [Preview Abstract] |
Thursday, October 30, 2014 2:12PM - 2:24PM |
UO4.00002: A survey of pulse shape options for a revised plastic ablator ignition design Daniel Clark, David Eder, Steven Haan, Denise Hinkel, Ogden Jones, Michael Marinak, Jose Milovich, Jayson Peterson, Harold Robey, Jay Salmonson, Vladimir Smalyuk, Christopher Weber Recent experimental results using the ``high foot'' pulse shape on the National Ignition Facility (NIF) have shown encouraging progress compared to earlier ``low foot'' experiments. These results strongly suggest that controlling ablation front instability growth can dramatically improve implosion performance, even in the presence of persistent, large, low-mode distortions. In parallel, Hydro. Growth Radiography experiments have so far validated the techniques used for modeling ablation front growth in NIF experiments. It is timely then to combine these two results and ask how current ignition pulse shapes could be modified so as to improve implosion performance, namely fuel compressibility, while maintaining the stability properties demonstrated with the high foot. This talk presents a survey of pulse shapes intermediate between the low and high foot extremes in search of a more optimal design. From the database of pulse shapes surveyed, a higher picket version of the original low foot pulse shape shows the most promise for improved compression without loss of stability. [Preview Abstract] |
Thursday, October 30, 2014 2:24PM - 2:36PM |
UO4.00003: Measurement of the Shock Velocity and Symmetry History in Decaying Shock Pulses Kevin Baker, Jose Milovich, Oggie Jones, Harry Robey, Vladimir Smalyuk, Daniel Casey, Peter Celliers, Dan Clark, Emilio Giraldez, Steve Haan, Alex Hamza, Laura Berzak-Hopkins, Ken Jancaitis, Jeremy Kroll, Kai Lafortune, Brian MacGowan, Andrew MacPhee, John Moody, Abbas Nikroo, Luc Peterson, Kumar Raman, Chris Weber, Clay Widmayer Decaying first shock pulses are predicted in simulations to provide more stable implosions and still achieve a low adiabat in the fuel, enabling a higher fuel compression similar to ``low foot'' laser pulses. The first step in testing these predictions was to measure the shock velocity for both a three shock and a four shock adiabat-shaped pulse in a keyhole experimental platform. We present measurements of the shock velocity history, including the decaying shock velocity inside the ablator, and compare it with simulations, as well as with previous low and high foot pulses. Using the measured pulse shape, the predicted adiabat from simulations is presented and compared with the calculated adiabat from low and high foot laser pulse shapes. This work was performed under the auspices of the U.S. Department of Energy by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Thursday, October 30, 2014 2:36PM - 2:48PM |
UO4.00004: Adiabat shape Laser Pulses for ablation front instability control and high fuel compression Jose Milovich, O.S. Jones, L. Berzak-Hopkins, D.S. Clark, K.L. Baker, D.T. Casey, A.G. MacPhee, J.L. Peterson, H.F. Robey, V.A. Smalyuk, C.R. Weber At the end of the NIC campaign a large body of experimental evidence showed that the point-design implosions driven by low-adiabat pulses had a high degree of mix [1]. To reduce instability a high-adiabat ($\sim$3x higher picket drive) design was fielded in the National Ignition Facility (NIF). The experimental results from this campaign [2] have shown considerable improvement in performance (10x neutron yields) over the point design with little evidence of mix. However, the adiabat of the implosions may be too high to achieve ignition for the available laser energy. To overcome this difficulty, and to take advantage of the high-picket drives, we have developed hybrid laser pulses that combined the virtue of both designs. These pulses can be thought of achieving adiabat shaping, where the ablator is set in a higher adiabat for instability control, while the fuel is maintained at a lower adiabat favoring higher fuel compression. Using these pulses, recent experiments at the NIF have indeed shown reduced growth rates. In this talk we will present the design of high-yield low-growth DT ignition experiments using these adiabat-shaped pulses.\\[4pt] [1] T. Ma, et al., Phys. Rev. Let. 111 085004\\[0pt] [2] O. Hurricane, et al., Phys. Plasmas 21, 056314 (2014) [Preview Abstract] |
Thursday, October 30, 2014 2:48PM - 3:00PM |
UO4.00005: Simulations of NIF 3-D capsule modulation growth experiments S.V. Weber, D.T. Casey, D.S. Clark, J. Field, S.W. Haan, V.A. Smalyuk, A. Nikroo, N. Rice Growth of 3-D modulations in NIF CH ablator capsules has been measured by inflight radiography of imploding shells through a reentrant keyhole. Gated images are taken near the time of peak shell velocity at a convergence ratio of about five. We have looked at shells with native roughness and with outer surface roughness enhanced by a factor of about four. Three-dimensional simulations have been carried out using surfaces reconstructed from measured power spectra and also from point by point metrology of the actual capsule surface. Comparison of measured and simulated radiographs tests our understanding of modulation growth as well as addressing whether there are significant seeds for growth that have been overlooked. [Preview Abstract] |
Thursday, October 30, 2014 3:00PM - 3:12PM |
UO4.00006: Measurements of hydrodynamic instability growth with 3-D capsule modulation at NIF V.A. Smalyuk, S.V. Weber, D.T. Casey, D.S. Clark, J. Field, S.W. Haan, H.F. Robey, A. Nikroo, N. Rice Repla Instability growth measurements of 3-D capsule modulations were performed at NIF. Ignition-scale, CH ablator capsules were imploded with ignition-relevant drives. Modulation growth was measured by inflight radiography through a reentrant keyhole near peak velocity of imploding shells. Shells with either native roughness or outer surface roughness enhanced by a factor of about four were used in the experiments. Comparison of measured and simulated radiographs will be presented. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Thursday, October 30, 2014 3:12PM - 3:24PM |
UO4.00007: Simulations of the ``tent'' and its signatures in NIF ignition implosions B.A. Hammel, R. Tommasini, H.A. Scott, V. Smalyuk NIF capsules are supported in the hohlraum by two thin $\sim$ 50 nm) Formvar films (``tent''). We report on highly-resolved Hydra simulations of the perturbation that develops on the capsule as a result of this support geometry. The simulations indicate that details of the geometry (e.g. the departure angle of the tent from the capsule surface) are important in determining the size of the final capsule areal density perturbation. Simulated diagnostic signatures of the capsule perturbation, including ``in-flight'' radiographs and the shape of the x-ray emission from the compressed core are in general agreement with experiments. We are designing dedicated measurements to fully validate the simulations. [Preview Abstract] |
Thursday, October 30, 2014 3:24PM - 3:36PM |
UO4.00008: Preparing for Polar-Drive Imprint Experiments at the National Ignition Facility A. Shvydky, M. Hohenberger, P.B. Radha, R.S. Craxton, V.N. Goncharov, J.A. Marozas, F.J. Marshall, P.W. McKenty, T.C. Sangster Control of laser-induced nonuniformities is critical for the success of polar-drive--ignition experiments at the National Ignition Facility (NIF). Laser-imprint studies in laser-driven spherical shell targets will be performed at the NIF in Q1 and Q2 of FY15. Corrugated spherical, cone-in-shell targets with sinusoidal surface modulations will provide a reference for calculating the equivalent surface roughness of the imprint using the current NIF phase plates and beam smoothing. \textit{DRACO} simulations are used to design the spherical-imprint experiments and set the initial shell thickness, cone geometry, laser pulse, and laser beam pointings. Results of \textit{DRACO} simulations will be presented, evaluating the expected experimental x-ray radiographs. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, October 30, 2014 3:36PM - 3:48PM |
UO4.00009: Measurements of Laser Imprinting Using 2-D Velocity Interferometry T.R. Boehly, G. Fiksel, S.X. Hu, V.N. Goncharov, T.C. Sangster, P.M. Celliers Evaluating laser imprinting and its effect on target performance is critical to direct-drive inertial confinement fusion research. Using high-resolution velocity interferometry, we measure modulations in the velocity of shock waves produced by the 351-nm beams on OMEGA. These modulations result from nonuniformities in the drive laser beams. We use these measurements to evaluate the effect on imprinting of multibeam irradiation and metal layers on both plastic and cryogenic deuterium targets driven with 100-ps pulses. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, October 30, 2014 3:48PM - 4:00PM |
UO4.00010: Simulation of instability growth on ICF capsule ablators Nicolas Niasse, Jeremy Chittenden It is believed that the ablation-front instabilities are mainly responsible for the hot-spot mix that impacts the performance of ICF capsules. Understanding the formation of these instabilities is therefore a first step towards a better control of the implosion dynamics and the optimization of the fusion yield. Using the Chimera code currently in development at Imperial College, we have performed several spherical wedge simulations of the low and high adiabat ablation phase pre-imposing different single-mode 2D and 3D perturbations on the capsule surface. Synthetic Sc, Fe and V X-ray backlighter images are generated by the Spk code and used to measure the growth of modes 30-160 with initial amplitude $\le $ 3.4 $\mu$m PTV. The growth of imposed 2D perturbations is assessed for both low-foot and high-foot radiation pulse shapes on the National Ignition Facility. Results showing the merger of spike and bubble structures in multi-mode perturbations in both 2D and 3D simulations are explored and preliminary assessments of the difference between 2D and 3D non-linear behaviour is discussed. The sensitivity of shock timing to NLTE changes in opacity is also assessed. [Preview Abstract] |
Thursday, October 30, 2014 4:00PM - 4:12PM |
UO4.00011: Measuring Mix in Direct-Drive Cryogenic DT Implosions Using Soft X-Ray Narrowband Backlighting C. Stoeckl, R. Epstein, G. Fiksel, V.N. Goncharov, S.X. Hu, D.W. Jacobs-Perkins, R.K. Jungquist, C. Mileham, P.M. Nilson, T.C. Sangster, W. Theobald Rayleigh--Taylor mix is widely seen as the major source of perturbations, which limit the performance of low-adiabat cryogenic implosions in both direct- and indirect-drive inertial confinement fusion experiments. Backlit images of cryogenic direct-drive implosions recorded with a narrowband x-ray imager using an aspherically bent quartz crystal for the Si He$_{\alpha}$ line at $\sim$ 1.86 keV show a clear signature of carbon from the CD outer shell of the cryogenic target mixing into the DT layer at the end of the acceleration phase. These implosions are driven on a low adiabat with a high in-flight aspect ratio (IFAR). Comparison with post-processed 1-D hydrodynamic simulations show that the absorption seen in the backlit images is $\sim 5\times $ larger than expected, consistent with mixing $\sim$ 0.2\% of carbon into the DT shell. Experiments with a slightly higher adiabat and lower IFAR match the predictions of clean 1-D simulations showing no signature of carbon mix. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, October 30, 2014 4:12PM - 4:24PM |
UO4.00012: Mix and instability growth seeded at the inner surface of CH-ablator implosions on the National Ignition Facility S.W. Haan, P.M. Celliers, G.W. Collins, C.D. Orth, D.S. Clark, P. Amendt, B.A. Hammel, H.F. Robey, H. Huang Mix and hydro instability growth are key issues in implosions of ignition targets on NIF. The implosions are designed so that the amplitude of perturbations is thought to be determined by initial seeds to the hydrodynamic instabilities, amplified by an instability growth factor. Experiments have indicated that growth factors can be calculated fairly well, but characterizing the initial seeds is an ongoing effort. Several threads of investigation this year have increased our understanding of growth seeded at the CH/DT interface. These include: more detailed characterization of the CH inner surface; possible other seeds, such as density irregularities either from fabrication defects or arising during the implosion; experiments on the Omega laser measuring velocity modulations on shock fronts shortly after breaking out from the CH, which can seed subsequent growth; and the possible significance of non-hydrodynamic effects such as plasma interpenetration or spall-like ejecta upon shock breakout. This presentation describes these developments, the relationships between them, and their implications for ignition target performance. [Preview Abstract] |
Thursday, October 30, 2014 4:24PM - 4:36PM |
UO4.00013: Measurements of ablation-front hydrodynamic instability growth in high-density carbon (HDC) ignition targets at the National Ignition Facility (NIF) D. Casey, V. Smalyuk, L. Peterson, L. Berzak Hopkins, T. Bunn, L. Carlson, S. Haan, D. Ho, D. Hoover, J. Kroll, O. Landen, S. Le Pape, A. MacKinnon, A. MacPhee, N. Meezan, J. Milovich, A. Nikroo, B. Remington, H. Robey, S. Ross High-density carbon (HDC) has emerged as a promising ablator for ignition experiments at the National Ignition Facility (NIF) partly because of its efficient coupling of the drive energy to fuel. Experiments during the National Ignition Campaign using a CH plastic ablator have shown that instability growth and the resultant mix of plastic into the hotspot was a significant source of overall the observed performance degradation. Likewise, mix of HDC ablator into the hotspot may also be a concern, as growth rates for HDC are comparable to CH and ablator/dopant is higher Z than CH ablators making the consequences potentially more severe. To help understand this issue, we plan to perform the first instability growth measurements of W-doped HDC implosions with preimposed mode 60 and mode 90 perturbations in convergent geometry using actual ignition targets and drives. These results will be presented and compared to ignition design simulations. [Preview Abstract] |
Thursday, October 30, 2014 4:36PM - 4:48PM |
UO4.00014: Modeling Mix in ICF Implosions C.R. Weber, D.S. Clark, B. Chang, D.C. Eder, S.W. Haan, O.S. Jones, M.M. Marinak, J.L. Peterson, H.F. Robey The observation of ablator material mixing into the hot spot of ICF implosions correlates with reduced yield in National Ignition Campaign (NIC) experiments. Higher Z ablator material radiatively cools the central hot spot, inhibiting thermonuclear burn. This talk focuses on modeling a ``high-mix'' implosion from the NIC, where greater than 1000 ng of ablator material was inferred to have mixed into the hot spot. Standard post-shot modeling of this implosion does not predict the large amounts of ablator mix necessary to explain the data. Other issues are explored in this talk and sensitivity to the method of radiation transport is found. Compared with radiation diffusion, Sn transport can increase ablation front growth and alter the blow-off dynamics of capsule dust. [Preview Abstract] |
Thursday, October 30, 2014 4:48PM - 5:00PM |
UO4.00015: Rayleigh--Taylor instability experiments in quasi-isentropically compressed Al targets at the Shenguang2 Laser Jiaqin Dong We present experiments on the Rayleigh-Taylor (RT) instability in pure Al foils at $\sim$ 34GPa pressure using a laser based, ramped-pressure acceleration technique. A line VISAR velocity diagnostic is developed to measure the drive on separate targets, and a X-rays K-B Microscope is used to measure the RT growth with 4.75keV Ti He-$\alpha $ x-ray backlighter. RT growth factors in solid state and melted Al, driven by approximate plasma loader, are measured and compared. Material strength suppresses the RT growth rate dramatically. [Preview Abstract] |
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