Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session GI3: ICF Preheat and DriveInvited
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Chair: Felicie Albert, Lawrence Livermore National Laboratory Room: Oglethorpe Auditorium |
Tuesday, November 17, 2015 9:30AM - 10:00AM |
GI3.00001: The Near Vacuum Hohlraum Campaign at the NIF: A New Approach Invited Speaker: Sebastien le Pape Hohlraums filled with helium \textgreater 1 mg/cc have been used with some success on the National Ignition Facility [1]. However challenges remain due to significant backscatter level, supra-thermal electron production and difficulties in modeling implosion symmetry via Cross Beam Energy Transfer (CBET) [2]. Near Vacuum Hohlraum (NVH, filled with \textless 0.1 mg/cc of helium) may provide a viable alternative with negligible laser plasma instabilities and high laser-to-hohlraum coupling [3]. In this reduced laser-plasma interaction system, implosion symmetry is controlled through direct adjustment of the laser beam power balance rather than through CBET. A significant challenge in extending this platform to higher convergence designs is achieving adequate symmetry control of the drive throughout the pulse. This talk will summarize experimental campaigns exploring laser pulse duration and power limits in three hohlraum size scales and two capsule size scales. Experiments with small capsules have shown good symmetry control using laser cone fraction tuning at convergence ratio (CR) of 18x and 7ns pulses. Results from higher convergence (CR $\sim$ 25x) cryogenic DT layered capsule implosions with $\sim$ 9ns pulses will be presented and implications for achieving conditions required for robust alpha heating with NVH driven implosions will be discussed. *Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. \\[4pt] [1] Hurricane, O. A., et al. ``Fuel gain exceeding unity in an inertially confined fusion implosion.'' Nature 506.7488 (2014): 343-348. \\[0pt] [2] J.D. Moody et al., ``The hohlraum Drive Campaign on the national Ignition facility,'' APS, DPP, 2013 \\[0pt] [3] L. F. Berzak Hopkins et. al., ``First high-convergence cryogenic implosion in a near-vacuum hohlraum,'' Phys. Rev. Lett. Phys. Rev. Lett.~114, 175001~(2015) [Preview Abstract] |
Tuesday, November 17, 2015 10:00AM - 10:30AM |
GI3.00002: Characterizing Hohlraum Plasma Conditions at the National Ignition Facility (NIF) Using X-ray Spectroscopy Invited Speaker: Maria Alejandra Barrios Improved hohlraums will have a significant impact on increasing the likelihood of indirect drive ignition at the NIF. In indirect-drive Inertial Confinement Fusion (ICF), a high-Z hohlraum converts laser power into a tailored x-ray flux that drives the implosion of a spherical capsule filled with D-T fuel. The x-radiation drive to capsule coupling sets the velocity, adiabat, and symmetry of the implosion. Previous experiments\footnote{S.A. Maclaren, et al, Phys. Rev Lett. 112, 105003 (2014).} in gas-filled hohlraums determined that the laser-hohlraum energy coupling is 20-25{\%} less than modeled, therefore identifying energy loss mechanisms that reduce the efficacy of the hohlraum drive is central to improving implosion performance. Characterizing the plasma conditions, particularly the plasma electron temperature (T$_{\mathrm{e}})$, is critical to understanding mechanism that affect the energy coupling such as the laser plasma interactions (LPI), hohlraum x-ray conversion efficiency, and dynamic drive symmetry. The first T$_{\mathrm{e}}$ measurements inside a NIF hohlraum, presented here, were achieved using K-shell X-ray spectroscopy of an Mn-Co tracer dot. The dot is deposited on a thin-walled CH capsule, centered on the hohlraum symmetry axis below the laser entrance hole (LEH) of a bottom-truncated hohlraum. The hohlraum x-ray drive ablates the dot and causes it to flow upward, towards the LEH, entering the hot laser deposition region. An absolutely calibrated streaked spectrometer with a line of sight into the LEH records the temporal history of the Mn and Co X-ray emission. The measured (interstage) Ly$_{\alpha}$/ He$_{\alpha}$ line ratios for Co and Mn and the Mn-He$_{\alpha}$/Co-He$_{\alpha}$ isoelectronic line ratio are used to infer the local plasma T$_{\mathrm{e}}$ from the atomic physics code SCRAM.\footnote{S.B. Hansen, \textit{et al.} High Energy Density Phys. 3, 109 (2007).} Time resovled x-ray images perpendicular to the hohlraum axis record the dot expansion and trajectory into the LEH region. The temporal evolution of the measured T$_{\mathrm{e}}$ and dot trajectory are compared with simulations from radiation-hydrodynamic codes. [Preview Abstract] |
Tuesday, November 17, 2015 10:30AM - 11:00AM |
GI3.00003: Scattered and Reflected Light Polarimetry as a Diagnostic of Multibeam Hohlraum Physics Invited Speaker: David Turnbull Scattered light provides a window into the complex laser-plasma interactions and hydrodynamics occurring within indirect-drive inertial confinement fusion (ICF) hohlraums. Understanding hohlraum physics is an important part of developing improved targets and increasing the likelihood of ignition. Measurements of the scattered light power and spectrum are routinely made on each cone of beams at the National Ignition Facility (NIF) in order to correct for coupling losses due to laser-plasma instabilities. The additional ability to probe scattered light polarization on a 30$^{\circ}$ incidence beam was recently added [1], which has produced a number of discoveries regarding multibeam hohlraum physics [2,3]. One particularly important insight is that the polarizations of an incident beam and its backscatter are affected by amplitude and phase modulations induced by crossing laser beams. The revised theory [3] describing this optical wave mixing has recently been validated by conducting a two beam pump-probe experiment under carefully controlled conditions. This effect could be utilized more generally to produce ultrafast, damage-resistant, and tunable laser-plasma wave plates, polarizers, or other photonic devices. It also enables remote polarimetry-based probing of plasma conditions such as electron temperature. To extract more quantitative feedback about crossed-beam energy transfer (CBET) from the polarimetry data in ICF experiments at the NIF, the diagnostic has been upgraded to measure the complete Stokes vector with temporal resolution [4]. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. \\[4pt] [1] D. Turnbull et al., ``Polarimetry of uncoupled light on the NIF,'' Rev. Sci. Inst. 85, 11E603 (2015).\\[0pt] [2] D. Turnbull et al., ``Multibeam seeded Brillouin sidescatter in inertial confinement fusion experiments,'' Phys. Rev. Lett. 114, 125001 (2015).\\[0pt] [3] P. Michel et al., ``Dynamic control of the polarization of intense laser beams via optical wave mixing in plasmas,'' Phys. Rev. Lett. 113, 205001 (2014).\\[0pt] [4] D. Turnbull et al., ``Complete time-resolved polarimetry of scattered light at the National Ignition Facility,'' submitted to SPIE (2015). [Preview Abstract] |
Tuesday, November 17, 2015 11:00AM - 11:30AM |
GI3.00004: Quantifying the Growth of Cross-Beam Energy Transfer in Polar-Direct-Drive Implosions at the Omega Laser and National Ignition Facilities Invited Speaker: A.K. Davis Direct-drive inertial confinement fusion requires multiple overlapping laser beams that can drive the cross-beam energy transfer (CBET) instability. This instability is of primary concern because it can reduce the laser energy coupling and can affect the symmetry in a polar-direct-drive (PDD) configuration. An experiment was designed to determine the CBET growth by measuring the angularly resolved mass ablation rate and ablation-front trajectory in a PDD configuration. Adding a thin layer of Si over a CH shell generates two peaks in x-ray self-emission images that are measured with a time-resolved pinhole imager. The inner peak is related to the position of the ablation front and the outer peak corresponds to the position of the interface of the two layers in the plasma. The emergence of the second peak is used to measure the time for the laser to burn through the outer layer, giving the average mass ablation rate of the material. The mass ablation rate was measured by varying the thickness of the outer silicon layer. The shell trajectory and mass ablation rate measured in PDD on the pole, where CBET has little effect, were compared with simulations to validate the electron thermal-transport model. Excellent agreement was obtained when using a 2-D nonlocal transport model, and these observables could not be reproduced with flux-limited models. A similar comparison was performed on the equator where the CBET growth is large. Without the CBET model, the shell velocity and mass ablation rate were significantly overestimated by the simulation. Adding the CBET model reduced the drive on the equator and reproduced the experimental results. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. In collaboration with, D. Cao, D. T. Michel, M. Hohenberger, R. Epstein, V. N. Goncharov, S. X. Hu, I. V. Igumenshchev, J. A. Marozas, D. D. Meyerhofer, P. B. Radha, S. P. Regan, T. C. Sangster, and D. H. Froula (Laboratory for Laser Energetics, U. of Rochester); M. Lafon (CEA). [Preview Abstract] |
Tuesday, November 17, 2015 11:30AM - 12:00PM |
GI3.00005: Investigating the laser heating of underdense plasmas at conditions relevant to MagLIF Invited Speaker: Adam Harvey-Thompson The magnetized Liner Inertial Fusion (MagLIF) scheme has achieved thermonuclear fusion yields on Sandia's Z Facility by imploding a cylindrical liner filled with D2 fuel that is preheated with a multi-kJ laser and pre-magnetized with an axial field B$_{\mathrm{z}}=$10 T. The challenge of fuel preheating in MagLIF is to deposit several kJ's of energy into an underdense (n$_{\mathrm{e}}$/n$_{\mathrm{crit}}$\textless 0.1) fusion fuel over $\sim$ 10 mm target length efficiently and without introducing contaminants that could contribute to unacceptable radiative losses during the implosion. Very little experimental work has previously been done to investigate laser heating of gas at densities, scale lengths, modest intensities (I$\lambda^{2}$ $\sim$ 10$^{14}$ watts-$\mu $m$^{2}$ /cm$^{2}$) and magnetization parameters ($\omega_{\mathrm{ce}}\tau_{\mathrm{e}}$ $\sim$ 10) necessary for MagLIF. In particular, magnetization of the preheated plasma suppresses electron thermal conduction, which can modify laser energy coupling. Providing an experimental dataset in this regime is essential to not only understand the dynamics of a MagLIF implosion and stagnation, but also to validate magnetized transport models and better understand the physics of laser propagation in magnetized plasmas. In this talk, we present data and analysis of several experiments conducted at OMEGA-EP and at Z to investigate laser propagation and plasma heating in underdense D$_{2}$ plasmas under a range of conditions, including densities (n$_{\mathrm{e}}=$ 0.05-0.1 n$_{\mathrm{c}})$ and magnetization parmaters ($\omega_{\mathrm{ce}}\tau_{\mathrm{e}}$ $\sim$ 0-10). The results show differences in the electron temperature of the heated plasma and the velocity of the laser burn wave with and without an applied magnetic field. We will show comparisons of these experimental results to 2D and 3D HYDRA simulations, which show that the effect of the magnetic field on the electron thermal conduction needs to be taken into account when modeling laser preheat. [Preview Abstract] |
Tuesday, November 17, 2015 12:00PM - 12:30PM |
GI3.00006: Fast Ignition Realization Experiment with High-Contrast Kilo-Joule Peta-Watt Laser ``LFEX'' and Strong External Magnetic Field Invited Speaker: Shinsuke Fujioka We report on progresses of the Fast Ignition Realization Experiment (FIREX) project that has been curried out at the Institute of Laser Engineering to assess the feasibility of high density core heating with a high-power, short-pulse laser including the construction of the Kilo-Joule, Petawatt class LFEX laser system. Our recent studies identify three scientific challenges to achieve high heating efficiency in the fast ignition (FI) scheme with the current GEKKO and LFEX laser systems: (i) control of energy distribution of relativistic electron beam (REB), (ii) guiding and focusing of REB to a fuel core, and (iii) formation of a high areal-density core. The control of the electron energy distribution has been experimentally confirmed by improving the intensity contrast of the LFEX laser up to \textgreater 10$^{9}$ and an ultra-high contrast of 10$^{11}$ with a plasma mirror. After the contrast improvement, 50{\%} of the total REB energy is carried by a low energy component of the REB, which slope temperature is close to the ponderomotive scaling value ($\sim$ 1 MeV). To guide the electron beam, we apply strong external magnetic field to the REB transport region. Guiding of the REB by 0.6 kT field in a planar geometry has already been demonstrated at LULI 2000 laser facility in a collaborative experiment lead by CELIA-Univ. Bordeaux. Considering more realistic FI scenario, we have performed a similar experiment using the Kilo-Joule LFEX laser to study the effect of guiding and magnetic mirror on the electron beam. A high density core of a laser-imploded 200 $\mu$m-diameter solid CD ball was radiographed with picosecond LFEX-produced K-alpha backlighter. Comparisons of the experimental results and integrated simulations using hydrodynamic and electron transport codes suggest that 10{\%} of the efficiency can be achievable with the current GEKKO and LFEX laser system with the success of the above challenges. [Preview Abstract] |
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