Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session YI3: ICF Physics; Postdeadline Invited |
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Chair: Craig Sangster, University of Rochester Room: Ballroom AC |
Friday, November 18, 2011 9:30AM - 10:00AM |
YI3.00001: Crossed-Beam Energy Transfer in Direct-Drive Implosions Invited Speaker: Direct-drive-implosion experiments on OMEGA have revealed the importance of crossed-beam energy transfer\footnote{I. V. Igumenshchev\textit{ et al.}, Phys. Plasmas \textbf{17}, 122708 (2010).} (CBET), which is caused by stimulated Brillouin scattering. The CBET reduces the laser absorption in a target corona by $\sim $10{\%} to 20{\%} and, therefore, decreases the implosion performance. The signature of CBET is observed in time-resolved, reflected-light spectra as a suppression of red-shifted light during the main laser pulse. Simulations without CBET typically predict an earlier bang time and overestimate the laser absorption in high-compression, low-adiabat implosions. Simulations using a CBET model and a nonlocal heat-transport model\footnote{V. N. Goncharov\textit{ et al.}, Phys. Plasmas \textbf{15}, 056310 (2008).} explain well the scattered-light and bang-timing measurements. This talk will summarize the possible mitigation strategies for CBET required for robust ignition designs. CBET most effectively scatters incoming light that interacts with outgoing light originated from laser beam edges. This makes it possible to mitigate CBET by reducing the beam diameter with respect to the target diameter. Implosion experiments using large 1400-\textit{$\mu $}m-diam plastic shells and in-focus and defocus laser beams have demonstrated the reduction of CBET in implosions with a smaller ratio of the beam-to-target diameters. Simulations predict the optimum range of this ratio to be 0.7 to 0.8. Another mitigation strategy involves splitting the incident light into two or more colors. This reduces CBET by shifting and suppressing the coupling resonances. The reduction in scattered light caused by CBET is predicted to be up to a factor of 2 when incident light colors are separated by \textit{$\delta \lambda $} $>$ 6 {\AA}. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. \\[4pt] In collaboration with W. Seka, D. H. Edgell, D. H. Froula, V. N. Goncharov, R. S. Craxton, R. L. McCrory, A. V. Maximov, D. D. Meyerhofer, J. F. Myatt, T. C. Sangster, A. Shvydky, S. Skupsky, and C. Stoeckl. [Preview Abstract] |
Friday, November 18, 2011 10:00AM - 10:30AM |
YI3.00002: Inertial Confinement Fusion Implosions with Seeded Magnetic Fields on OMEGA Invited Speaker: Experiments applying laser-driven magnetic-flux compression to inertial confinement fusion (ICF) experiments to enhance the fuel-assembly performance are described. Spherical CH targets filled with 10 atm of deuterium gas were imploded by the OMEGA laser in polar-drive geometry. The targets were embedded with an 80-kG magnetic seed field. Upon laser irradiation, the high-implosion velocities and ionization of the target fill lead to trapping of the magnetic field inside the capsule and its amplification through flux compression to up to tens of megagauss. At such strong magnetic fields, the hot spot inside a spherical target becomes strongly magnetized, reducing the heat losses through electron confinement. The experimentally observed ion temperature was enhanced by 15{\%} and the neutron yield was increased by 30{\%}, compared to nonmagnetized implosions. This represents the first experimental verification of performance enhancement resulting from embedding a strong magnetic field into an ICF capsule. The compressed field was probed via proton deflectometry using the 14.7-MeV protons generated in the D+$^{3}$He fusion reactions from a laser-imploded glass microballoon. Experimental data for the fuel-assembly performance and magnetic field are compared to numerical results from combining the 1-D hydrodynamics code \textit{LILAC} with a 2-D, azimuthal symmetry MHD postprocessor. This work was supported by the U.S. Department of Energy under Cooperative Agreement No. DE-FC02-04ER54789 and DE-FC52-08NA28302. \\[4pt] In collaboration with P.-Y. Chang, G. Fiksel, J. P. Knauer, R. Betti, F. J. Marshall, and D. D. Meyerhofer (Laboratory for Laser Energetics, Univ. of Rochester), and F. H. S\'{e}guin and R. D. Petrasso (PSFC, MIT). [Preview Abstract] |
Friday, November 18, 2011 10:30AM - 11:00AM |
YI3.00003: Nonlocal electron energy transport and flux inhibition in laser produced plasmas in one and two dimensions Invited Speaker: As the mean free path of the heat conducting electrons in laser produced plasmas can, at certain points, be greater than the temperature gradient scale length, the classical, local model can be invalid. More energetic electrons can advance ahead of the main heat front and preheat the fusion target. Also, experiments show that the main heat front does not propagate as rapidly as classical theory would predict, so there is heat flux inhibition. This latter effect is usually treated by limiting the flux to some arbitrary fraction f of the free streaming flux; f's have ranged from 0.03 to 0.3. However the choice of flux limit is arbitrary and the choice affects plasma temperature, which in turn affects thresholds for laser plasma instabilities; too low a limit has given too high a temperature and false optimism regarding instability threshold. We have developed a velocity dependent Krook model for nonlocal electron energy transport. It shows preheat and flux limitation are not separate effects, but are two sides of the same coin. The model gives an analytic solution for the nonlocal electron energy flux, and it is relatively simple and inexpensive to incorporate in a fluid simulation run at the ion time scale. It shows that in some sense, preheat is subtracted from the main electron energy flux, thereby giving rise to flux limitation. We have developed the theory and compared it with Fokker Planck simulations of simple configurations. We have incorporated the model into our code FAST2D and used it to model foil acceleration and evaluate and compare a number of competing physical effects in one and two dimensions, and compared with experiments. We have investigated the effect on spherical implosions, especially the effect on corona temperature, pressure, fuel adiabat and preheat, and ultimately gain. [Preview Abstract] |
Friday, November 18, 2011 11:00AM - 11:30AM |
YI3.00004: Fast Magnetic Reconnection in High-Energy-Density Laser-Produced Plasmas Invited Speaker: Recent experiments have observed magnetic reconnection in high-energy-density, laser-produced plasma bubbles [1,2], with reconnection rates observed to be much higher than can be explained by classical theory. This is a novel regime for magnetic reconnection study, characterized by extremely high magnetic fields, high plasma beta and strong, supersonic plasma inflow. Reconnection in this regime is investigated with particle-in-cell simulations. Collisionless simulations have identified two key ingredients, simultaneously present for the first time: two-fluid reconnection mediated by collisionless effects (that is, the Hall current and electron pressure tensor), and strong flux pile-up of the inflowing magnetic field [3]. These effects combine to yield reconnection rates independent of the nominal Alfv\'{e}n speed (based on the magnetic field before interaction), and simply given by the dynamic time $L/V$, in qualitative agreement with the experiments. We present detailed simulations spanning the parameter ranges of the experiments, and further compare the results of simulations with and without binary collisions, in 2D and 3D. Finally we discuss plans for future laser-driven reconnection experiments.\\[4pt] [1] P. M. Nilson, \textit{et al}, Phys. Rev. Lett. \textbf{97}, 255001 (2006).\\[0pt] [2] C. K. Li, \textit{et al}, Phys. Rev. Lett. \textbf{99}, 055001 (2007).\\[0pt] [3] W. Fox, A. Bhattacharjee, K. Germaschewski, Phys. Rev. Lett. \textbf{106}, 215003 (2011). [Preview Abstract] |
Friday, November 18, 2011 11:30AM - 12:00PM |
YI3.00005: Generation and focusing of short pulse high intensity laser accelerated protons Invited Speaker: Mark E. Foord Much progress has recently been reported in generating MeV energy protons from intense laser-matter interactions, having potential applications in areas such as radiography, oncology, and ion-proton beam fast ignition. Experiments were conducted on the sub-ps LANL Trident laser, where we systematically investigated proton focusing and conversion efficiency from curved surface targets in both open and closed cone-shaped target geometries. We clearly show that the focusing is strongly affected by the electric fields in the beam, bending the trajectories near the axis. We also find that in the cone geometry, a sheath electric field effectively ``channels'' the proton beam through the cone tip, substantially improving the beam focusing properties. The far-field energy and angular distribution of the proton beam were measured using a mesh that images the beam onto a RCF detector. For the cone-shaped targets using a 300 $\mu$m-radius curved surface foil, a 60 $\mu$m diameter proton spot was determined. Proton generation and focusing were modeled using 2-D hybrid PIC simulations, which compared well with RCF data. The proton conversion efficiency varied strongly with the target geometry. Simulations indicate this is due to that charge flow on the structure and the coupling to the hot electrons and electric fields in the plasma. [Preview Abstract] |
Friday, November 18, 2011 12:00PM - 12:30PM |
YI3.00006: A New High Performance Field-Reversed Configuration Operating Regime in the C-2 Device Invited Speaker: Michel Tuszewski Large-size hot FRCs are produced in the C-2 device by merging two dynamically-formed, high-beta, Compact Toroids.\footnote{M.W. Binderbauer et al., Phys. Rev. Lett. \textbf{105}, 045003 (2010).} The good confinement properties of these merged FRCs must be further improved to achieve the C-2 goal of FRC sustainment with neutral beam injection and pellet fuelling. Recently, an AMBAL plasma gun\footnote{T. Akhmetov et al., Trans. Fusion Science and Technology \textbf{43}, 58 (2003).} and 2 T magnetic mirror end plugs were installed on C-2 to attempt electric field control of the plasma sheath outside of the FRC separatrix.\footnote{M. Tuszewski, A. Smirnov et al., Trans. Fusion Science and Technology 5\textbf{9}, 23 (2010).} The combined effects of the gun, mirror plugs, and neutral beams yielded the following important new results. First, the gun produced a radially inward electric field that countered the usual FRC spin-up and mitigated the dangerous n = 2 rotational instability without multipole magnetic fields. Better plasma centering was also obtained, presumably from line-tying to the gun electrodes. Second, longer (up to 2.5 ms) FRC lifetimes, with improved FRC flux confinement and rotational stability, were obtained with perpendicular (to B) neutral beam injection. Third, a factor 2 improvement of the FRC particle and global energy confinement times was obtained. These exciting new results will be detailed. [Preview Abstract] |
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