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 YO6: Fast Ignition and Electron Transport |
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Chair: Chris McGuffey, University of California, San Diego Room: 201/202 |
Friday, November 20, 2015 9:30AM - 9:42AM |
YO6.00001: Controlling the Angular Distribution of Fast Electron Beams using Magnetic Conical Guided generated by Engineered Resistivity Gradients Alex Robinson, Holger Schmitz, Max Tabak In recent work [1,2] we have shown that a solid target consisting of a wire-like element with an inverse conical taper embeded in less resistive (lower $Z$) material will confine and guide laser-generated fast electrons due to the resistive generation of an azimuthal magnetic field around the guide element, {\em and} progressively reduce the angular distribution/spread of the fast electron beam. The reduction of the angular spread being the novel element of this recent work. The reduction in the angular spread comes from the fast electrons undergoing specular, but oblique reflections from the magnetic field (which follows the inverse conical taper). In our more recent studies we have investigated the extent to which the shrinkage in the angular spread might be optimized and the potential applications this might lead to. \\[4pt] [1] A.P.L.Robinson, H.Schmitz, J.S.Green, C.P.Ridgers, and N.Booth PPCF 57 064004 (2015) \\[0pt] [2] A.P.L.Robinson, H.Schmitz, J.S.Green, C.P.Ridgers, N.Booth, and J.Pasley PoP 22 043118 (2015) [Preview Abstract] |
Friday, November 20, 2015 9:42AM - 9:54AM |
YO6.00002: Optimization of Cone Wall Thickness to Reduce High Energy Electron Generation for Fast-Ignition Scheme Sadaoki Kojima, Zhang Zhe, Hiroshi Sawada In Fast Ignition Inertial Confinement Fusion, optimization of relativistic electron beam (REB) accelerated by a high-intensity laser pulse is critical for the efficient core heating. The high-energy tail of the electron spectrum is generated by the laser interaction with a long-scale-length plasma and does not efficiently couple to a fuel core. In the cone-in-shell scheme, long-scale-length plasmas can be produced inside the cone by the pedestal of a high-intensity laser, radiation heating of the inner cone wall and shock wave from an implosion core. We have investigated a relation between the presence of pre-plasma inside the cone and the REB energy distribution using the Gekko XII and 2kJ-PW LFEX laser at the Institute of Laser Engineering. The condition of an inner cone wall was monitored using VISAR and SOP systems on a cone-in-shell implosion. The generation of the REB was measured with an electron energy analyzer and a hard x-ray spectrometer on a separate shot by injecting the LFEX laser in an imploded target. The result shows the strong correlation between the preheat and high-energy tail generation. Optimization of cone-wall thickness for the fast-ignition will be discussed. [Preview Abstract] |
Friday, November 20, 2015 9:54AM - 10:06AM |
YO6.00003: Laser ablation and target acceleration under the strong magnetic field H. Nagatomo, K. Matsuo, J. Breil, P. Nicolai, J-L. Feugeas, T. Asahina, A. Sunahara, T. Johzaki, S. Fujioka, T. Sano, K. Mima Various discussion and experiments have been made about the laser plasma phenomena under the strong magnetic field recently. One of the advantage is guiding electron beam for heating core plasma in last phase of Fast Ignition scheme. However, the implosion dynamics in FI is influenced by the magnetic field due to the anisotropic of electron heat conduction [1]. Some simple experiments where target is accelerated by laser driven ablation under the strong magnetic field were conducted to benchmark the simulation code. Related to the experiment, we focus on the early stage of the acceleration in this study. 2-D radiative MHD code (PINOCO-MHD) is used for the simulation. In the simulation magnetic field transport, diffusion and Braginskii coefficient for electron heat conduction are taken account. In preliminary simulation result suggests that the magnetic pressure may have an influence on the target surface and/or ablated plasma at very early phase. The effect of the magnetic pressure is very sensitive to the vacuum, initial and boundary conditions, and they should be treated carefully. These numerical conditions will be discussed as well.\\[4pt] [1] H. Nagatomo \textit{et al. Nuclear Fusion (to be published.)} [Preview Abstract] |
Friday, November 20, 2015 10:06AM - 10:18AM |
YO6.00004: Experimental study of transport of relativistic electron beams in strong magnetic mirror field Shohei Sakata, Kotaro Kondo, Mathiu Bailly-Grandvaux, Claudio Bellei, Joao Santos Relativistic electron beams REB produced by ultra high intense laser pulses have generally a large divergence angle that results in degradation of energy coupling between the REB and a fuel core in the fast ignition scheme. Guiding and focusing of the REB by a strong external magnetic field was proposed to achieve high efficiency. We investigated REB transport through 50 $\mu$m or 250 $\mu$m thick plastic foils CuI doped under external magnetic fields, in magnetic mirror configurations of 1.2 or 4 mirror ratio. The experiment was carried out at the GEKKO XII and LFEX laser facility. Spatial pattern of the REB was measured by coherent transition radiation and/or Cu Ka x ray emission from the rear surface of the foil targets. Strong collimation of the REB by the external magnetic field was observed with 50 $\mu$m thick plastic targets, while the REB scattered in 250 $\mu$m thick targets. The experimental results are compared with computer simulations to understand the physical mechanisms of the REB transport in the external magnetic field. [Preview Abstract] |
Friday, November 20, 2015 10:18AM - 10:30AM |
YO6.00005: Magnetic Guiding of Electron Beam in Imploded Spherical Solid Targets Tomoyuki Johzaki, Yasuhiko Sentoku, Hideo Nagatomo, Atsushi Sunahara, Hitoshi Sakagami, Shinsuke Fujioka, Hiroyuki Shiraga, Takuma Endo In fast ignition, the large divergence of electron beam is one of the most critical issues for efficient core heating. For improving the efficiency in FIREX project, we proposed the electron beam guiding by externally applied kT-class longitudinal magnetic fields. The 2D collisional PIC simulations [1] showed that the electron beam can be successfully focused by the moderately-converging fields (mirror ratio RM $<$ 20). On the other hand, in the implosion simulation [2] for a cone-attached CD shell target with B-field, the mirror ratio reaches RM $>$ 100 at the maximum compression, which is too high for efficient guiding. Recently, we introduced a spherical solid target, where the mirror ratio is moderate since the density compressibility stays low ($\sim$ 30) and the magnetic-field compressibility will also be low. In the conference, we will show the integrated simulation results for core heating by fast electron beam with large beam divergence under the compressed core and magnetic fields formed through implosion of a solid spherical target. [1] T. Johzaki et al., Nucl. Fusion 55, 053022 (2015). [2] H. Nagatomo et al., to be published to Nucl. Fusion. [Preview Abstract] |
Friday, November 20, 2015 10:30AM - 10:42AM |
YO6.00006: Investigation of resistive guiding of fast electrons in ultra-intense laser-solid interactions James Green, Nicola Booth, Alex Robinson, Kate Lancaster, Chris Murphy, Chris Ridgers A key issue in realising the development of a number of high-intensity laser-plasma applications is the critical problem of fast electron divergence. Previous experimental measurements have indicated that the electron divergence angle is considerable at relativistic intensities ($>$ $10^{18} Wcm^{-2}$) and that self-pinching of the electron beam will not be sufficient to produce the collimated propagation that is required for applications such as WDM studies or bright, short-pulse X-ray sources. A number of concepts have been proposed to improve fast electron collimation, with one promising approach being to exploit resistivity gradients inside targets to magnetically guide fast electrons. Here we present experimental work using a novel conical target geometry that uses a high/low Z interface to produce such guiding. A range of target designs have been tested using the Vulcan Petawatt laser to investigate improvements in fast electron transport and collimation. Preliminary results will be presented from a number of complementary diagnostics in order to assess the degree and robustness of the focusing mechanism. [Preview Abstract] |
Friday, November 20, 2015 10:42AM - 10:54AM |
YO6.00007: Resistivity and anisotropic return currents in warm dense plasmas Nigel Woolsey, Nicola Booth, A. Robinson, P. Hakel, R. Clarke, R. Dance, D. Doia, L. Gizzi, G. Gregori, P. Koester, L. Labate, B. Li, M. Makita, R. Mancini, J. Pasley, P. Rajeev, D. Riley, E. Wagenaars, J. Waugh In an ultra-intense laser interaction with a solid, the electrons from the hot plasma are accelerated by the laser streaming into the solid behind, creating a dense plasma in the bulk. This provides a laboratory for creating warm dense matter in a parameter range where the material resistivity and equation of states are complex and mostly untested. Here we describe an experimental study of electron transport in a low atomic number (plastic) material at solid density and temperatures of 200 eV. The plastic is doped with sulphur as a diagnostic tracer to enable the observation of emission spectra. Through observing high positive polarisation in this emission it is possible to infer \textit{in situ} anisotropic currents driving the heat transport. Matching the current anisotropy enables tests of resistivity models in these complex plasmas. Results show that the background resistivity at these conditions is high than expected from commonly used models. [Preview Abstract] |
Friday, November 20, 2015 10:54AM - 11:06AM |
YO6.00008: Laboratory measurements of the resistivity of warm dense plasmas Nicola Booth, Alex Robinson, Peter Hakel, Ginaluca Gregori, Pattathil Rajeev, Nigel Woolsey In this talk we will present a method for studying material resistivity in warm dense plasmas in the laboratory in which we interrogate the microphysics of the low energy electron distributions associated with an anisotropic return current. Through experimental measurements of the polarization of the Ly-$\alpha $ doublet emission (2$s_{1/2}$--2$p_{1/2}$,$_{3/2}$ transitions) of sulphur, we determine the resistivity of a sulphur-doped plastic target heated to warm dense conditions by an ultra-intense laser at relativistic intensities, $I \approx 5 \times 10^{20}$Wcm$^{-2}$. We describe a method of exploiting classical x-ray scattering to separately measure both the $\pi $- and $\sigma $- polarizations of Ly-$\alpha _{1}$ spectral emission in a single shot. These measurements make it possible to explore fundamental material properties such as resistivity in warm and hot dense plasmas through matching plasma physics modelling to atomic physics calculations of the experimentally measured large, positive, polarisation. [Preview Abstract] |
Friday, November 20, 2015 11:06AM - 11:18AM |
YO6.00009: Proton radiography of petawatt-driven channel formation in a plasma gradient Matthew Hill, Nathan Sircombe, Martin Ramsay, Colin Brown, Lauren Hobbs, Peter Allan, Steven James, Peter Norreys, Naren Ratan, Luke Ceurvorst Channel formation by ultra-intense laser pulses in underdense plasmas is a challenging simulation problem with direct relevance to many areas of current research. Recent experiments at the Orion laser facility have used high-energy proton radiography (\textgreater 40 MeV) driven by a 1$\omega $ petawatt beam to directly probe the interaction of another petawatt beam with a well-characterized plasma density gradient. The interaction plasma was generated using a 3$\omega $ long-pulse beam and diagnosed using a 2$\omega $ optical probe, simultaneously imaged onto four gated optical imagers and two streak cameras. The unique capabilities of the Orion facility allowed a comparison of the channels generated by intense 1$\omega $ (1 $\mu$m, 100-500 J, 0.6 ps, 10$^{21}$ W/cm$^{2}$, f/3 parabola) and 2$\omega $ (0.5 $\mu$m, 100 J, 0.6 ps, 10$^{20}$ W/cm$^{2}$, f/6 parabola) pulses. Proton radiographs of these channels are presented along with PIC simulations performed using the EPOCH code, supported by K-$\alpha $ measurements of hot electron beam divergence and magnetic spectrometer data. Together these provide a solid foundation for improvements to hydrodynamic and PIC simulations, further developing the predictive capabilities required to optimize future experiments. [Preview Abstract] |
Friday, November 20, 2015 11:18AM - 11:30AM |
YO6.00010: In-target electron thermalization by the Weibel instability during intense irradiation of a thin aluminum foil J. Fuchs, C. Ruyer, B. Albertazzi, L. Lancia, V. Dervieux, P. Antici, J. Bocker, S.N. Chen, M. Nakatsutsumi, L. Romagnani, R. Shepherd, M. Swantusch, M. Borghesi, O. Willi, H. Pepin, M. Grech, C. Riconda, L. Gremillet Proton-radiography of the electromagnetic fields developing after irradiation of a 3$\mu$m-thick Al foil by a high-intensity laser($5 \times 10^{19}$W.cm$^{-2}$, 700fs, 8$\mu$m focal spot) was performed at the Titan facility. The obtained radiographs evidence filamentary structures which develop inside the dense target, $300\mu$m away from the focal spot, a few picoseconds after the laser drive. We will demonstrate that the radiographs' structures are due to magnetic fields triggered by the so-called Weibel instability, inside the dense target. For this purpose, large scale particle-in-cell simulations of hot electrons thermalization in a dense, cold and collisional target have been performed. They demonstrate the ability of the laser-heated electrons to sustain a strong temperature anisotropy during their relaxation in the thin foil. This hot electron anisotropy results in a Weibel instability, thus triggering magnetic fluctuations of spectrum consistent with the experiment over 10 picoseconds. [Preview Abstract] |
Friday, November 20, 2015 11:30AM - 11:42AM |
YO6.00011: Study of Pre-Plasma Effects on Fast Electron Generation and Transport using the Split Pulse Titan Laser J. Peebles, C.M. Krauland, C. McGuffey, A. Sorokovikova, R. Hua, M.S. Wei, S. Kerr, C. Curry, H. Sio, P. Forestier-Colleoni, J. Santos, H.S. McLean, S. Krasheninnikov, F.N. Beg Relativistic laser plasma interactions (LPI) could facilitate interesting and useful applications, such as table top particle acceleration and high energy K-alpha and gamma ray sources. In recent experiments it has been shown that the presence of an underdense, pre-formed plasma at the target surface has a significant heating effect. PIC simulations have shown that an electrostatic potential well forms on the target surface in this pre-plasma, which traps electrons and allows them to be excited to very high energy. Here we present results from an experiment conducted on the high intensity Titan laser at the Jupiter Laser Facility to further examine the role of pre-plasma in electron heating. We utilized the split beam, short pulse capability of the Titan system to generate and diagnose an interaction via proton radiography. The region was altered with a controlled pre-plasma generated by a wide focus, long pulse beam with variable energy. These experiments show that in the presence of pre-plasma, a hotter secondary population of electrons was generated. This work performed under the auspices of the US DOE Office of Sciences Program under contracts DE-NA0001858 [Preview Abstract] |
Friday, November 20, 2015 11:42AM - 11:54AM |
YO6.00012: Hot-Electron and Strong-Shock Generation at Shock-Ignition--Relevant Laser Intensities W. Theobald, R. Betti, R. Nora, W. Seka, M. Lafon, D.T. Michel, C. Stoeckl, A. Casner, J. Peebles, F.N. Beg, X. Ribeyre, A. Vallet, M.S. Wei The effect of hot electrons on the formation of spherical shocks in solid targets was studied in direct-illumination experiments on OMEGA at incident laser intensities of up to $6 \times 10^{15}$ W/cm$^{2}$. The experiments investigated the interaction physics in various ablator materials (Be, C, CH, and SiO$_{2}$) and under various beam-focusing conditions, which are relevant to developing a shock-ignition target design for the National Ignition Facility. The hot-electron production and the temperature of the distribution varied with the focal spot and beam overlap with values between 40 to 90 keV and instantaneous conversion efficiencies of laser power into hot-electron power of up to $\sim$ 15\%. A significant increase in hot-electron population was observed with CH ablators that was correlated with higher shock strength, exceeding 400 Mbar in the ablation layer and reaching Gbars upon convergence in the center of the spherical target. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944 and DE-FC02-04ER54789 (Fusion Science Center). [Preview Abstract] |
Friday, November 20, 2015 11:54AM - 12:06PM |
YO6.00013: ABSTRACT WITHDRAWN |
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