Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session KI2: Plasma Based Accelerators and Sources |
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Chair: Wim Leemans, Lawrence Berkeley National Laboratory Room: Rosen Centre Hotel Salon 3/4 |
Tuesday, November 13, 2007 3:00PM - 3:30PM |
KI2.00001: Laser Wakefield Structures and Electron Acceleration in Gas Jet and Capillary Discharge Plasmas Invited Speaker: Laser-driven plasma wakefield accelerators have the potential to become the next generation of particle accelerators because of the very high acceleration gradients. The beam quality from such accelerators depends critically on the details plasma wave spatial structures. In experiments at the University of Michigan it was possible in a single shot by frequency domain holography (FDH) to visualize individual plasma waves produced by the 40 TW, 30 fs Hercules laser focused to the intensity of 10$^{19}$ W/cm$^{2}$ onto a supersonic He gas jet [1]. These holographic ``snapshots'' capture the evolution of multiple wake periods, and resolve wavefront curvature seen previously only in simulations. High-energy quasi-monoenergetic electron beams for plasma density in the specific range 1.5$\times $10$^{19}\le $n$_{e}\le $3.5$\times $10$^{19}$ cm$^{-3 }$were generated [2]. The experiments show that the energy, charge, divergence and pointing stability of the beam can be controlled by changing n$_{e}$, and that higher electron energies and more stable beams are produced for lower densities. An optimized quasi-monoenergetic beam of over 300 MeV and 10 mrad angular divergence is demonstrated at a plasma density of n$_{e}$=1.5$\times $10$^{19}$ cm$^{-3}$. The resulted relativistic electron beams have been used to perform gamma-neutron activation of $^{12}$C and $^{63}$Cu and photo-fission of $^{238}$U with a record high reaction yields of $\sim $5x10$^{5}$/Joule [3]. Experiments performed with ablative capillary discharge plasma demonstrate stable guiding for laser power up to 10 TW with the transmission of 50{\%} and guided intensity of $\sim $10$^{17}$ W/cm$^{2}$. Study of the staged electron acceleration have been performed which uses ablated plasma in front of the capillary to inject electrons into the wakefield structures. \newline \newline [1] N. H. Matlis et. al., Nature Physics \textbf{2,} 749 (2006). \newline [2] A. Maksimchuk et. al., Journal de Physique IV \textbf{133}, 1123 (2006). \newline [3] S. A. Reed et. al., Appl. Phys. Lett. \textbf{89}, 231107 (2006). [Preview Abstract] |
Tuesday, November 13, 2007 3:30PM - 4:00PM |
KI2.00002: Ultra-high-order harmonic generation in cavitated plasmas Invited Speaker: High-harmonic generation (HHG) using short pulse lasers in gases is a compact method for producing ultrafast, coherent light, but has been limited to the soft x-ray and extreme-ultraviolet spectral regions. The energy of the HHG photons can be increased by increasing the laser intensity and/or the ionization potential of the atom (or ion). At high laser intensities, however, HHG is suppressed by ionization and plasma production, limiting the coherence length via plasma-induced phase slippage. Phase-matching to overcome the plasma-induced slippage has been a critical challenge to further development of HHG x-ray sources. In this talk a novel method for producing hard x-rays via HHG from highly-stripped ions using ultra-intense lasers is described. The method relies on electron cavitation and ion channel formation by the ponderomotive force of an ultra-intense laser pulse or the space-charge force of a relativistic (laser-plasma-accelerated) electron beam. An intense, short-pulse laser propagating in the electron-free ion cavity can produce laser harmonics. A counter-propagating laser pulse train is proposed for quasi-phase matching via periodic suppression of the longitudinal electron motion owing to the magnetic component of the nonlinear Lorentz force for relativistic laser intensities. This method enables the reach of HHG to be extended to the sub-$\textrm{\AA}$ regime. [Preview Abstract] |
Tuesday, November 13, 2007 4:00PM - 4:30PM |
KI2.00003: Interaction of intense ultrashort pulse lasers with clusters. Invited Speaker: The last ten years have witnessed an explosion of activity involving the interaction of clusters with intense ultrashort pulse lasers. Atomic or molecular clusters are targets with unique properties, as they are halfway between solid and gases. The intense laser radiation creates hot dense plasma, which can provide a compact source of x-rays and energetic particles. The focus of this investigation is to understand the salient features of energy absorption and Coulomb explosion by clusters. The evolution of clusters is modeled with a relativistic time-dependent 3D Molecular Dynamics (MD) model [1]. The Coulomb interaction between particles is handled by a fast tree algorithm, which allows large number of particles to be used in simulations [2]. The time histories of all particles in a cluster are followed in time and space. The model accounts for ionization-ignition effects (enhancement of the laser field in the vicinity of ions) and a variety of elementary processes for free electrons and charged ions, such as optical field and collisional ionization, outer ionization and electron recapture. The MD model was applied to study small clusters (1-20 nm) irradiated by a high-intensity (10$^{16}$-10$^{20}$ W/cm$^{2})$ sub-picosecond laser pulse. We studied fundamental cluster features such as energy absorption, x-ray emission, particle distribution, average charge per atom, and cluster explosion as a function of initial cluster radius, laser peak intensity and wavelength. Simulations of novel applications, such as table-top nuclear fusion from exploding deuterium clusters [3] and high power synchrotron radiation for biological applications and imaging [4] have been performed. The application for nuclear fusion was motivated by the efficient absorption of laser energy ($\sim $100{\%}) and its high conversion efficiency into ion kinetic energy ($\sim $50{\%}), resulting in neutron yield of 10$^{6}$ neutrons/Joule laser energy. Contributors: J. Davis and A. L. Velikovich. [1] G. M. Petrov, \textit{et al} \textit{Phys. Plasmas} \textbf{12} 063103 (2005); \textbf{13 }033106 (2006) [2] G. M. Petrov, J. Davis, \textit{European Phys. J. D} \textbf{41} 629 (2007) [3] G. M. Petrov, J. Davis, A. L. Velikovich, \textit{Plasma Phys. Contr. Fusion} \textbf{48} 1721 (2006) [4] G. M. Petrov, J. Davis, A. L. Velikovich, \textit{J. Phys.} B 39 4617 (2006) [Preview Abstract] |
Tuesday, November 13, 2007 4:30PM - 5:00PM |
KI2.00004: Increased Efficiency of Short-Pulse Laser Generated Proton Beams from Novel Flat-Top Cone Targets Invited Speaker: Ion-driven Fast Ignition (IFI) may have significant advantages over electron-driven FI (EFI) due to a large reduction in the ignitor beam and laser driver energy requirements. Recent experiments at the LANL Trident facility, using novel flat-top cones made by Nanolabz in Reno Nevada, have yielded a 4 fold increase in laser-ion conversion efficiency, a 13 fold increase in the number of ions above 10 MeV, and a two fold increase in the maximum proton energy as compared to Au flat-foil targets. If efficiencies scale with intensity, in accordance with flat-foils, then IFI would have an even bigger advantage over EFI. At a modest intensity of 10$^{19}$ W/cm$^{2}$ with 20 Joules in 600 fs protons with at least 30 MeV were observed from the cone targets. Particle in Cell (PIC) simulations show that the maximum cutoff energy could have been as high as 40 MeV. The simulations indicate that the observed energy and efficiency increase can be attributed to the cone's ability guide and focus the laser, allowing more laser-light to be absorbed into the electrons. The cone's geometry then funnels the electrons to the flat-top. The small size also limits the number of electrons, allowing more to be heated to high temperatures, creating a hotter, denser sheath. The PIC simulations elucidate the critical parameters in obtaining superior proton acceleration such as the dependence on laser contrast/preplasma-fill and longitudinal and transverse laser pointing. In addition, these cones have the potential to revolutionize ICF target design and fabrication via mass production. [Preview Abstract] |
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