Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session UI2: Laser Wakefield Acceleration and Solid Target Interactions |
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Chair: Michael Downer, University of Texas Room: Centennial I |
Thursday, November 5, 2009 2:00PM - 2:30PM |
UI2.00001: Controlled electron injection in laser wakefield accelerators using axially tailored plasmas Invited Speaker: Controlling injection of electrons in laser plasma accelerators (LPA's) is crucial for improving the beam quality and enabling applications such as free electron lasers (FEL's). In addition, techniques are needed to control the amount of charge, energy and energy spread. To date, LPA's have typically operated in a highly nonlinear regime in which electrons are self-injected into a laser-excited plasma density wave. Although percent level energy spread beams have been demonstrated experimentally [1-4], production of lower energy spread beams will require accurate control of the injection process. In order to avoid self-trapping, an LPA would have to operate with lower wake amplitude, whether linear or non-linear. This also necessitates the use of a laser guiding structure to overcome diffraction of the laser beam. Such guiding structures have been obtained by transversely shaping the plasma density profile and they have successfully been used in experiments using laser-produced [2] or capillary-based channels [4]. In this talk, experimental results are presented that demonstrate the use of a longitudinally tailored plasma density profile in a capillary discharge waveguide to control trapping, significantly improving LPA performance. A gas jet was embedded in the capillary to locally alter the plasma density. It was found that electrons can be trapped and accelerated to hundreds of MeV using plasma densities in the capillary lower than in previous experiments, where no stable self-trapped electron beams were obtained in previous experiments [5]. It is found that using a longitudinally tailored density profile improves and increases control over electron beam properties. \\[4pt] [1] Mangles et al., Nature 431, 535 (2004)\\[0pt] [2] Geddes et al., Nature 431, 538 (2004)\\[0pt] [3] Faure et al., Nature 431, 541 (2004)\\[0pt] [4] Leemans et al., Nat. Phys. 2, 696 (2006)\\[0pt] [5] Nakamura et al., Phys. Plasmas 14, 056708 (2007) [Preview Abstract] |
Thursday, November 5, 2009 2:30PM - 3:00PM |
UI2.00002: Laser Wakefield Acceleration in the Self-Guided Regime Invited Speaker: Recent experimental results of laser wakefield acceleration demonstrate self-guiding as means to achieve GeV scale electron energies. Experimental results using Helium gas jet and gas cell targets ranging in length from 3 mm to 14 mm produced electron energies to beyond 700 MeV. Energy measurements relied on a unique two-screen method to eliminate error due to angular deviation of the electron beam. To achieve such high-energy electrons in the self-guided regime, a 200 TW 60 fs laser pulse was focused to a spot size of 15 microns and propagated through underdense plasmas with densities ranging from 10$^{18}$ to 10$^{19 }$cm$^{-3}$. The power threshold for self-trapping of electrons was found to be a strong function of the laser pulse power compared with the critical power for relativistic self-focusing in plasmas. Full 3D Particle-In-Cell simulations show excellent agreement with experimental results. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and a Department of Energy Grant No. DEFG02-92ER40727 and was partially funded by the Laboratory Directed Research and Development Program under tracking code 06-ERD-056. [Preview Abstract] |
Thursday, November 5, 2009 3:00PM - 3:30PM |
UI2.00003: Boosted frame PIC simulations of LWFA: ultra-fast modeling of current experiments and first studies of acceleration towards the energy frontier Invited Speaker: The development of new laser systems, in the 10PW range, will push Laser Wakefield Accelerators (LWFA) to a new qualitative regime, for which theoretical scalings predict the possibility to accelerate electron bunches close to the energy frontier, with self-injected electrons in excess of 10 GeV, and above 50GeV bunches with externally injected electrons. As in the past, numerical simulations will certainly play an important role in testing, probing and optimizing the physical parameters and setup of these upscale experiments. The distances involved in these numerical experiments, however, are very demanding in terms of computational resources, so that three-dimensional fully kinetic simulations are not yet possible to (easily) accomplish. Following the work on optimized Lorentz frames by J.-L. Vay [PRL 98, 130405 (2007)], the Lorentz transformation for a boosted frame was implemented in OSIRIS [R. A. Fonseca et al, LNCS 2329, III-342 (Springer-Verlag, 2002)], leading to a dramatic change in the computational resources required to model LWFA. The critical implementation details will be described, and the main difficulties discussed. Quantitative benchmarks will be presented between boosted frame and laboratory frame simulations, and also with experimental results from Imperial College and Lawrence Livermore National Laboratory, with emphasis on the boosted frame scheme as a tool for faster design and modeling of current experiments. Finally, simulations for the scenarios possible with the next generation of laser systems will be presented, including the confirmation of electron bunch acceleration to the energy frontier. [Preview Abstract] |
Thursday, November 5, 2009 3:30PM - 4:00PM |
UI2.00004: Measurements and modeling of radiation from laser wakefield accelerators Invited Speaker: Electron beams produced by laser wakefield acceleration are characterized by having relatively high current density in short and quasi-monoenergetic bunches. Oscillations of these electrons, in the electromagnetic fields of electron plasma cavities, created by laser driven ponderomotive expulsion, can lead to extremely bright sources of x-rays. Radiation is also emitted in the form of coherent scattering of the background electrons and other emission from the highly dynamic electron motions in the field structure. Presented here is a summary of recent experimental measurements and numerical modeling towards laser wakefield driven compact radiation sources, conducted at the Center for Ultrafast Optical Science at the University of Michigan. Experiments, on the 30~fs, 300~TW {\sc Hercules} laser, studying tunneling ionization assisted electron trapping, demonstrate an enhancement to the electron beam through the addition of a high-Z contaminant to the Helium background gas. The nature of the effect of the ionization depends on the energy level structure of the contaminant. Electron beams are also measured and characterized from structured gas density profiles, including a density step- function to increase trapping. Measurements of x-rays produced by betatron oscillations in the wakefield demonstrate x-rays with peak brightness comparable to third generation synchrotron sources, from a micron-scale source size. Experimental measurements of side-scattered light from the wakefield interaction yields information on the evolution of the pulse and plasma bubble. Numerical models of radiation emission are compared with the experimental results. These include calculations using the radiation code {\it Radampeltrac}, which indicate the role of the laser pulse in modifying the radiation distribution. [Preview Abstract] |
Thursday, November 5, 2009 4:00PM - 4:30PM |
UI2.00005: Generation and Transport of Hot Electrons in Cone-Wire Targets Invited Speaker: We present results from a series of experiments where cone-wire targets in various configurations were employed both to assess hot electron coupling efficiency, and to reveal the source temperature of the hot electrons. Initial experiments were performed on the Vulcan petawatt laser at the Rutherford Appleton Laboratory and Titan laser at the Lawrence Livermore National Laboratory. Results with aluminum cones joined to Cu wires of diameters from 10 to 40 $\mu $m show that the laser coupling efficiency to electron energy within the wire is proportional to the cross sectional area of the wire. In addition, coupling into the wire was observed to decrease with the laser prepulse and cone-wall thickness. More recently, this study was extended, using the OMEGA EP laser. The resulting changes in coupling energy give indications of the scaling as we approach FI-relevant conditions. Requirements for FI scale fast ignition cone parameters: tip thickness, wall thickness, laser prepulse and laser pulse length, will be discussed. In collaboration with T. Yabuuchi, T. Ma, D. Higginson, H. Sawada, J. King, M.H. Key, K.U. Akli, Al Elsholz, D. Batani, H. Chen, R.R. Freeman, L. Gizzi, J. Green, S. Hatchett, D. Hey, P. Jaanimagi, J. Koch, K. L. Lancaster, D.Larson, A.J. MacKinnon, H. McLean, A. MacPhee, P.A. Norreys, P.K Patel, R. B. Stephens, W. Theobald, R. Town, M. Wei, S. Wilks, Roger Van Maren, B. Westover and L. VanWoerkom. [Preview Abstract] |
Thursday, November 5, 2009 4:30PM - 5:00PM |
UI2.00006: Integrated Kinetic Simulation of Laser-Plasma Interactions, Fast-electron Generation and Transport in Fast Ignition Invited Speaker: We present new results on the physics of short-pulse laser-matter interaction of kilojoule-picosecond pulses at full spatial and temporal scale, using a new approach that combines a 3D collisional electromagnetic Particle-in-Cell code with an MHD-hybrid model of high-density plasma. In the latter, collisions damp out plasma waves so the displacement current can be neglected; and an Ohm's law with electron inertia effects neglected determines the electric field. In addition to yielding orders of magnitude in speed-up while avoiding numerical instabilities, this allows us to model the whole problem in a single unified framework: the laser-plasma interaction at sub-critical densities, energy deposition at relativistic critical densities, and fast-electron transport in solid densities. Key questions such as the multi-picosecond temporal evolution of the laser energy conversion into hot electrons, the impact of return currents on the laser-plasma interaction, and the effect of self-generated electric and magnetic fields on electron transport will be addressed. We will report applications to current experiments at LLNL's Titan laser and Omega EP, and to a Fast-Ignition point design for forthcoming experiments on NIF-ARC. [Preview Abstract] |
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