Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session CO7: Laser Wakefield Accelerators |
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Chair: Wim Leemans, Lawrence Berkeley National Laboratory Room: Galerie 6 |
Monday, October 27, 2014 2:00PM - 2:12PM |
CO7.00001: Direct Laser Acceleration of Electrons in a Plasma Wakefield with Ionization Injection J.L. Shaw, F.S. Tsung, N. Lemos, K.A. Marsh, N. Vafaei-Najafabadi, W.B. Mori, C. Joshi We show through experiments and supporting simulations the role of direct laser acceleration (DLA) of electrons in a plasma accelerator when ionization injection of electrons is employed to inject charge into the laser-produced wake. If the laser pulse is intense enough to expel most of the plasma electrons but is nevertheless long enough to overlap the electrons trapped in the first accelerating potential well (bucket) of the wakefield, then the betatron oscillations of the electrons in the plane of the laser polarization in the presence of an ion column can lead to an energy transfer from the laser pulse to the electrons. DLA can be a major contributor to the maximum electron energy, and the energy gain due to DLA can exceed that due to laser wakefield acceleration for certain laser and plasma parameters. [Preview Abstract] |
Monday, October 27, 2014 2:12PM - 2:24PM |
CO7.00002: GeV Electrons due to a Transition from Laser Wakefield Acceleration to Plasma Wakefield Acceleration M.Z. Mo, P.-E. Masson-Laborde, A. Ali, S. Fourmaux, P. Lassonde, J.-C. Kieffer, W. Rozmus, D. Teychenn\'{e}, R. Fedosejevs The Laser Wakefield Acceleration (LWFA) experiments performed with the 200 TW laser system located at the Canadian Advanced Laser Light Source facility at INRS, Varennes (Qu\'{e}bec) observed at relatively high plasma densities (1x10$^{19}$cm$^{-3}$) electron bunches of GeV energy gain, more than double of the predicted energy using Lu's scaling law. This energy boost phenomena can be attributed to a transition from LWFA regime to a plasma wakefield acceleration (PWFA) regime. In the first stage, the acceleration mechanism is dominated by the bubble created by the laser in the regime of LWFA, leading to an injection of a large number of electrons. After propagation beyond the depletion length, where the laser pulse is depleted and it can no longer sustain the bubble anymore, the dense bunch of high energy electrons propagating inside the bubble will drive its own wakefield in the PWFA regime that can trap and accelerate a secondary population of electrons up to the GeV level. 3D particle-in-cell simulations support this analysis, and confirm the scenario. [Preview Abstract] |
Monday, October 27, 2014 2:24PM - 2:36PM |
CO7.00003: Single-shot visualization of evolving laser wakefields using an all-optical streak camera Zhengyan Li, Hai-En Tsai, Xi Zhang, Chih-Hao Pai, Yen-Yu Chang, Rafal Zgadzaj, Xiaoming Wang, Vladimir Khudik, Gennady Shvets, Michael Downer We visualize ps-time-scale evolution of an electron density bubble, a wake structure created in atmospheric density plasma by an intense ultrashort laser pulse, from the phase ``streak'' that the bubble imprints onto a probe pulse that crosses its path obliquely. Phase streaks, recovered in one shot using frequency-domain interferometry, reveal formation, propagation and coalescence of the bubble within a 3 mm long ionized helium gas target. 3D particle-in-cell (PIC) simulations validate the observed density-dependent bubble evolution, and correlate it with generation of a quasi-monoenergetic $\sim$ 100 MeV electron beam. The results provide a basis for understanding optimized electron acceleration at plasma density ne $\sim$ 2e19 cm$^{-3}$, at which the bubble formed and persisted until the jet exit, enabling acceleration over a distance slightly exceeding the dephasing length. In contrast, at lower density, electrons accelerated inefficiently due to weak laser self-focusing and late bubble formation. At higher density, overly strong self-focusing also led to low quality electrons due to early bubble formation and strong dephasing. Bubble coalescence due to beam loading further degraded electron acceleration. [Preview Abstract] |
Monday, October 27, 2014 2:36PM - 2:48PM |
CO7.00004: Manipulating ionization-injection trapping in laser wakefield accelerators Nuno Lemos, Jessica Shaw, C.J. Zhang, K.A. Marsh, Chan Joshi Experiments have shown that when using tunneling ionization as an injection mechanism in laser wakefield acceleration (LWFA), electrons can be trapped and accelerated using roughly four times less laser power than required to self-trap electrons. Using the three-dimensional (3D) scaling laws for LWFAs in the blowout regime, it was found that injecting electrons directly into the wakefield significantly increases the potential difference for the electron to become trapped. This study further explores this injection mechanism in order to lower the electron energy spread and increase the available normalized wake potential. Two and 3D particle-in-cell simulations show that by changing the laser pulse duration and plasma density, one can control the trapping condition and energy spread. [Preview Abstract] |
Monday, October 27, 2014 2:48PM - 3:00PM |
CO7.00005: A two-stage --injector-accelerator-- plasma wakefield accelerator at FACET Navid Vafaei-Najafabadi, C.E. Clayton, K.A. Marsh, W. An, W.B. Mori, C. Joshi, W. Lu, E. Adli, S. Corde, J. Frederico, S.Z. Green, M. Litos, S. Gessner, D. Walz, C.I. Clarke, M.J. Hogan, V. Yakimenko, P. Muggli Ionization injection is important for a beam-driven plasma-wakefield-accelerator because it can be used to embed electrons within a highly-relativistic wake. Furthermore, the placement of an acceleration stage following such an injector opens the possibility of controlling the charge, emittance, and energy spread of the beam. Such two-stage accelerator experiments have been carried out at the FACET facility at SLAC. The ionization injection stage is formed by a 10 cm density up-ramp of Li vapor, which overlaps with a density down-ramp of He. The He atoms provide a source of electrons for injection into the wake that is created by the 3nC, 20 GeV FACET electron beam. The injected electrons are then accelerated by the wakefield generated in either a 30 cm, 2.5x10$^{17}$ cm$^{-3}$ or a 130 cm, 8x10$^{16}$ cm$^{-3}$ Li plasma. Narrow-divergence electron bunches with energies as high as 30 GeV attributable to He electrons are observed. [Preview Abstract] |
Monday, October 27, 2014 3:00PM - 3:12PM |
CO7.00006: First Results from Hollow-Channel Plasma Wakefield Acceleration Experiments with Positron Beams at FACET Spencer Gessner, Erik Adli, James Allen, Christine Clarke, Chris Clayton, Joel Frederico, Selina Green, Mark Hogan, Chan Joshi, Michael Litos, Ken Marsh, Sebastien Corde, Navid Vafaei, Vitaly Yakimenko, Weiming An We report on the first results from a hollow-channel plasma wakefield acceleration experiment using positron beams at FACET. A meter-scale plasma channel is created by field ionizing lithium vapor using an intense laser pulse that has a transverse J8 Bessel profile. The plasma channel is roughly 600 $\mu$m in diameter and has unionized vapor at its center. A 20.35 GeV positron beam with spot size of roughly 50 $\mu$m was sent through the channel. We observed the transverse beam profile while varying the position of the beam relative to the channel. Our measurements clearly indicate that a plasma channel was formed. We characterize the strength of the wake and discuss plans for subsequent experiments. [Preview Abstract] |
Monday, October 27, 2014 3:12PM - 3:24PM |
CO7.00007: Simulation studies on electron beam formation in high density plasmas in Laser Wake Field Acceleration Bhavesh Patel, Chandrashekhar Joshi In recent experimental work based on Laser Wakefield Acceleration, Rao et al. [1] have demonstrated production of monoenergetic, 35 MeV electron bunch using 3 TW pulse and high density, 5.8x10cm$^{-3}$ plasma. The electron beam formation in such scenario relies greatly on physical processes like relativistic self-focusing and modulation instability. Further, in view of the fact that the laser pulse has a pulse-length several times the plasma wavelength, it may be surmised that the beam electrons may gain energy by direct laser acceleration in addition to that from the longitudinal fields. In present work, laser wakefield acceleration and electron bunch formation for this relatively low intensity laser pulse and a high density plasma is studied using particle-in-cell code OSIRIS. The objective here is to decipher the role of various physical mechanisms responsible for production of the surprisingly narrow energy electron bunch.The electrons are trapped only after the laser pulse is longitudinally compressed such that there is little overlap between the trapped electrons and the laser field. Thus the acceleration of beam electrons is due to the wakefield. \\[4pt] [1] B.S. Rao. et al. High-quality stable electron beams from laser wakefield acceleration in high density plasma. Phys. Rev. ST Accel. Beams 17, 011301 (2014). [Preview Abstract] |
Monday, October 27, 2014 3:24PM - 3:36PM |
CO7.00008: Optical Probing of Meter Scale Beam Driven Plasma Wakefield Accelerator Rafal Zgadzaj, Zhengyan Li, M.C. Downer, Spencer Gesner, Sebastien Corde, Mike Litos, Christine Clarke, Margaux Schmeltz, James Allen, Selina Green, Mark Hogan, Vitaly Yakimenko We report results of optical visualization experiment at the FACET/SLAC user facility. Experiment E224, carried out in parallel to the ongoing e-beam driven wakefield experimental campaign at FACET, has the aim of optically observing both the short term and long term plasma structure produced by the e-beam driver. The SLAC plasma wakefield experiments have demonstrated the highest energy gain to date and continue work on further optimization. Direct visualization of the plasma wake structure would aid in the understanding of the dynamics of the beam plasma interaction and acceleration, and its optimization. It also would serve to benchmark simulations results which have been so far the main window into visualizing the beam plasma interaction. We will describe the optical probing geometry used in this initial run, a variation of a method previously developed in our group [1], as governed by the unique experimental challenges of the FACET beam driven experiments in their current configuration. We will discuss the current results, the limitations of the current experimental configuration, and the changes planned for future experiments. \\[4pt] [1] Z. Li, et al., ``Single-shot visualization of evolving, light-speed structures by multiobject-plane phase-contrast imaging,''Opt. Lett. 38, 5157-5160 (2013). [Preview Abstract] |
Monday, October 27, 2014 3:36PM - 3:48PM |
CO7.00009: Coherent phase space matching of staging plasma and traditional accelerator using longitudinally tailored plasma structure Xinlu Xu, Wei Lu, Warren Mori, Chan Joshi, Mark Hogan For the further development of plasma based accelerators, phase space matching between plasma acceleration stages and between plasma stages and traditional accelerator components becomes a very critical issue for high quality high energy acceleration and its applications in light sources and colliders. Without proper matching, catastrophic emittance growth in the presence of finite energy spread may occur when the beam propagating through different stages and components due to the drastic differences of transverse focusing strength. In this paper we propose to use longitudinally tailored plasma structures as phase space matching components to properly guide the beam through stages. Theoretical analysis and full 3-dimensional particle-in-cell simulations are utilized to show clearly how these structures may work in four different scenarios. Very good agreements between theory and simulations are obtained. [Preview Abstract] |
Monday, October 27, 2014 3:48PM - 4:00PM |
CO7.00010: Emittance Pressure Dominated Regimes for Resonant PWFA Experiments at SPARC Lab Alberto Marocchino, Stefano Atzeni, Enrica Chiadroni, Massimo Ferrario, Claudio Gatti, Pasquale Londrillo, Andrea Mostacci, Francesco Massimo, Luigi Palumbo, Andrea Rossi, Stefano Sinigardi Considerable interest has been shown in the last few years in compact plasma accelerators characterized by extremely high accelerating gradients generated, e.g., by high brightness particle beams. PWFA is currently under investigation at SPARC Lab test facility (Frascati, Italy). Despite 1D model are too simple and limited to catch the whole underlying physics, they offer a simple and fast tool to assess possible working points. We discuss how these models can be analytically modified to extend their validity in the quasi-non-linear regime to phenomenologically account for damping effects. We also present 3D PIC simulations for emittance pressure dominated regimes. We discuss how elongated bunch, with a transverse dimension smaller than the longitudinal dimension, suffer from being drawn into the self-generated bubble and seed the two-stream instability in the witness bunch. A possible mechanism to reduce such an effect consists in using emittance-dominated bunch that can contrast the self-focusing force produced by the surrounding bubble. [Preview Abstract] |
Monday, October 27, 2014 4:00PM - 4:12PM |
CO7.00011: High brightness electron beam generation through plasma density variation induced injection in beam or laser driven 3D nonlinear wakes Wei Lu, Fei Li, Xinlu Xu, Warren Mori, Chan Joshi High brightness electron beam generation is critical for the development of plasma wakefield accelerators. The generation of high current and low emittance electron beam through plasma density variation induced injection in charged beam or laser driven 3D nonlinear wakes is explored using full 3-dimensional particle-in-cell simulations. It is found out that the radial selection in the injection process and the transverse dynamics when the injected electrons move in the electron sheath of the wake determine the final beam quality, e.g., emittance, current and energy spread. Simulations show that brightness as high as 5 $\times$ 10$^{20}$Am$^{-2}$rad$^{-2}$ could be generated under proper condition. [Preview Abstract] |
Monday, October 27, 2014 4:12PM - 4:24PM |
CO7.00012: Electron trapping condition of transverse ellipsoidal bubble in laser wakefield accelerator MyungHoon Cho, YoungKuk Kim, MinSup Hur We present the condition of electron trapping in an ellipsoidal bubble in the LWFA, which is not well explained by the spherical bubble model. The formation of an ellipsoidal bubble, which is elongated transversely, frequently occurs when the spot size of the laser pulse is large compared to the plasma wavelength. First we introduce the relation between the bubble size and the field slope inside the bubble in longitudinal and transverse directions. Then we provide an ellipsoidal model of the bubble potential and investigate the electron trapping condition by numerical integration of the equations of motion. If the field slop in longitudinal direction reaches the maximum earlier than that in the transverse direction, the trapping condition is determined only by a transverse bubble radius. This gives a significantly less restrictive trapping condition than the spherical bubble model. The trapping condition is compared with three-dimensional particle-in-cell simulations and the electron trajectory in test potential simulation, from which we confirm the simulation result is consistent with the theoretical expectations. [Preview Abstract] |
Monday, October 27, 2014 4:24PM - 4:36PM |
CO7.00013: Positron acceleration in doughnut wakefields in the blowout regime Jorge Vieira, Jose Mendonca, Ricardo Fonseca, Luis Silva Most important plasma acceleration results were reached in the so called bubble or blowout regime. Although ideally suited for electron acceleration, it has been recognized that non-linear regimes are not adequate to accelerate positrons. New configurations enabling positron acceleration in non-linear regimes would therefore open new research paths for future plasma based collider configurations. In this work, we explore, analytically and through 3D OSIRIS simulations, a novel configuration for positron acceleration in strongly non-linear laser wakefield excitation regimes using Laguerre-Gaussian laser drivers to drive doughnut shaped wakefields with positron focusing and accelerating fields [J. Vieira and J.T. Mendonca, PRL 112 215001 (2014)]. We demonstrate that positron focusing-fields can be up to an order of magnitude larger than electron focusing in the spherical blowout regime. The amplitude of the accelerating fields is similar to the spherical blowout. Simulations demonstrate laser self-guiding and stable positron acceleration until the laser energy has been exhausted to the plasma. Other realisations of the scheme, using two Gaussian laser pulses, will also be explored. [Preview Abstract] |
Monday, October 27, 2014 4:36PM - 4:48PM |
CO7.00014: Laser pulse group velocity in electrically-discharged capillary waveguides Joost Daniels, Jeroen van Tilborg, Anthony Gonsalves, Carlo Benedetti, Carl Schroeder, Eric Esarey, Wim Leemans Plasma channels are critical in maintaining high intensity laser fields over extended distances, such as required in efficient laser-plasma accelerators (LPAs). In LPAs, the background plasma electron density is a critical parameter as it influences the dephasing length - the distance of optimum acceleration - as well as laser guiding, accelerating wakefield amplitude and particle injection. In this talk a novel method is presented that measures the group velocity in the plasma through two-pulse spectral interferometry, from which the on-axis electron density can be determined. We will show results on parabolic plasma channels created in discharged capillaries, for a range of pressures as relevant for current-day LPAs. The obtained knowledge on the group velocity allows us to improve the design of guiding channels and accelerator structures. [Preview Abstract] |
Monday, October 27, 2014 4:48PM - 5:00PM |
CO7.00015: Laser-Plasma Accelerator based compact, narrow bandwidth Thomson photon sources C.G.R. Geddes, S.G. Rykovanov, J.-L. Vay, A. Bonatto, C.B. Schroeder, E. Esarey, W.P. Leemans Compact, high-quality photon sources at MeV energies can be enabled by recent advances in Laser-Plasma Accelerator (LPA) beam quality (e.g. 1$\%$ level energy spread, low emittance) together with photon production strategies which take advantage of unique beam and plasma capabilities. LPA experiments will be reviewed which establish the basis for such a source. Simulations show that for electron beam parameters achieved in LPAs, plasma optics can be used to tailor beam divergence and minimize photon source bandwidth in a compact package. Source yield can be increased, for realistic laser parameters, via use of plasma channels to guide the scattering laser and/or control of laser pulse shape and chirp. The LPA can further be used to de-accelerate the electron beam after photon production to reduce undesired radiation. This is crucial to a laboratory or field operable source. The combination of these elements will be presented, towards a complete LPA-based high-flux photon source which is compact. [Preview Abstract] |
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