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 TO6: Radiation Sources, Particle Beams, and Positron Production |
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Chair: Zulfikar Najmudin, Imperial College, UK Room: Galerie 3 |
Thursday, October 30, 2014 9:30AM - 9:42AM |
TO6.00001: X-ray radiation from a laser-wakefield accelerator in the self-modulated regime Felicie Albert, Bradley Pollock, John Ruby, Mickael Klem, Arthur Pak, Frederico Fiuza, Joseph Ralph, John Moody, Jessica Shaw, Nuno Lemos, Ken Marsh, Chris Clayton, Chan Joshi, Benjamin Galloway, William Schumaker, Siegfried Glenzer We will present recent experiments performed using the Titan laser (150 J, 1 ps) at the Jupiter Laser Facility, LLNL. When a 0.5-1 ps laser pulse with an intensity approaching 10$^{20}$ W/cm$^{2}$ is focused on a gas target (electron density 10$^{19}$ cm$^{-3}$), electrons can be accelerated via the self-modulated laser wakefield (SMLWF) regime and the direct laser acceleration (DLA) regime. In SMLWF acceleration, electrons are accelerated by the plasma wave created in the wake of the light pulse, whereas in DLA, electrons are accelerated from the interaction of the laser field with the focusing force of the plasma channel. In our experiments, the SMLWF mechanism dominates, (\textless 10$^{20}$ W/cm$^{2})$, and the transmitted laser spectrum exhibits intense Raman satellites which measured shifts depend on the electron plasma density. The high charge, $\sim$ 100 MeV electrons measured in our experiments are also a source of bright multi-keV x-ray beams of interest of future high energy density science applications. [Preview Abstract] |
Thursday, October 30, 2014 9:42AM - 9:54AM |
TO6.00002: Ultra-bright X-rays using Plasma Wigglers Jimmy Holloway, Peter Norreys, Riccardo Bartolini, Robert Bingham, John Nydell, Raoul Trines, Richard Walker, Matthew Wing Fourth generation light sources have advanced many fields of the natural sciences by providing extraordinary reductions in X-ray pulse lengths and increases in brightness. In this paper, we will present a novel numerical study, showing that existing third generation synchrotron light sources can produce X-ray pulses with equally unique properties, by undulating their electron beams within plasma wakefields. We have conducted a full scale two dimensional particle-in-cell study that shows that by micro-bunching a realistic electron beam longitudinally generates an X-ray pulse of comparable brightness to fourth generation free electron laser sources is possible. The production mechanism ensures the pulses are radially polarized on creation. We also demonstrate that the micro-bunched electron beam is also an effective wakefield driver, one that provides in itself another potential new cost-effective route to a reliable intense X-ray source. [Preview Abstract] |
Thursday, October 30, 2014 9:54AM - 10:06AM |
TO6.00003: Multi-GeV electron beam and high brightness betatron x-ray generation in recent Texas Petawatt laser-driven plasma accelerator experiments Xiaoming Wang, Rafal Zgadzaj, Neil Fazel, Zhengyan Li, Xi Zhang, Watson Henderson, Yen-Yu Chang, Rick Korzekwa, H.-E. Tsai, Hernan Quevedo, Gilliss Dyer, Erhard Gaul, Mikael Martinez, Aaron Bernstein, Michael Spinks, Joseph Gordan, Michael Donovan, Vladimir Khudik, Gennady Shvets, Todd Ditmire, Michael Downer Compact laser-plasma accelerators (LPAs) driven by petawatt (PW) lasers have produced highly collimated, quasi-monoenergetic multi-GeV electron bunches with $\sim$ 100 pC charge [1], which are promising sources of ultrafast x-rays. Here we report three recent advances in PW-LPA performance brought about by optimizing the focal volume of the Texas PW laser with a deformable mirror. First, we accelerated electrons up to 3 GeV with hundreds of pC over 1GeV and \textless 0.5mrad divergence. Second, we significantly improved shot-to-shot reproducibility (90{\%} shots \textgreater 1GeV, 10{\%} \textgreater 2GeV). Third, by introducing a double-peaked laser focus, creating a ``double bubble'' that subsequently merged [2], we significantly increased electron charge (0.5 nC) above 1 GeV, while producing brighter (10$^{22}$photon/mm$^{2}$/rad/0.1{\%}), harder (up to 30keV) betatron x-rays, characterized by a multi-metal filter pack and phase-contrast imaging. We observe evidence of dimuon production [3] by irradiating a high-Z target with this high-charge, GeV electron beam. [1] Wang \textit{et a}l., Nature Commun. \textbf{4}, 1988 (2013); Kim \textit{et al}., Phys. Rev. Lett. 11, 165002( 2013). [2] Wen \textit{et al}., Phys. Plasmas \textbf{17}, 103113 (2010). [3] Titov \textit{et al}., Phys. Rev.-ST AB \textbf{12}, 111301 (2009). [Preview Abstract] |
Thursday, October 30, 2014 10:06AM - 10:18AM |
TO6.00004: High-brightness, high-energy radiation generation from non-linear Thomson scattering of laser wakefield accelerated electrons W. Schumaker, Z. Zhao, A.G.R. Thomas, K. Krushelnick, G. Sarri, D. Corvan, M. Zepf, J. Cole, S.P.D. Mangles, Z. Najmudin To date, all-optical sources of high-energy ($>MeV$) photons have only been reported in the linear ($a_0 < 1$) regime of Thomson scattering using laser wakefield acceleration (LWFA). We present novel results of high-brightness, high-energy photons generated via non-linear Thomson scattering using the two-beam Astra-Gemini laser facility. With one $300$ $TW$ beam, electrons were first accelerated to $500$ $MeV$ energies inside gas cells through the process of LWFA. A second $300$ $TW$ laser pulse focused to $a_0 = 2$ was subsequently scattered off these electrons, resulting in a highly directional, small source size, and short pulse beam of photons with $>$10 $MeV$ energies. The photon beam was propagated through a low-$Z$ converter and produced Compton-scattered electrons that were spectrally measured by magnetic deflection and correlated with the incident photons. The measured photon yield at $15$ $MeV$ was $2 \times 10^6$ photons/$MeV$ and, when coupled with the small source size, divergence, and pulse duration, results in a record peak brightness of $2 \times 10^{19}$ photons/s/mm$^2$/mrad$^2$/0.1\%bandwidth at $15$ $MeV$ photon energy. [Preview Abstract] |
Thursday, October 30, 2014 10:18AM - 10:30AM |
TO6.00005: Compton Backscattered X-rays from Self-Generated Laser Wiggler in a Laser Wakefield Accelerator Antonio Ting, Dmitri Kaganovich, Bahman Hafizi, John Palastro, Michael Helle, Daniel Gordon, Yu-hsin Chen, John Seely A unique Compton backscattering configuration for generating monochromatic, short pulse, and potentially coherent x-rays in a Laser Wakefield Accelerator (LWFA) is being studied at the Naval Research Laboratory. Reflection mechanisms such as stimulated Raman scattering and shock-created density gradients in a plasma can generate the required backward-travelling laser pulse directly from the same laser pulse used in the LWFA, i.e., the high energy electron beam and the counter-propagating photon beam are both self-generated by an ultrashort laser pulse in plasma. The automatic alignment of the counter-propagating electrons and photons together with the extended interaction distance and tightly guided beam sizes in a LWFA can lead to a high-gain situation for the Doppler upshifted forward propagating x-rays. Possibilities for exponential gain to achieve coherent generation of the x-rays are under investigation. Theoretical, numerical, and preliminary experimental results will be presented. [Preview Abstract] |
Thursday, October 30, 2014 10:30AM - 10:42AM |
TO6.00006: Ring-Shaped Distributions of Monoenergetic Electron Beams Generated via Density Discontinuities in a Two-Stage Gas Cell Zhen Zhao, Keegan Behm, Bixue Hou, Vladimir Chvykov, Anatoly Maksimchuk, Victor Yanovsky, Alexander Thomas, Karl Krushelnick Using two-stage gas cells for laser wakefield acceleration experiments, we measure clear ring-shaped angular distributions of monoenergetic electron beams.~ These ``halo''-like structures are observed both on an on-axis and a magnet spectrometer imaging system.~ Initial assessment of the beam-halos suggests that they are most consistently generated in a gas cell where opposing flows create a type of density discontinuity between the stages.~ Generating such well-defined angular distributions of mono-energetic electrons may be useful for plasma-based X-ray sources. [Preview Abstract] |
Thursday, October 30, 2014 10:42AM - 10:54AM |
TO6.00007: X-ray Emission Characteristics of Ultra-High Energy Density Relativistic Plasmas Created by Ultrafast Laser Irradiation of Nanowire Arrays R.C. Hollinger, C. Bargsten, V.N. Shlyaptsev, A. Pukhov, M.A. Purvis, A. Townsend, D. Keiss, Y. Wang, S. Wang, A. Prieto, J.J. Rocca Irradiation of ordered nanowire arrays with high contrast femtosecond laser pulses of relativistic intensity creates volumetrically heated near solid density plasmas characterized by multi-KeV temperatures and extreme degrees of ionization.\footnote{M.Purvis et al Nature Photonics \textbf{7,}796 (2013).} The large hydrodynamic-to-radiative lifetime ratio of these plasmas results in very efficient X-ray generation. Au nanowire array plasmas irradiated at I 5x10$^{18}$ Wcm$^{-2}$ are measured to convert $\sim$ 5 percent of the laser energy into h$\nu $\textgreater 0.9 KeV X-rays, and \textgreater 1 x 10$^{-4}$ into h$\nu $\textgreater 9 KeV photons, creating bright picosecond X-ray sources. The angular distribution of the higher energy photons is measured to change from isotropic into annular as the intensity increases, while softer X-ray emission (h$\nu $ \textgreater 1 KeV) remains isotropic and nearly unchanged. Model simulations suggest the unexpected annular distribution of the hard X-rays might result from bremsstrahlung of fast electrons confined in a high aspect ratio near solid density plasma in which the electron-ion collision mean free-path is of the order of the plasma thickness. [Preview Abstract] |
Thursday, October 30, 2014 10:54AM - 11:06AM |
TO6.00008: Volumetric Heating of Ultra-High Energy Density Relativistic Plasmas by Ultrafast Laser Irradiation of Aligned Nanowire Arrays Clayton Bargsten, Reed Hollinger, Vyacheslav Shlyaptsev, Alexander Pukhov, David Keiss, Amanda Townsend, Yong Wang, Shoujun Wang, Amy Prieto, Jorge Rocca We have demonstrated the volumetric heating of near-solid density plasmas to keV temperatures by ultra-high contrast femtosecond laser irradiation of arrays of vertically aligned nanowires with an average density up to 30{\%} solid density. X-ray spectra show that irradiation of Ni and Au nanowire arrays with laser pulses of relativistic intensities ionizes plasma volumes several micrometers in depth to the He-like and Co-like (Au 52$+)$ stages respectively.\footnote{M. Purvis \textit{et al.}, Nature Photonics 7, 796 (2013).} The penetration depth of the heat into the nanowire array was measured monitoring He-like Co lines from irradiated arrays in which the nanowires are composed of a Co segment buried under a selected length of Ni. The measurement shows the ionization reaches He-like Co for depth of up to 5 $\mu$m within the target. This volumetric plasma heating approach creates a new laboratory plasma regime in which extreme plasma parameters can be accessed with table-top lasers. Scaling to higher laser intensities promises to create plasmas with temperatures and pressures approaching those in the center of the sun.\footnote{Purvis (2013)} [Preview Abstract] |
Thursday, October 30, 2014 11:06AM - 11:18AM |
TO6.00009: Characterizing relativistic petawatt-laser-generated particle beams on Orion Matthew Hill, Peter Allan, Colin Brown, Ray Edwards, Edward Gumbrell, David Hoarty, Lauren Hobbs, Steven James, Hui Chen, Andy Hazi, Edward Marley, Ronnie Shepherd, Jackson Williams The Orion laser facility at AWE has been used to irradiate a variety of metal and plastic targets with up to 600 J of 1.054$\mu$m laser light at pulse lengths ranging from 0.5 ps to 8 ps and intensities above 10$^{21}$ W/cm$^{2}$. The particle beams produced from these targets include considerable numbers of relativistic electrons (up to 250 MeV) as well as positrons, protons and heavy ions (up to 60 MeV). Magnetic spectrometers, radiochromic film stacks and a Thomson parabola suggest strong sheath field acceleration of both positrons and ions, as well as very hot electron distributions (T$_{\mathrm{hot}}$ \textgreater 18 MeV) indicating efficient laser-plasma coupling at high intensities. Simultaneous proton radiography and heating have been accomplished on metal foils and foams, showing promise for diagnosing short-pulse laser-plasma interactions as well as fields within extended target objects. We report on the latest progress in charged particle diagnostics systems and experimental platforms on the Orion facility. Supporting work performed at LLNL under the auspices of the U.S. DoE under contract DE-AC52-07NA27344. [Preview Abstract] |
Thursday, October 30, 2014 11:18AM - 11:30AM |
TO6.00010: Optical Guiding and Electron Acceleration in Programmably Modulated Plasma Waveguides George Hine, Andrew Goers, Jennifer Elle, Linus Feder, Howard Milchberg We demonstrate the guiding of relativistically intense laser pulses through programmably structured plasma waveguides. The structure of the waveguide is dictated electronically using a Spatial Light Modulator(SLM). The waveguides are generated by sending a radially patterned intense laser pulse through an axicon in a clustered gas medium, efficiently ionizing and heating a column of plasma which expands to form an optical guiding structure. Intensity modulations at the line focus produce density modulations as the waveguide evolves. Patterning of the intense laser pulse is achieved using the SLM in an interferometric configuration. This SLM patterning technique allows for \textit{in situ} sculpting of waveguides with arbitrary density structures. Density ramps are generated for electron injection, and periodic structures are formed to quasi-phasematch laser wakefield acceleration and direct laser acceleration. [Preview Abstract] |
Thursday, October 30, 2014 11:30AM - 11:42AM |
TO6.00011: Positron Production Using a Laser-Wakefield Electron Source G. Jackson Williams, Felicie Albert, Hui Chen, Jaebum Park, Bradley Pollock Positron generation using wakefield-accelerated electrons driven into a second mm-scale target was investigated using the Callisto Laser at the Jupiter Laser Facility at Lawrence Livermore National Laboratory. This technique\footnote{G. Sarri et al., Phys. Rev. Lett., 110:255002, Jun 2013. } is in contrast to previous experiments that use direct laser-target interactions to create positron-electron pairs,\footnote{H. Chen et al., Phys. Rev. Lett., 105:015003, Jul 2010} and has the potential to make laser-produced positron sources widely available to smaller scale laboratories. Monte Carlo simulations show a near-collimated ($<$10 mrad) wakefield electron beam produces a positron beam with a significantly larger divergence angle ($>$100 mrad) due to multiple small angle coulomb scattering, resulting in an emitted pair density of $10^{13}$ particles/cm$^3$. At the Callisto Laser, we did not observe a signal consistent with positrons using two different charged particle spectrometers. This could be due to a high noise environment and a large detection threshold. [Preview Abstract] |
Thursday, October 30, 2014 11:42AM - 11:54AM |
TO6.00012: Monte-Carlo Simulations of the Creation of High Energy Gamma Rays and Electron/Positron Pairs and Experiments on the Texas Petawatt Laser Alexander Henderson, Edison Liang, Pablo Yepes, Gilliss Dyer, Nathan Riley, Kristina Serratto High intensity (\textgreater 10$^{18}$ W/cm$^{2}$) lasers incident on high-Z, solid targets produce a large number of high-energy electrons, which in turn produce gamma-rays and electron-positron pairs. We have used GEANT4 Monte-Carlo simulation to characterize the production of these particles in and their passage through thick (\textgreater 1 mm) Au and Pt targets. The general results of these simulations have been validated in experiments conducted from 2011 to 2013 on the Texas Petawatt Laser (TPW), and refinements have been made to the simulation to help design future experiments. In addition, this simulation was used in the design and calibration of spectrometers used in these experiments. In particular, we have designed and deployed a Forward Compton Electron Spectrometer (FCES) which is more compact and cheaper than previous spectrometers build on the same principle, with only a minor reduction in resolution and applicable range. We were able to characterize the angular distribution of the gamma-rays as a Gaussian cone with a Full-Width-at-Half-Maximum (FWHM) of 35 degrees. The laser -to-gamma-ray-energy yield was around 2{\%}, The gamma-ray spectra fit a two-temperature model, with mean temperatures of 2.1 MeV at low energies (up to 5 MeV) and a mean temperature of 6 MeV at high energies (above 10 MeV). In the future, we hope to explore the astrophysical implication of these systems. [Preview Abstract] |
Thursday, October 30, 2014 11:54AM - 12:06PM |
TO6.00013: Collimation of a Positron Beam Using an Externally Applied Magnetic Field D.H. Barnak, P.-Y. Chang, R. Betti, G. Fiksel, D.D. Meyerhofer, G.J. Williams, S. Kerr, H. Chen Positron beam collimation using externally applied magnetic fields has been demonstrated on the OMEGA EP Laser System at the University of Rochester's Laboratory for Laser Energetics and the Titan laser at Lawrence Livermore National Laboratory. A positron jet with a divergence of $\sim 20^{\circ}$ is produced by irradiating a high-$Z$ target with an infrared short-pulse (10-ps) laser. The beam is then collimated into an electron--positron spectrometer by an 8-T magnetic field produced with a small (12-mm-diam) coil powered by a pulsed magnetic-field device. The positron density in the collimated beam is increased by a factor of $\sim 40,$ measured 0.6 m from the source. At a given target-to-coil distance, the collimation depends on the beam energy and the magnetic-field strength. Experiments show higher peak energies in the positron spectrum than simulation for a given field strength; several potential causes will be discussed. Target and coil alignment is critical to the collimation, and the effects of misalignment are calculated numerically. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944, the Office of Fusion Energy Sciences Number DE-FG02-04ER54786, and by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Thursday, October 30, 2014 12:06PM - 12:18PM |
TO6.00014: Generation of a Strong Terahertz Radiation by Counter-Propagating Laser Pulses in a Magnetized Plasma Min Sup Hur, Myung-Hoon Cho, Young-Kuk Kim A novel scheme of terahertz emission from a laser-plasma system was studied by theory and PIC simulations. In this new scheme, two counter-propagating laser pulses collide in a magnetized plasma. The strong ponderomotive force of the colliding pulses induces longitudinal current, which again is partially converted to a transverse one via the external magnetic field. This current actually plays the role radiating antenna. Since the ponderomotive force of the colliding pulses is generally much stronger than that from the single pulse, the intensity of the terahertz emission from the suggested scheme can be enhanced by tens of times from the single-pulse-driven Cherenkov wake scheme. Theoretically it was found that the terahertz amplitude scales with the P-square of the driving pulse instead of just P. More than that, an interesting physics of the electric field diffusion near the cutoff was observed in the simulations and fully described theoretically. One direct result of such a driven-diffusion of the electric field is the growth of the central field, leading to increased terahertz emission with the plasma density gradient. [Preview Abstract] |
Thursday, October 30, 2014 12:18PM - 12:30PM |
TO6.00015: Liquid crystals as on-demand, variable thickness targets for intense laser applications Patrick L. Poole, C. David Andereck, Douglass W. Schumacher Laser-based ion acceleration is currently studied for its applications to advanced imaging and cancer therapy, among others. Targets for these and other high-intensity laser experiments are often small metallic foils with few to sub-micron thicknesses, where the thickness determines the physics of the dominant acceleration mechanism. We have developed liquid crystal films that preserve the planar target geometry advantageous to ion acceleration schemes while providing on-demand thickness variation between 50 and 5000 nm. This thickness control is obtained in part by varying the temperature at which films are formed, which governs the phase (and hence molecular ordering) of the liquid crystal material. Liquid crystals typically have vapor pressures well below the 10$^{\mathrm{-6}}$ Torr operating pressures of intense laser target chambers, and films formed in air maintain their thickness during chamber evacuation. Additionally, the minute volume that comprises each film makes the cost of each target well below one cent, in stark contrast to many standard solid targets. We will discuss the details of liquid crystal film control and formation, as well as characterization experiments performed at the Scarlet laser facility. [Preview Abstract] |
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