Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session UO6: Radiation Sources and Underdense Ion Acceleration |
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Chair: Patrick Poole, Ohio State University Room: 230 C |
Thursday, November 3, 2016 2:00PM - 2:12PM |
UO6.00001: Medical imaging using a laser-wakefield driven x-ray source Jason Cole, Jonathan Wood, Nelson Lopes, Kristjan Poder, Christos Kamperidis, Saleh Alatabi, Jonathan Bryant, Stefan Kneip, Katalin Mecseki, Dominic Norris, Lydia Teboul, Henrik Westerburg, Richard Abel, Andi Jin, Dan Symes, Stuart Mangles, Zulfikar Najmudin Laser-wakefield accelerators driven by high-intensity laser pulses are a proven centimetre-scale source of GeV electron beams. One of the proposed uses for these accelerators is the driving of compact hard x-ray synchrotron light sources. Such sources have been shown to be bright, have small source size and high photon energy, and are therefore interesting for imaging applications. By doubling the focal length at the Astra-Gemini laser facility of the Rutherford Appleton Laboratory, UK, we have significantly improved the average betatron x-ray flux compared to previous experiments. This fact, coupled to the stability of the radiation source, facilitated the acquisition of full 3D tomograms of hard bone tissue and soft mouse neonates, the latter requiring the recording of over 500 successive radiographs. Such multimodal performance is unprecedented in the betatron field and indicates the usefulness of these sources in clinical imaging applications, scalable to very high photon flux without compromising source size or photon energy. [Preview Abstract] |
Thursday, November 3, 2016 2:12PM - 2:24PM |
UO6.00002: Compact Laser-Compton X-ray Source at LLNL Yoonwoo Hwang, Roark Marsh, David Gibson, Gerald Anderson, Christopher Barty, Toshiki Tajima The scaling of laser-Compton X-ray and gamma-ray sources is dependent upon high-current, low-emittance accelerator operation and implementation of efficient laser-electron interaction architectures. Laser-Compton X-rays have been produced using the unique compact X-band linear accelerator at LLNL operated in a novel multibunch mode, and results agree extremely well with modeling predictions. An Andor X-ray CCD camera and image plates have been calibrated and used to characterize the 30 keV laser-Compton X-ray beam. The X-ray source size and the effect of scintillator blur have been measured. K-edge absorption measurements using thin metallic foils confirm the production of narrow energy spread X-rays and results validate X-ray image simulations. Future plans for medically relevant imaging will be discussed with facility upgrades to enable 250 keV X-ray production. [Preview Abstract] |
Thursday, November 3, 2016 2:24PM - 2:36PM |
UO6.00003: Dynamic Thomson Scattering from Nonlinear Electron Plasma Waves in a Raman Plasma Amplifier A. Davies, J. Katz, S. Bucht, D. Haberberger, J. Bromage, J.D. Zuegel, D.H. Froula, R. Trines, R. Bingham, J. Sadler, P.A. Norreys Electron plasma waves (EPW's) can be used to transfer significant energy from a long-pulse laser to a short-pulse seed laser through the Raman scattering instability. Successful implementation of Raman amplification could open an avenue to producing high-intensity pulses beyond the capabilities of current laser technology $\left( {{\sim 10^{22}\mbox{\thinspace W}} \mathord{\left/ {\vphantom {{\sim 10^{22}\mbox{\thinspace W}} {\mbox{cm}^{2}}}} \right. \kern-\nulldelimiterspace} {\mbox{cm}^{2}}} \right).$ This three-wave interaction takes advantage of the plasma's ability to sustain large-amplitude plasma waves. Having complete knowledge of the EPW amplitude is essential to establishing optimal parameters for high-efficiency Raman amplification. A dynamic Thomson-scattering diagnostic is being developed to spatially and temporally resolve the amplitude of the driven and thermal EPW's. By imaging the scattered probe light onto a novel pulse-front tilt compensated streaked optical spectrometer, the diffraction efficiency of this plasma wave can be measured as a function of space and time. These data will be used in conjunction with particle-in-cell simulations to determine the EPW's spatial and temporal profile. This will allow the effect of the EPW profile on Raman scattering to be experimentally determined. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 2:36PM - 2:48PM |
UO6.00004: Experimental observation of multiphoton Thomson scattering Wenchao Yan, Grigory Golovin, Colton Fruhling, Daniel Haden, Ping Zhang, Jun Zhang, Baozhen Zhao, Cheng Liu, Shouyuan Chen, Sudeep Banerjee, Donald Umstadter With the advent of high-power lasers, several multiphoton processes have been reported involving electrons in strong fields. For electrons that were initially bound to atoms, both multiphoton ionization and scattering have been reported. However, for free electrons, only low-order harmonic generation has been observed until now. This limitation stems from past difficulty in achieving the required ultra-high-field strengths in scattering experiments. Highly relativistic laser intensities are required to reach the multiphoton regime of Thomson scattering, and generate high harmonics from free electrons. The scaling parameter is the normalized vector potential (a0). Previous experiments have observed phenomena in the weakly relativistic case (a0$\ge $ 1). In ultra-intense fields (a0 \textgreater \textgreater 1), the anomalous electron trajectory is predicted to produce a spectrum characterized by the merging of multiple high-order harmonic generation into a continuum. This may be viewed as the multiphoton Thomson scattering regime analogous to the wiggler of a synchrotron. Thus, the light produced reflects the electrons behavior in an ultra-intense lase field. We discuss the first experiments in the highly relativistic case (a0 \textasciitilde 15). [Preview Abstract] |
Thursday, November 3, 2016 2:48PM - 3:00PM |
UO6.00005: Probing lattice dynamics in silicon with laser-wakefield accelerated electrons John Nees, Z-H He, A.G.R. Thomas, Karl Krushelnick, S. Scott, M. Legally, B. Beaurepaire, G. Gallé, J. Faure Laser wakefield acceleration is the key technology in a new breed of electron and photon beam sources that operate in the ultrafast domain. We show that the spatial and temporal properties of wakefield-generated electron beams can be manipulated to enable them interrogate ultrafast lattice dynamics in freestanding single-crystal silicon membranes, while maintaining spatial resolution on the atomic scale. In particular, picosecond resolution of Si lattice dynamics is obtained by recording streaked electron diffraction peaks using static magnetic fields. We will also discuss the role of wave front control in establishing optimal beam characteristics and the significance of single-shot measurements. [Preview Abstract] |
Thursday, November 3, 2016 3:00PM - 3:12PM |
UO6.00006: Ultrafast small-angle x-ray scattering from laser-produced plasmas using an x-ray free electron laser Christian Roedel, Alexander Pelka, Thomas Kluge, Melanie Roedel, Thomas Cowan, Andreas Kemp, Luke Fletcher, Will Schumaker, Sebastian Goede, Eric Galtier, Hae Ja Lee, Siegfried Glenzer Small-angle x-ray scattering (SAXS) using ultrashort x-ray pulses from free electron lasers has the potential to resolve transient phenomena in dense laser-produced plasmas with nanometer spatial and femtosecond temporal resolution. As a proof-of-principle experiment, we demonstrated ultrafast SAXS from a laser-irradiated wire target using the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS). A 5 \textmu m Al wire was irradiated with a high-intensity laser pulse (up to 200 mJ, 50 fs) leading to a rapidly expanding laser plasma. X-ray pulses from the free-electron laser (60 fs, 5.5 keV) probe the laser produced plasma 80 ps after the interaction. The SAXS data reveals that an indentation of the dense plasma is initiated due to plasma expansion. The measurements will be discussed using two-dimensional particle-in-cell simulations of the laser plasma interaction. [Preview Abstract] |
Thursday, November 3, 2016 3:12PM - 3:24PM |
UO6.00007: Ultrafast-electron-diffraction studies of predamaged tungsten excited by femtosecond optical pulses. M. Mo, Z. Chen, R. Li, Y. Wang, X. Shen, M. Dunning, S. Weathersby, I. Makasyuk, R. Coffee, Q. Zhen, J. Kim, A. Reid, K. Jobe, C. Hast, Y. Tsui, X. Wang, S. Glenzer Tungsten is considered as the main candidate material for use in the divertor of magnetic confinement fusion reactors. However, radiation damage is expected to occur because of its direct exposure to the high flux of hot plasma and energetic neutrons in fusion environment. Hence, understanding the material behaviors of W under these adverse conditions is central to the design of magnetic fusion reactors. To do that, we have recently developed an MeV ultrafast electron diffraction probe to resolve the structural evolution of optically excited tungsten. To simulate the radiation damage effect, the tungsten samples were bombarded with 500 keV Cu ions. The pre-damaged and pristine W's were excited by 130fs, 400nm laser pulses, and the subsequent heated system was probed with 3.2MeV electrons. The pump probe measurement shows that the ion bombardment to the W leads to larger decay in Bragg peak intensities as compared to pristine W, which may be due to a phonon softening effect. The measurement also shows that pre-damaged W transitions into complete liquid phase for conditions where pristine W stays solid. Our new capability is able to test the theories of structural dynamics of W under conditions relevant to fusion reactor environment. [Preview Abstract] |
Thursday, November 3, 2016 3:24PM - 3:36PM |
UO6.00008: Relativistic flying mirrors in the nonlinear and large wavelength difference regime James Koga, Sergei Bulanov, Timur Esirkepov, Masaki Kando The up-shifting and longitudinal compression of electromagnetic waves has been shown possible with relativistic mirrors (see review [1]). These relativistic mirrors have been generated with ultra-high power laser pulses (driver pulses) propagating in plasma and generating breaking plasma waves. Ultra-short high frequency laser pulses were shown to be generated by counter-propagating lower power laser pulses (source pulses) with the relativistic mirrors both theoretically and experimentally (see review [1]). The source pulses had low enough intensity so that they did not significantly modify the mirror and nearly the same wavelength as the driver. Here, we investigate a new regime where the source pulse is of high enough power to significantly modify the mirror and has significantly longer wavelength than the driver. In this case the driver pulse can be in a significantly underdense plasma resulting in high upshift factor while the source is in a near critical plasma allowing for greater reflection with relatively small density perturbations. Results from 1D particle-in-cell simulations and theory will be presented. [1] S. V. Bulanov,el al., Phys. Usp. 56, 429 (2013). [Preview Abstract] |
Thursday, November 3, 2016 3:36PM - 3:48PM |
UO6.00009: Enhanced Narrow-band, Coherent Emission from a Current Source Immersed in Cut-off of a Plasma-like Medium Min Sup Hur, Bernhard Ersfeld, Adam Noble, Hyyong Suk, Dino Jaroszynski In plasma-like media sharing a similar dispersion relation, there exists a cut-off frequency to make the wave number zero. This particular situation has been understood classically in a way that the radiation impedance becomes infinite, resulting in a total reflection of an incident wave. However, in this framework of understanding the cut-off, a pure current source immersed in the cut-off region leads to infinite radiation power from Ohm's law. This is obviously unphysical and requires a different approach to address the problem. In this presentation, we show that by solving the driven time-dependent Schr\"{o}dinger equation, the radiation at the cut-off frequency can be selectively enhanced by several times the pure vacuum-emission. Important question here is whether such current sources are available in practical systems. We find that quasi-current sources are actually ubiquitous as long as the conversion efficiency from the current driver to the radiation emission is low. We demonstrate two such cases by PIC simulations; THz radiation from a plasma driven by colliding laser pulses, and THz from two-color lasers enclosed by a tapered waveguide. We also discuss the previous experimental results in terms of this enhanced emission concept. [Preview Abstract] |
Thursday, November 3, 2016 3:48PM - 4:00PM |
UO6.00010: High-Power Tunable Laser Pulse Driven Terahertz Generation in Corrugated Plasma Waveguides Chenlong Miao, John Palastro, Thomas Antonsen Excitation of terahertz radiation by the interaction of an ultra-short laser pulse and the fields of a miniature, corrugated plasma waveguide is considered. Plasma structures of this type have been realized experimentally [1] and they can support electromagnetic (EM) channel modes with properties that allow for radiation generation. In particular, the mode have subluminal field components, thus allowing phase matching between the generated THz modes and the ponderomotive potential of the laser pulse. Theoretical analysis and full format PIC simulations are conducted. We find THz generated by this slow wave phase matching mechanism is characterized by lateral emission and a coherent, narrow band, tunable spectrum with relatively high power and conversion efficiency. We investigated two different types of channels, and a range of realistic laser pulses and plasma profile parameters are considered with the goal of increasing the conversion of optical energy to THz radiation. We find high laser intensities strongly modify the THz spectrum by exciting higher order channel modes. Enhancement of a specific channel mode can be realized by using an optimum pulse duration and plasma density. As an example, a fixed drive pulse (0.55 J) with spot size of 15 \textmu m and pulse duration of 15 fs excites 37.8 mJ of THz radiation in a 1.5 cm corrugated plasma waveguide with on axis average density of 1.4\texttimes 10$^{\mathrm{18~}}$cm$^{\mathrm{-3}}$, conversion efficiency exceeding 8{\%} is achieved. [1] B. D. Layer et. al., Phys. Rev. Lett. 99, 035001 (2007). [Preview Abstract] |
Thursday, November 3, 2016 4:00PM - 4:12PM |
UO6.00011: Stability of Brillouin Flow in Slow-Wave Structures David Simon, Y.Y. Lau, Geoffrey Greening, Patrick Wong, Ronald Gilgenbach, Brad Hoff For the first time, we include a slow-wave structure (SWS) to study the stability of Brillouin flow in the conventional, planar, and inverted magnetron geometry. The resonant interaction of the SWS circuit mode and the corresponding smooth-bore diocotron-like mode is found to be the dominant cause for instability, overwhelming the intrinsic negative (positive) mass property of electrons in the inverted (conventional) magnetron geometry [1]. It severely restricts the wavenumber for instability to the narrow range in which the cold tube frequency of the SWS is within a few percent of the corresponding smooth bore diocotron-like mode in the Brillouin flow. This resonant interaction is absent in a smooth bore magnetron. [1] D. H. Simon, et al., \textit{Physics of Plasmas} \textbf{22}, 82104 (2015). [Preview Abstract] |
Thursday, November 3, 2016 4:12PM - 4:24PM |
UO6.00012: Proton acceleration using doped Argon plasma density gradient interacting with relativistic CO2-laser pulse Aakash Sahai, Oliver Ettlinger, George Hicks, Emma-Jane Ditter, Zulfikar Najmudin We investigate proton and light-ion acceleration driven by the interaction of relativistic $CO_2$ laser pulses with overdense Argon or other heavy-ion gas targets doped with lighter-ion species. Optically shaping the gas targets allows tuning of the pre-plasma scale-length from a few to several laser wavelengths, allowing the laser to efficiently drive a propagating snowplow through the bunching in the electron density. Preliminary PIC-based modeling shows that the lighter-ion species is accelerated even without any significant motion of the heavier ions which is a signature of the Relativistically Induced Transparency Acceleration mechanism. Some outlines of possible experiments at the TW $CO_2$ laser at the Accelerator Test Facility at Brookhaven National Laboratory are presented. [Preview Abstract] |
Thursday, November 3, 2016 4:24PM - 4:36PM |
UO6.00013: Effects of laser polarization on electrostatic shock ion acceleration in near-critical plasmas Young-Kuk Kim, Teyoun Kang, Min Sup Hur Ion acceleration from laser-driven collisionless electrostatic shock (CES) is attracting much attention, as quasi-monoenergetic, tens of MeV ion beams are expected to be available from relatively moderate laser power and near-critical density plasmas. For generation of a high-speed shock by a laser pulse, it is important to compress a high-contrast density layer by hole-boring process, and to heat the electrons in the upstream, where the hole-boring speed should match the Mach number condition 1.5 \textless M \textless 3.7. In this presentation, we compare the formation of CES and shock ion acceleration by ultrashort LP and CP pulses using PIC simulations. Owing to the better ability of CP pulses in density compression, the CP-driven shock is generated more efficiently even in low density plasmas than the LP-driven shocks. As the hole-boring speed is higher in lower density plasmas, we observed consistently higher speed of the shock and accelerated ion energy when driven by CP pulses. Interesting point is that the CP-shock generation is determined predominantly by the transmittance only, while the LP-shock formation depends on other parameters such as plasma scale length. In 2D simulations, we found that Weibel instability is less effective in CP than LP, which enables more stable shock formation for given conditions of the laser and plasma. [Preview Abstract] |
Thursday, November 3, 2016 4:36PM - 4:48PM |
UO6.00014: Density bunching effects in a laser-driven, near-critical density plasma for ion acceleration Oliver Ettlinger, Aakash Sahai, George Hicks, Emma-Jane Ditter, Nicholas Dover, Yu-hsin Chen, Michael Helle, Daniel Gordon, Antonio Ting, Mikhail Polyanskiy, Igor Pogorelsky, Marcus Babzien, Zulfikar Najmudin We present work investigating the interaction of relativistic laser pulses with near-critical density gas targets exhibiting pre-plasma scale lengths of several laser wavelengths. Analytical and computational modelling suggest that the interaction dynamics in a low-Z plasma is a direct result of induced density bunching up to the critical surface. In fact, these bunches can themselves become overcritical and experience significant radiation pressure, accelerating ions to higher energies compared to an ``idealised'' plasma slab target. This work will be used to help explain the observation of ion energies exceeding those predicted by radiation pressure driven hole-boring in recent experiments using the TW CO$_{\mathrm{2}}$ laser at the Accelerator Test Facility at Brookhaven National Laboratory. [Preview Abstract] |
Thursday, November 3, 2016 4:48PM - 5:00PM |
UO6.00015: Shock-Wave Acceleration of Protons on OMEGA EP D. Haberberger, D.H. Froula, A. Pak, A. Link, P. Patel, F. Fiuza, S. Tochitsky, C. Joshi The creation of an electrostatic shock wave and ensuing ion acceleration is studied on the OMEGA EP Laser System at the Laboratory for Laser Energetics. Previous work using a 10-$\mu $m CO$_{\mathrm{2}}$ laser in a H$_{\mathrm{2}}$ gas jet shows promising results for obtaining narrow spectral features in the accelerated proton spectra.\footnote{D. Haberberger\textit{ et al}., Nat. Phys. \textbf{8}, 95 (2012).\par } Scaling the shock-wave acceleration mechanism to the 1-$\mu $m-wavelength drive laser makes it possible to use petawatt-scale laser systems such as OMEGA-EP, but involves tailoring of the plasma profile. To accomplish the necessitated sharp rise to near-critical plasma density and a long exponential fall, an $\sim 1\mbox{-}\mu \mbox{m-thick}$ CH foil is illuminated on the back side by thermal x rays produced from an irradiated gold foil. The plasma density is measured using the fourth-harmonic probe system, the accelerating fields are probed using an orthogonal proton source, and the accelerated protons and ions are detected with a Thomson parabola. These results will be presented and compared with particle-in-cell simulations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944 and LLNL's Laboratory Directed Research and Development program under project 15-LW-095. [Preview Abstract] |
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