Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session TO7: Coherent Radiation Sources |
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Chair: Victor Granatstein, University of Maryland Room: Ballroom H |
Thursday, November 17, 2011 9:30AM - 9:42AM |
TO7.00001: Numerical studies of startup scenarios in a 1.5 MW, 110 GHz gyrotron operating in short pulses Oleksandr Sinitsyn, Gregory Nusinovich, Thomas Antonsen, Jr., David Tax, Richard Temkin Megawatt class gyrotrons operate in very high-order modes which form a very dense spectrum. In order to excite the operating mode in the presence of many competitors and drive it to the nominal operating point, careful control of the gyrotron's startup is necessary. Our studies are focused on the startup scenarios of the 110 GHz MIT gyrotron designed for operation at 1.5 MW power level in short pulses. Nominal parameters of the electron beam are: 96 kV, 42 A and orbital-to-axial velocity ratio $\alpha $ = 1.4. Previous numerical studies of the startup of this gyrotron had shown that at low voltages (at about 62 kV), first, the high-frequency TE$_{23,6}$ mode was excited and then, at higher voltages (at about 74 kV), it was replaced by the desired TE$_{22,6}$ mode. However, during a series of recent experiments at MIT it was shown that instead of the TE$_{23,6}$ mode a low-frequency TE$_{21,6}$ mode was excited during the voltage rise and persisted up to a voltage of 70 kV. In this work the authors make an attempt to simulate and explain this result with the help of self-consistent time-dependent code MAGY. [Preview Abstract] |
Thursday, November 17, 2011 9:42AM - 9:54AM |
TO7.00002: Propagation Effects on THz Generation from Ionizing Two Color Laser Pulses Luke Johnson, Thomas Antonsen, John Palastro, Ki-Yong Kim Coherent mixing of an ultrashort laser pulse and its second harmonic in a gas cell produces THz radiation due to the nonlinear dependence of the ionization rate on field strength [1]. As this is a coherent process, propagation effects, including self-phase modulation, dispersion, and diffraction are important in determining the THz yield and are considered in this work. The laser pulses modeled have wavelengths of 800nm and 400nm, pulse widths of 50fs, and intensities of 10$^{14}$ W/cm$^2$. The laser pulse propagation is modeled using the split-step Fourier method [2] to solve a first order differential equation for the forward (+z direction) propagating spectral components. Further, we have used ADK ionization to describe the ionization of N$_2$. The ionization current evolution is determined by solving a microscopic model where the free electrons are driven by the laser fields. To date, a 1D code with transverse fields and currents has been demonstrated. Future work includes extending the model to include diffraction and refraction. \newline [1] K. Y. Kim, ``Generation of coherent terahertz radiation in ultrafast laser-gas interactions,'' Physics of Plasmas, vol. 16, 2009, p. 056706. \newline [2] M. Kolesik and J. Moloney, ``Nonlinear optical pulse propagation simulation: From Maxwell's to unidirectional equations,'' Physical Review E, vol. 70, Sep. 2004, p. 036604. [Preview Abstract] |
Thursday, November 17, 2011 9:54AM - 10:06AM |
TO7.00003: Effect of Surface Roughness on Metallic Conductivity in the Terahertz Regime Matt Kirley, Benjamin B. Yang, John H. Booske The design of sources and components for terahertz (THz) regime radiation is influenced by dissipation losses in conducting surfaces. In this regime, however, predictive capability of surface loss is lacking. A high quality factor quasi-optical (QO) hemispherical resonator operating at 400 GHz to 650 GHz is designed and implemented for the measurement of electronic properties of conductors and low-loss dielectrics. Large area, periodic, textured gold surfaces are fabricated using nanoimprinted resist as a mold for an electroforming process. A comparison of the measured effective conductivity between the samples shows the increased resistivity of the metal due to the surface features. The regular geometry of these textures enables straightforward computational simulation and direct comparison to experiment. In addition to the nanofabricated structures, we will also present measured conductivity of samples that are lapped to different average roughness values. Computational simulation of such surfaces is less straightforward than for the patterned case. Progress in this area will be reported. [Preview Abstract] |
Thursday, November 17, 2011 10:06AM - 10:18AM |
TO7.00004: High energy T-ray pulses from table-top laser driven Ion accelerators Amrutha Gopal, Sven Herzer, Albrecht Schmidt, Andreas Reinhard, Wolfgang Ziegler, Gerhard Paulus, Stefano Minardi, Torsten May, Marco Schubert, Ulrich Dillner, Hans Georg Meyer, Hans Peter Gemuend We present the first experimental observation of energetic non-collinear T-rays (Terahertz pulses) from laser driven ion accelerators. The experimental and 2D Particle-In-Cell simulation results show that the observed T-rays are mostly emitted at large angles to the target normal. Two dimensional particle-in-cell (PIC) simulations point out that the emission originates from a micron-scale plasma sheath at the rear surface of the target, which is also responsible for the ion acceleration. This opens a perspective for the application of the T-ray detection for on-site diagnostic of particle acceleration in laser produced plasmas. [Preview Abstract] |
Thursday, November 17, 2011 10:18AM - 10:30AM |
TO7.00005: Three-dimensional kinetic analysis of longitudinal space-charge waves in a relativistic electron beam Agostino Marinelli, Erik Hemsing, James Rosenzweig We develop a three-dimensional analysis of longitudinal space-charge waves in a relativistic electron beam. Our analysis includes the effects of longitudinal thermal motion due to energy spread and emittance, transverse betatron oscillations and edge effects due to the finite transverse size of the beam. We reduce the system of coupled Vlasov/Maxwell equations to an integro-differential eigenvalue equation which represents the three-dimensional dispersion relation for the plasma oscillation eigenmodes. The dispersion relation can be expressed in terms of four dimensionless scaling parameters. We solve the dispersion relation by means of an approximate variational method as well as a numerical discretization method and use the universally scaled solutions to explain the main physical features of the problem. Finally, the initial value problem is solved by means of a bi-orthogonal mode expansion of an arbitrary initial perturbation in six-dimensional phase space and used to describe space-charge induced amplification and suppression of beam microbunching. Possible experimental applications of this analysis in the context of free-electron laser injectors are also discussed. [Preview Abstract] |
Thursday, November 17, 2011 10:30AM - 10:42AM |
TO7.00006: Demonstration of a Gain-Saturated 8.8 nm Table-Top Soft X-Ray Laser at 1 Hz Repetition Rate Jorge Rocca, David Alessi, Yong Wang, Bradley Luther, Liang Yin, Dale Martz, Marx Berrill, Vyacheslav Shlyaptsev, Mark Woolston There is significant interest in extending compact plasma-based lasers to sub-10 nm wavelengths to be able to realize table-top experiments with high intensity laser pulses at these wavelengths. However, the steep wavelength scaling of the optical pump laser energy necessary imposes a challenge and has limited gain-saturated table-top lasers that can operated at high repetition rate to wavelength above 10.9 nm [1]. We report the demonstration of gain-saturated laser operation at $\lambda $= 8.85 nm in the 4d$^{1}$S$_{0}\to $4p$^{1}$P$_{1}$ line of nickel-like lanthanum ions in a plasma column created at 1 Hz repetition rate by focusing pulses from a Ti:Sapphire laser with a total of 7.5 J energy onto a lanthanum slab. We have observed strong lasing at 8.5 nm in nickel-like cerium. \\[4pt] [1] D. Alessi, D. H. Martz, Y. Wang, M. Berrill, B. M. Luther, and J. J. Rocca. Optics. Lett. Vol. 35, 414 (2010). [Preview Abstract] |
Thursday, November 17, 2011 10:42AM - 10:54AM |
TO7.00007: Generation of high energy density plasmas in ultrafast micro-capillary discharges Gonzalo Avaria, Michael Grisham, Fernando Tomasel, Vyacheslav N. Shlyaptsev, Jorge J. Rocca Important plasma applications depend on the ability to generate hot and dense plasma columns of high homogeneity. We are investigating an scheme for the generation of very hot and dense plasma columns consisting in the rapid excitation of micro-capillary channels with ultrafast current pulses ($<$ 3 ns 10-90\% risetime). Model simulations suggest this concept has the potential to heat dense homogenous plasma columns to electron temperatures of 500-1000 eV using modest currents (eg. 30-40 kA). We have conducted experiments in 500 $\mu m$ diameter capillaries filled with different gases that support these predictions. End-on soft x-ray emission was detected by means of Rowland circle and flat field spectrometers with gated detectors. Xenon plasma column with soft x-ray spectra dominated by Xe XXVIII lines were generated. [Preview Abstract] |
Thursday, November 17, 2011 10:54AM - 11:06AM |
TO7.00008: Simulation methods for laser-plasma based x-ray sources Benjamin Cowan, Estelle Cormier-Michel, David Bruhwiler, Serguei Kalmykov, Bradley Shadwick, Kyle Bunkers, Donald Umstadter Laser-plasma accelerators (LPAs) hold great promise for compact, bright x-ray sources due to the extraordinary field strengths available. The longitudinal fields in a relativistic plasma wave enable acceleration of high-quality electron bunches up to several hundred MeV in just a few mm of plasma. The strong transverse focusing fields enable betatron radiation in the x-ray regime. Design of these x-ray sources requires large-scale particle-in-cell simulations. We describe new algorithms that improve the accuracy and reliability of PIC simulations of LPA x-ray sources. These include a perfect dispersion algorithm, which allows use of lower resolution without loss of accuracy for greater efficiency; post-processing routines for evaluation of betatron radiation, and methods for reducing the statistical noise in the simulation of the self-injection process. [Preview Abstract] |
Thursday, November 17, 2011 11:06AM - 11:18AM |
TO7.00009: ABSTRACT WITHDRAWN |
Thursday, November 17, 2011 11:18AM - 11:30AM |
TO7.00010: Computational Methods for Simulating the Generation Synchrotron Radiation Generation in Laser Wakefield Acceleration Experiments Paul Cummings, Alec Thomas A promising application of laser-wakefield acceleration (LWFA) technology is as a tunable source of x-ray and gamma radiation via synchrotron radiation. Such a source could serve as a valuable tool for detecting hidden nuclear material, radiation-based cancer therapy, and microscopic imaging of advanced materials. Consequently, the generation of synchrotron radiation in LWFA experiments is investigated computationally using the particle-in-cell simulation code OSIRIS 2.0. A novel computational algorithm for explicitly simulating synchrotron radiation, involving the generation of particle-like ``macrophotons,'' is derived. A skeleton particle tracking code is developed to validate this model, and the results of this validation are presented and discussed. Results from simulations of Thomson scattering using this code are presented and discussed. Potential applications for integrating this algorithm into OSIRIS 2.0 are presented and discussed. Specifically, the utilization of this algorithm, in conjunction with earlier work implementing the explicit simulation of optical aberrations, to study the experimentally-observed relationship between the comatic aberration and the synchrotron spectrum critical frequency, is discussed. [Preview Abstract] |
Thursday, November 17, 2011 11:30AM - 11:42AM |
TO7.00011: Laser Wakefield Accelerators towards table-top accelerators and bright synchrotron sources M. Bloom, M.J. Streeter, S. Kneip, R. Bendoyro, A. D\"opp, J. Holloway, J. Jiang, N. Lopes, Z. Najmudin, H. Nakamura, D. Symes, S.P.D. Mangles Laser wakefield accelerators (LWFA) have the potential to lead a new generation of compact table top accelerators. In particular, they could be an attractive driver for a whole range of light source applications. Recently it has been demonstrated that LWFAs can be a source of very bright x-rays. These x-rays are generated due to the transverse (betatron) oscillations of the electron beam as it is being accelerated. Experiments with 100 TW class lasers have shown the peak brightness of this betatron source to be comparable to 3rd generation light sources (S. Kneip et al, Nature Physics 6, 980-983 (2010)). I will present results from recent experiments on x-ray production using the 250 TW Astra Gemini Laser located at the Rutherford Appleton Laboratory. [Preview Abstract] |
Thursday, November 17, 2011 11:42AM - 11:54AM |
TO7.00012: Shaping of Ultraintense Pulse Front from an Overdense Plasma for Generation of a Concave Flying Electron Mirror and X-ray Focusing Min Sup Hur, Young-Kuk Kim, Victor Kulagin, Hyyong Suk Shaping of a petawatt laser pulses using relativistic transparency in a nonuniform overdense plasma has been studied. The resultant pulse shape is the Concave and sharp-cut pulse front. As an application of such shaping, we consider the generation of a relativistic electron mirror, which is concave to the propagation direction, so that it can be used in generating and focusing X-ray by reflecting a counterpropagating laser pulses. For this study, for the first time, we derived an analytic formula to calculate the channeling speed of a linearly polarized pulse in an overdense plasma. Using that formula it was possible to design an optimal plasma density profile in transverse direction to make the pulse front not only sharp-cut, but also concave to the propagation direction. By accelerating a nano-film with such a concave sharp-cut pulse, we expect a concave relativistic electron mirror can be generated. The concavity of the electron mirror can be used in focusing X-rays into a very small high-energy-density regime. [Preview Abstract] |
Thursday, November 17, 2011 11:54AM - 12:06PM |
TO7.00013: Fundamental limits on the brightness of Compton scattering light sources Fred Hartemann, Felicie Albert, Sheldon Wu Fundamental limits on the brightness of Compton scattering light sources are studied by considering the interaction of a single electron with a fully 6-dimensional, Fourier transform-limited incident laser phase space in the weakly nonlinear regime [1]. It is shown that the number of scattered photons and the nonlinear dephasing due to the ponderomotive force in the laser field both scale as the square of the normalized vector potential and the normalized effective interaction length; this fundamentally limits the spectral brightness of the source. Mitigation strategies are proposed, based on optimum temporal and transverse laser shaping. Implications for practical design of Compton scattering light sources are outlined. \\[4pt] [1] F.V. Hartemann, \textit{et al}., Phys. Rev. Lett. 105, 130801 (2010) [Preview Abstract] |
Thursday, November 17, 2011 12:06PM - 12:18PM |
TO7.00014: ABSTRACT WITHDRAWN Ashutosh Sharma This abstract has been removed. [Preview Abstract] |
Thursday, November 17, 2011 12:18PM - 12:30PM |
TO7.00015: A comparative study of Terahertz radiation generation by beating of two spatial-Gaussian / super-Gaussian lasers Hitendra K. Malik, Anil K. Malik The subject \textit{terahertz (THz) radiation generation} has got worldwide attention due to its diverse applications in nonlinear THz spectroscopy, imaging, topography, material characterization, etc. For generation of THz radiation, short pulse lasers have widely been employed for their interaction with semiconductors, air, gases and plasmas. In the present work, we propose to achieve THz radiation based on beating of two spatial-Gaussian or super-Gaussian lasers having different frequencies and wave numbers but the same electric field amplitudes in a periodic density plasma. In this situation, a ponderomotive force is attained along the direction of propagation and also in the transverse direction. Hence, the transverse component of current resonantly excites the radiation with efficiency of about 10$^{-3}$. The importance of laser-beam-width, amplitude and periodicity of density structure is discussed and a comparative study is made for the two types of lasers. [Preview Abstract] |
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