Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session NO7: Coherent and Incoherent Radiation Sources |
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Chair: John Verbonvoeur, University of California, Berkeley Room: Rosen Centre Hotel Salon 7/8 |
Wednesday, November 14, 2007 9:30AM - 9:42AM |
NO7.00001: Investigation of Improved Magnetron Characteristics with Transparent Cathode Sarita Prasad, Herman Bosman, Mikhail Fuks, Edl Schamiloglu In [1] we showed: (a) fast start and rate of build-up of oscillations, (b) magnetron operation in the stable 2pi mode over a wide range of magnetic field, and (c) very high output power levels ($\sim $ 1GW), when the transparent cathode was used. This work is an attempt to explain the reasons for improvement of the magnetron characteristics in terms of priming and/or the large amplitude of the azimuthal component of the RF electric field in the electron flow region. Different cathode designs were studied in computer simulations to investigate these two effects. \newline \newline [1] H. Bosman et al. IEEE Trans. Plasma Sci, 34 (4), 606 (2006). [Preview Abstract] |
Wednesday, November 14, 2007 9:42AM - 9:54AM |
NO7.00002: Dielectric window breakdown in oxygen gas: from vacuum multipactor to collisional microwave discharge S.K. Nam, C. Lim, J.P. Verboncoeur, H.C. Kim, Y.Y. Lau The major limiting factor in the transmission of high power microwave is dielectric window breakdown. Using a PIC/MCC model, dielectric window breakdown from vacuum multipactor to collisional microwave discharge in noble gases has been investigated [1]. In contrast with noble gases, however, the oxygen in air produces negative ions, which can reach five or more times the electron density with a strong impact on the breakdown. In this work, breakdown of a dielectric window in contact with oxygen gas is investigated over a wide pressure range, and extension of the theoretical scaling law for breakdown in noble gases [2] is sought for oxygen and air. \newline \newline [1] H.C. Kim \textit{et al}., \textit{Phys. Plasmas} \textbf{13}, 123506 (2006). \newline [2] Y.Y. Lau \textit{et al}., \textit{Appl. Phys. Lett.} \textbf{89}, 261501 (2006). [Preview Abstract] |
Wednesday, November 14, 2007 9:54AM - 10:06AM |
NO7.00003: Fast Opening Microwave Barrier and Independency of Polarization in Plasma tubes Ted Anderson, Igor Alexeff, Esmaeil Farshi, Fred Dyer, Jeffry Peck, Eric P. Pradeep, Nanditha Pulsani, Naresh Karnam Plasma barriers are used to protect sensitive microwave apparatus from potentially damaging electronic warfare signals. We have found both experimentally and theoretically that we can open such a barrier on a time scale of microseconds instead of typically many milliseconds. We do this by increasing the plasma density rather than waiting for it to decay. We produce a standing wave between the two layers that results in microwave transmission, analogous to the transmission found in an optical Fabry-Perot Resonator. The plasma tubes work extremely well in intercepting microwave radiation when the incident wave electric field is parallel to the tubes. However, if the electric field is perpendicular to the tubes, the normally induced plasma current cannot flow, and the plasma effects are not expected to appear. To our surprise, when the plasma tubes were experimentally tested with the electric field perpendicular to the tubes, the plasma tubes not only intercepted the microwave signal, but the observed cut-off with a pulsed plasma lasted about twice as long. The effect appears to be due to an electrostatic resonance, and preliminary calculations suggest that a normally ignored term in Maxwell's Equations is responsible. [Preview Abstract] |
Wednesday, November 14, 2007 10:06AM - 10:18AM |
NO7.00004: Interpretation of the nonlinear mode excitation in the ITER gyrotron. Oleksandr Sinitsyn, Gregory Nusinovich Recently, the first 170 GHz gyrotron delivering 1 MW continuous-wave power (CW) for the International Thermonuclear Experimental Reactor (ITER) has been developed at the Japan Atomic Energy Agency (JAEA) [K. Sakamoto et al., ``Achievement of robust high-efficiency 1 MW oscillation in the hard self-excitation region by a 170 GHz continuous wave gyrotron,'' Nature Physics, vol. 3, pp. 411- 414, 2007]. In that work the hysteresis phenomenon in excitation of two modes was described. It was found that the operating mode can be excited in the region of hard self-excitation when the parasitic mode is present. The interpretation of this effect is given in the present paper. [Preview Abstract] |
Wednesday, November 14, 2007 10:18AM - 10:30AM |
NO7.00005: Mode switching in a gyrotron with azimuthally corrugated resonator Gregory Nusinovich, Oleksandr Sinitsyn, Thomas Antonsen Operation of a gyrotron having a cylindrical resonator with an azimuthally corrugated wall is analyzed. In such device, wall corrugation cancels degeneracy of modes with azimuthally standing patterns. The coupling between these modes depends on the radius of electron beam. It is shown that such gyrotron can be easily switched from one mode to another. When the switching is done with the repetition frequency equal to the rotational frequency of magnetic islands, this sort of operation can be used for suppression of neoclassical tearing modes in large-scale tokamaks and stellarators. [Preview Abstract] |
Wednesday, November 14, 2007 10:30AM - 10:42AM |
NO7.00006: Propagation of cyclotron maser radiation in inhomogeneous magnetic fields. Robert Bingham, Alan Cairns, Irena Vorgul, Barry Kellett, Alan Phelps, Kevin Ronald, David Speirs, Sandra McConville, Adrian Cross, Craig Robertson, Craig Whyte Cyclotron masers are important laboratory devices~and play a major role in planetary and stellar radio emission. Recently we have shown that a cyclotron maser instability driven by a horseshoe shaped distribution in velocity space~may be responsible for the observations. A long standing problem though is how the radiation generated at frequencies below the upper hybrid resonance, gets onto the higher frequency branch of the dispersion~curve that connects to the vacuum propagation branch. Here we consider some of the dispersion properties of waves in the presence of energetic particle populations in the shape of a horseshoe and ring distribution in velocity space. The analysis is carried out in a homogeneous and~an inhomogeneous magnetic field and demonstrates that the extraordinary mode that is initially driven unstable by the energetic particles can couple to the vacuum regime and escape the region. [Preview Abstract] |
Wednesday, November 14, 2007 10:42AM - 10:54AM |
NO7.00007: Fiber-Based, Spatially and Temporally Shaped Picosecond UV Laser for Advanced RF Gun Applications C. Siders, S. Anderson, S. Betts, D. Gibson, J. Hernandez, M. Johnson, I. Jovanovic, D. McNabb, M. Messerly, J. Pruet, M. Shverdin, A. Tremaine, F. Hartemann, C.P.J. Barty The UV laser system has been specifically designed for advanced rf gun applications, with a special emphasis on the production of high-brightness electron beams for free-electron lasers and Compton scattering light sources. The laser pulse can be shaped to a flat-top in both space and time with a duration of 10 ps FWHM, rise and fall times under 1 ps, and pulse energy of 50 micro-joules at 261.75 nm. A fiber oscillator and amplifier system generates a chirped pump pulse at 1047 nm; stretching is achieved in a chirped fiber Bragg grating; recompression to 1 ps FWHM is achieved with a single multi-layer dielectric grating based compressor. A two stage harmonic converter frequency quadruples the beam. Temporal shaping is accomplished with a Michelson-based ultrafast pulse stacking device with nearly 100{\%} throughput. [Preview Abstract] |
Wednesday, November 14, 2007 10:54AM - 11:06AM |
NO7.00008: Electromagnetic Simulation Studies of Photocathode Sources Mark Hess, Chong Shik Park We present the results of electromagnetic simulation studies on space-charge dominated electron beams produced by photocathode sources. In particular, we demonstrate the computational requirements on the Green's function based simulation code IRPSS (Indiana Rf Photocathode Source Simulator) for obtaining relative space-charge electromagnetic field errors of at most 1{\%}, and show how these fields compare with electrostatic based field solver methods. We also present the results of a multislice method used within IRPSS for modeling electron bunches, which approximates a local region of beam density as a zero longitudinal thickness slice. We show how these results can be applied to realistic photocathode experiments, such as the 1.3 GHz AWA photoinjector experiment at Argonne National Laboratory [1]. \newline [1] P. Schoessow, et al, The Argonne Wakefield Accelerator Overview and Status, Proceedings of PAC'03, Washington, D.C., p.2596. [Preview Abstract] |
Wednesday, November 14, 2007 11:06AM - 11:18AM |
NO7.00009: Relativistic Electron Beam Tunneling Through an Overdense Plasma A.G. Sgro 3D simulations of an electron beam encountering an over dense background plasma show that at early times the beam is disrupted by the beam plasma instability. As the beam source continues to emit, background electrons from regions progressively further from the beam source are ejected radially leaving the background ions in the region of the beam. These ions neutralize the beam charge and allow the beam to regain its identity and continue to propagate through the plasma. Thus over long timescales the beam basically digs a channel through the over dense plasma through which it then propagates with approximately its initial average radius without any evident further instability. Previously, this effect has been shown in 2D simulations [1] which resolved only axisymmetric modes, and then in 3D with coarse spatial resolution [2]. These new finely resolved 3D simulations show that non axisymmetric modes grow to larger amplitudes than axisymmetric ones. The result, then, is that the beam acquires a significant non axisymmetric structure while, over long timescales, digging a channel through the background and propagating through it. (LAUR-07-4723) \newline [1] A. G. Sgro and T. J. T. Kwan, Phys. Plasmas 10, 849 (2003) \newline [2] A. G. Sgro, Bull. Am. Phys. Soc. 49 (11), 96 (2004). [Preview Abstract] |
Wednesday, November 14, 2007 11:18AM - 11:30AM |
NO7.00010: Design considerations and parameter study for a 0.5TeV PWFA afterburner Chengkun Huang, W. Lu, M. Tzoufras, M. Zhou, V.K. Decyk, C. Joshi, W.B. Mori A recent plasma wakefield acceleration (PWFA) experiment using short ($\sim $100fs), high peak current ($>$10KA) electron beam as driver has demonstrated sustained acceleration gradients of $\sim $50GeV/m over 85 cm distance [1]. The rapid progress of PWFA experiments has attracted interest regarding the possibility of making an afterburner for a linear collider. In the afterburner concept, an electron beam is placed into the wakefield to extract energy deposited in the wake. We investigate the afterburner concept based on the present understanding of the key physics. Possible design scenarios such as single stage acceleration or integrated design with plasma lens final focusing are studied. The final energy, charges, emittance, energy spread and energy stability of the accelerated electron beam are taken as intrinsic design considerations. Parameters are suggested for a 0.5 TeV afterburner. We also present full scale 3D particle-in-cell simulations of the possible design using a highly efficient and accurate quasi-static code QuickPIC. \newline [1] Blumenfeld et. al., Nature 445, 741 (2007). [Preview Abstract] |
Wednesday, November 14, 2007 11:30AM - 11:42AM |
NO7.00011: Intense coherent THz radiation from two-color laser-gas interactions Ki-Yong Kim, Balakishore Yellampalle, James Glownia, Antoinette Taylor, George Rodriguez Strong terahertz (THz) electromagnetic pulse generation is of current interest for applications in rapid THz imaging and nonlinear THz spectroscopy. While intense THz fields exceeding MV/cm can be obtained from large facility sources such as free electron lasers and synchrotron-based sources, there is a demand for high-power tabletop-scale THz sources. Recently, strong THz pulse generation was observed upon mixing the fundamental and its second harmonic laser fields in air. The underlying mechanism has been examined and understood in the context of a plasma current model, in which a directional transverse plasma current can be produced when the bound electrons of gas atoms are liberated via phase-sensitive tunneling ionization and accelerated in the asymmetric laser field, such as a mixed two-color field. This current surge can occur on the timescale of laser pulse duration ($<$50 fs), thus producing an electromagnetic pulse at THz frequencies in the far-field. We have investigated the THz generation mechanism using our 0.5 TW Ti:sapphire laser system. We have also observed intense THz radiation with peak field amplitude of 150 kV/cm with 2 THz bandwidth filtering, and energy of $>$4 microjoule per pulse with a spectral bandwidth in access of 70 THz. [Preview Abstract] |
Wednesday, November 14, 2007 11:42AM - 11:54AM |
NO7.00012: Second Harmonic Generation in the Blowout Regime of Laser Wakefield Accelerators Daniel Gordon, Bahman Hafizi, Antonio Ting, Dmitri Kaganovich, Phil Sprangle Analysis and simulations of the blowout regime of laser wakefield accelerators reveal that the density gradients associated with electron cavitation can lead to strong conical emission of radiation at the second harmonic of the laser frequency. The frequency spectrum and angular distribution of this radiation carries information about the structure of the cavitation region. This information might be useful as a diagnostic of the ``plasma bubble'' which is observed in simulations of quasi-monoenergetic acceleration in laser wakefields. [Preview Abstract] |
Wednesday, November 14, 2007 11:54AM - 12:06PM |
NO7.00013: Coherent and incoherent radiation from ultra-intense laser interaction with nanostructured nickel nanowire (`velvet') targets Robin Marjoribanks, Marina Servol, Paul Forrester, Hart Levy, Luke McKinney, Brett Teeple, Yves Candela, Jean-Claude Kieffer, Simon Le Moal, Gabor Kulcsar, John Sipe, Patrick Audebert, Jean-Paul Geindre, Anne Heron, Jean-Claude Adam Nickel nanowires (`velvet') are a pure metallic anisotropic nanostructured material, averaging as much as one-quarter of solid density, that does not support material polarization- or current-densities required for Fresnel reflection. Since they present $>90\%$ absorption and an effective skin-depth on the order of $1 \mu$m for intense laser light, they have been shown to be efficient x-ray converters. We show theoretical and experimental results of their behaviour under a range of irradiation conditions, from small-signal up to very clean pulses of relativistic-intensity laser light, including their transition from an effective dielectric to an effective metal, as the result of the generation of relativistic Brunel electrons. [Preview Abstract] |
Wednesday, November 14, 2007 12:06PM - 12:18PM |
NO7.00014: Compton Scattering in Ignited Thermonuclear Plasmas Frederic Hartemann, Craig Siders, Chris Barty Inertially confined, ignited thermonuclear D-T plasmas will produce intense blackbody radiation at temperatures T $\sim $ 20 keV; it is shown that the injection of GeV electrons into the burning core can efficiently generate high-energy Compton scattering photons. Moreover, the spectrum scattered in a small solid angle can be remarkably monochromatic, due to kinematic pileup; peak brightness in excess of 10$^{29}$ photons/(mm$^{2}$ x mrad$^{2}$ x s x 0.1{\%} bandwidth) are predicted. Electron focusing of the $\gamma $-rays could produce electromagnetic fields exceeding the Schwinger critical field. [Preview Abstract] |
Wednesday, November 14, 2007 12:18PM - 12:30PM |
NO7.00015: Effect of radiation back-reaction on the interaction of super-strong laser fields with plasmas. Igor V. Sokolov, John A. Nees, Victor P. Yanovsky, Natalia M. Naumova, Gerard A. Mourou We analyze the effect of self-force on a single electron and on plasma electrons giving attention to the electromagnetic energy generated by non-linear Thomson scattering. The effect is essential if the scattered energy is comparable with the rest-mass energy of the electron in the frame of reference where the electron was initially at rest. We develop a method for solving the Lorentz-Abraham-Dirac equation and accounting for radiation in a self-consistent manner. The solution is then applied to the interactions of super-strong laser fields with an electron and a plasma layer including the presence of strong charge separation fields. This scheme allows a simulation of resulting radiation with spatial and spectral distributions, and demonstrates the possibility of efficient conversion (up to several {\%}) of incident radiation to $\gamma $-ray emission at intensities of $\sim $10$^{22}$W/cm$^{2}$, recently achieved in experiments. [Preview Abstract] |
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