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Feb 2013

Volume 20, Issue 2, Articles (02xxxx)

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Phys. Plasmas 20, 022303 (2013); http://dx.doi.org/10.1063/1.4790639 (12 pages)

Julio J. Martinell and Diego del-Castillo-Negrete
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back to top Lasers, Particle Beams, Accelerators, Radiation Generation

Three-dimensional nonlinear efficiency enhancement analysis in free-electron laser amplifier with prebunched electron beam and ion-channel guiding

F. Jafari Bahman and B. Maraghechi

Phys. Plasmas 20, 023101 (2013); http://dx.doi.org/10.1063/1.4776682 (10 pages)

Online Publication Date: 5 February 2013

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Efficiency enhancement in free-electron laser is studied by three-dimensional and nonlinear simulation using tapered helical wiggler magnetic field or tapered ion-channel density. In order to reduce the saturation length, prebunched electron beam is used. A set of nonlinear and coupled differential equations are derived that provides the self-consistent description of the evolution of both an ensemble of electrons and the electromagnetic radiation. These equations are solved numerically to show that the combined effect of tapering and prebunching results in significant enhancement of power and considerable reduction of the saturation length. To have a deeper insight into the problem, an analytical treatment is also presented that uses the small signal theory to derive a modified pendulum equation.
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41.60.Cr Free-electron lasers
41.20.Jb Electromagnetic wave propagation; radiowave propagation

Suppressing longitudinal double-layer oscillations by using elliptically polarized laser pulses in the hole-boring radiation pressure acceleration regime

Dong Wu, C. Y. Zheng, C. T. Zhou, X. Q. Yan, M. Y. Yu, and X. T. He

Phys. Plasmas 20, 023102 (2013); http://dx.doi.org/10.1063/1.4791654 (5 pages)

Online Publication Date: 11 February 2013

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It is shown that well collimated mono-energetic ion beams with a large particle number can be generated in the hole-boring radiation pressure acceleration regime by using an elliptically polarized laser pulse with appropriate theoretically determined laser polarization ratio. Due to the J × B effect, the double-layer charge separation region is imbued with hot electrons that prevent ion pileup, thus suppressing the double-layer oscillations. The proposed mechanism is well confirmed by Particle-in-Cell simulations, and after suppressing the longitudinal double-layer oscillations, the ion beams driven by the elliptically polarized lasers own much better energy spectrum than those by circularly polarized lasers.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.40.Kh Plasma sheaths
52.65.Rr Particle-in-cell method
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Optimization of laser-target interaction for proton acceleration

E. d'Humières, A. Brantov, V. Yu. Bychenkov, and V. T. Tikhonchuk

Phys. Plasmas 20, 023103 (2013); http://dx.doi.org/10.1063/1.4791655 (8 pages)

Online Publication Date: 12 February 2013

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The aim of this study is to optimize the characteristics, in particular, the maximum energy and the number of high energy protons accelerated by a high intensity laser from different targets by using two- and three-dimensional particle-in-cell simulations. Two principal ways are considered to increase the maximum proton energy: (1) the use of an optimized target design (by choosing its atomic composition, density, structure, thickness, and transverse size); (2) variation of the laser pulse parameters (duration, power, intensity, focal spot size, polarization, pulse shape, etc.). Our analysis demonstrates a possibility to accelerate protons to the energies exceeding 200 MeV with 20 J laser pulses with appropriately chosen focusing and duration.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.65.Rr Particle-in-cell method

Acceleration and dynamics of an electron in the degenerate and magnetized plasma elliptical waveguide

A. Abdoli-Arani, B. Jazi, and B. Shokri

Phys. Plasmas 20, 023104 (2013); http://dx.doi.org/10.1063/1.4792261 (9 pages) | Cited 1 time

Online Publication Date: 14 February 2013

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The dynamics and energy gain of an electron in the field of a transverse magnetic wave propagating inside an elliptical degenerate plasma waveguide is analytically investigated by finding the field components of the TMmr mode in this waveguide. Besides, by solving the relativistic momentum and energy equations the deflection angle and the acceleration gradient of the electron in the waveguide are obtained. Furthermore, the field components of the hybrid mode and the transferred power in the presence of the magnetic field in this waveguide are found. Also by applying the boundary conditions at the plasma-conductor interface, we calculate the dispersion relation. It is shown that the cutoff frequency of this mode is dependent on the plasma density but independent of the magnetic field. Then, a single-electron model for numerical calculations of the electron deflection angle and acceleration gradient inside the magnetized plasma-filled elliptical waveguide is generally presented to be used as a cascading process for the acceleration purposes.
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52.40.Fd Plasma interactions with antennas; plasma-filled waveguides
52.27.Ny Relativistic plasmas
52.25.Fi Transport properties
02.60.-x Numerical approximation and analysis
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma

A high efficient relativistic backward wave oscillator with coaxial nonuniform slow-wave structure and depth-tunable extractor

Xingjun Ge, Huihuang Zhong, Jun Zhang, and Baoliang Qian

Phys. Plasmas 20, 023105 (2013); http://dx.doi.org/10.1063/1.4776711 (4 pages)

Online Publication Date: 22 February 2013

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A high efficient relativistic backward wave oscillator with coaxial nonuniform slow-wave structures (SWSs) and depth-tunable extractor is presented. The physical mechanism to increase the power efficiency is investigated theoretically and experimentally. It is shown that the nonuniform SWSs, the guiding magnetic field distribution, and the coaxial extractor depth play key roles in the enhancement of the beam-wave power conversion efficiency. The experimental results show that a 1.609 GHz, 2.3 GW microwave can be generated when the diode voltage is 890 kV and the beam current is 7.7 kA. The corresponding power efficiency reaches 33.6%.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
41.60.Bq Cherenkov radiation
41.75.Ht Relativistic electron and positron beams

Thomson parabola spectrometry for gold laser-generated plasmas

L. Torrisi, M. Cutroneo, L. Andò, and J. Ullschmied

Phys. Plasmas 20, 023106 (2013); http://dx.doi.org/10.1063/1.4793454 (7 pages)

Online Publication Date: 26 February 2013

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The plasma generated from thin gold films irradiated in high vacuum at high intensity (∼1015 W/cm2) laser shot is characterized in terms of ion generation through time-of-flight techniques and Thomson parabola spectrometry. Gold ions and protons, accelerated in forward direction by the electric field developed in non-equilibrium plasma, have been investigated. Measurements, performed at PALS laboratory, give information about the gold charge states distributions, the ion energy distributions and the proton acceleration driven as a function of film thickness, laser parameters, and angular emission. The ion diagnostics of produced plasma in forward direction permits to understand some mechanisms developed during its expansion kinetics. The role of the focal position of a laser beam with respect to the target surface, plasma properties, and the possibility to accelerate protons up to energies above 3 MeV has been presented and discussed.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Fi Transport properties
52.38.Kd Laser-plasma acceleration of electrons and ions

Tunability of the terahertz space-charge modulation in a vacuum microdiode

P. Jonsson, Marjan Ilkov, A. Manolescu, A. Pedersen, and A. Valfells

Phys. Plasmas 20, 023107 (2013); http://dx.doi.org/10.1063/1.4793451 (7 pages)

Online Publication Date: 28 February 2013

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Under certain conditions, space-charge limited emission in vacuum microdiodes manifests as clearly defined bunches of charge with a regular size and interval. The frequency corresponding to this interval is in the terahertz range. In this computational study, it is demonstrated that, for a range of parameters, conducive to generating THz frequency oscillations, the frequency is dependant only on the cold cathode electric field and on the emitter area. For a planar micro-diode of given dimension, the modulation frequency can be easily tuned simply by varying the applied potential. Simulations of the microdiode are done for 84 different combinations of emitter area, applied voltage, and gap spacing, using a molecular dynamics based code with exact Coulomb interaction between all electrons in the vacuum gap, which is of the order 100. It is found, for a fixed emitter area, that the frequency of the pulse train is solely dependant on the vacuum electric field in the diode, described by a simple power law. It is also found that, for a fixed value of the electric field, the frequency increases with diminishing size of the emitting spot on the cathode. Some observations are made on the spectral quality, and how it is affected by the gap spacing in the diode and the initial velocity of the electrons.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.65.Yy Molecular dynamics methods
52.75.Fk Magnetohydrodynamic generators and thermionic convertors; plasma diodes
52.80.Vp Discharge in vacuum
52.25.Fi Transport properties

Observation of emission process in hydrogen-like nitrogen Z-pinch discharge with time integrated soft X-ray spectrum pinhole image

Y. Sakai, J. Rosenzweig, H. Kumai, Y. Nakanishi, Y. Ishizuka, S. Takahashi, T. Komatsu, Y. Xiao, H. Bin, Z. Quishi, Y. Hayashi, I. Song, T. Kawamura, M. Watanabe, and E. Hotta

Phys. Plasmas 20, 023108 (2013); http://dx.doi.org/10.1063/1.4789617 (6 pages)

Online Publication Date: 28 February 2013

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The emission spectra of hydrogen-like nitrogen Balmer at the wavelength of 13.4 nm in capillary Z-pinch discharge plasma are experimentally examined. Ionization to fully strip nitrogen at the pinch maximum, and subsequent rapid expansion cooling are required to establish the population inversion between the principal quantum number of n = 2 and n = 3. The ionization and recombination processes with estimated plasma parameters are evaluated by utilizing a time integrated spectrum pinhole image containing radial spatial information. A cylindrical capillary plasma is pinched by a triangular pulsed current with peak amplitude of 50 kA and pulse width of 50 ns.
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52.58.Lq Z-pinches, plasma focus, and other pinch devices
51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.)
52.40.Mj Particle beam interactions in plasmas
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

Dispersion properties of a 2D magnetized plasma metallic photonic crystal

T. Fu, Z. Yang, Z. Shi, F. Lan, D. Li, and X. Gao

Phys. Plasmas 20, 023109 (2013); http://dx.doi.org/10.1063/1.4792264 (5 pages)

Online Publication Date: 28 February 2013

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This is a study on a 2D magnetized plasma-filled metal photonic crystal (PMPC). We analyze the dispersion relation of the magnetized PMPC by using the finite-difference time-domain method. Results show a cutoff frequency for the PMPC, and two flat bands and new forbidden band gaps appear due to the external magnetic field. Adjusting the external magnetic field can control the positions of the flat bands, cutoff frequency, and location and width of the local gap. These results provide theoretical basis for designing tunable photonic crystal devices.
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52.27.Lw Dusty or complex plasmas; plasma crystals
02.10.-v Logic, set theory, and algebra
02.70.Bf Finite-difference methods
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