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Mar 2013

Volume 20, Issue 3, Articles (03xxxx)

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Phys. Plasmas 20, 032106 (2013); http://dx.doi.org/10.1063/1.4794320 (10 pages)

M. Raghunathan and R. Ganesh
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back to top Lasers, Particle Beams, Accelerators, Radiation Generation

Breather-like penetration of ultrashort linearly polarized laser into over-dense plasmas

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

Phys. Plasmas 20, 033101 (2013); http://dx.doi.org/10.1063/1.4794197 (5 pages)

Online Publication Date: 4 March 2013

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The penetration of an ultrashort laser pulse into an overdense plasma in the relativistic transparency regime is reexamined. The interaction is governed by the Maxwell and relativistic hydrodynamic equations and investigated using a fully implicit energy-conserving numerical scheme. It is found that for a circularly polarized laser, the penetrated pulse has the expected soliton structure. However, for a linearly polarized laser, the penetrated light exhibits a breather structure, and energy exchange between it and the plasma is at twice the laser frequency.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.27.Ny Relativistic plasmas
52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)
52.40.Mj Particle beam interactions in plasmas

High-charge energetic electron bunch generated by intersecting laser pulses

Lei Yang, Zhigang Deng, C. T. Zhou, M. Y. Yu, and Xingang Wang

Phys. Plasmas 20, 033102 (2013); http://dx.doi.org/10.1063/1.4794352 (6 pages)

Online Publication Date: 6 March 2013

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The interaction of two energetic electron bunches generated in the wakefields of two intense intersecting laser pulses in rarefied plasmas is investigated using particle-in-cell simulations. It is found that, with suitable intersection angle between the two laser pulses, the initially independent wakefield accelerated electron bunches can merged into a single one with high charge, energy, and narrow energy spread. The dynamics of the laser-pulse intersection and wake-bubble merging process is also investigated, and the crucial roles of the intersection angle are pointed out and analyzed.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.65.Rr Particle-in-cell method
29.20.Ej Linear accelerators
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Low-voltage gyrotrons

M. Yu. Glyavin, N. A. Zavolskiy, A. S. Sedov, and G. S. Nusinovich

Phys. Plasmas 20, 033103 (2013); http://dx.doi.org/10.1063/1.4791663 (7 pages)

Online Publication Date: 8 March 2013

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For a long time, the gyrotrons were primarily developed for electron cyclotron heating and current drive of plasmas in controlled fusion reactors where a multi-megawatt, quasi-continuous millimeter-wave power is required. In addition to this important application, there are other applications (and their number increases with time) which do not require a very high power level, but such issues as the ability to operate at low voltages and have compact devices are very important. For example, gyrotrons are of interest for a dynamic nuclear polarization, which improves the sensitivity of the nuclear magnetic resonance spectroscopy. In this paper, some issues important for operation of gyrotrons driven by low-voltage electron beams are analyzed. An emphasis is made on the efficiency of low-voltage gyrotron operation at the fundamental and higher cyclotron harmonics. These efficiencies calculated with the account for ohmic losses were, first, determined in the framework of the generalized gyrotron theory based on the cold-cavity approximation. Then, more accurate, self-consistent calculations for the fundamental and second harmonic low-voltage sub-THz gyrotron designs were carried out. Results of these calculations are presented and discussed. It is shown that operation of the fundamental and second harmonic gyrotrons with noticeable efficiencies is possible even at voltages as low as 5–10 kV. Even the third harmonic gyrotrons can operate at voltages about 15 kV, albeit with rather low efficiency (1%–2% in the submillimeter wavelength region).
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84.40.Ik Masers; gyrotrons (cyclotron-resonance masers)
52.40.Mj Particle beam interactions in plasmas
52.50.Nr Plasma heating by DC fields; ohmic heating, arcs

Coupling of laser energy into hot-electrons in high-contrast relativistic laser-plasma interactions

G. E. Kemp, A. Link, Y. Ping, D. W. Schumacher, R. R. Freeman, and P. K. Patel

Phys. Plasmas 20, 033104 (2013); http://dx.doi.org/10.1063/1.4794961 (9 pages)

Online Publication Date: 13 March 2013

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We use particle-in-cell simulations to explain the mechanisms responsible for the coupling of laser energy into relativistic electrons for the case of sharp interface, solid density metal targets free of pre-plasma. For perfectly flat interfaces, the accelerated electron trajectories are dominated by the standing-wave (SW) field structure formed by interference between incident and reflected pulses. We find that quasi-static magnetic fields that develop near the interface play only a minor role in perturbing the relativistic electron trajectories but can contribute to enhanced absorption. Target surfaces that are structured exhibit enhanced absorption, and the acceleration mechanism deviates from the clean standing-wave acceleration mechanism leading to more stochastic electron heating and larger divergence angles.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.65.Rr Particle-in-cell method
52.27.Ny Relativistic plasmas
52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)

Propagation of a short-pulse laser-driven electron beam in matter

L. Volpe, D. Batani, G. Birindelli, A. Morace, P. Carpeggiani, M. H. Xu, F. Liu, Y. Zhang, Z. Zhang, X. X. Lin, F. Liu, S. J. Wang, P. F. Zhu, L. M. Meng, Z. H. Wang, et al.

Phys. Plasmas 20, 033105 (2013); http://dx.doi.org/10.1063/1.4793453 (10 pages)

Online Publication Date: 14 March 2013

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We studied the transport of an intense electron beam produced by high intensity laser pulses through metals and insulators. Targets were irradiated at two different intensities, 1017 W/cm2 and 1019 W/cm2, at the laser facility Xtreme Light XL-III in Beijing, a Ti:Sa laser source emitting 40 fs pulses at 800 nm. The main diagnostic was Cu-Kα fluorescence imaging. Images of Kα spots have been collected for those two laser intensities, for different target thickness, and for different materials. Experimental results are analyzed taking into account both collisional and collective effects as well as refluxing at the edge of the target. The target temperature is evaluated to be Tc ∼ 6 eV for intensity I = 1017 W/cm2 (for all the tested materials: plastic, aluminium, and copper), and Tc ∼ 60 eV in aluminium and 120 eV in titanium for intensity I = 1019 W/cm2.
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61.80.Fe Electron and positron radiation effects
61.82.Bg Metals and alloys
61.82.Ms Insulators
78.70.En X-ray emission spectra and fluorescence

Competition between coherent emission and broadband spontaneous emission in the quantum free electron laser

G. R. M. Robb and R. Bonifacio

Phys. Plasmas 20, 033106 (2013); http://dx.doi.org/10.1063/1.4794731 (5 pages)

Online Publication Date: 15 March 2013

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We extend previous analyses of spontaneous emission in a quantum free electron laser (QFEL) and competition between spontaneous and coherent QFEL emission to include a broad distribution of photon frequencies and momenta appropriate for spontaneous undulator radiation. We show that although the predictions of monochromatic and broadband models predict different electron momentum distributions for the quantum regime due to spontaneous emission alone after many photon emissions, the inclusion of broadband spontaneous emission has a negligible effect on the competition between spontaneous and coherent emission in the QFEL. Numerical results from both models are well described by the same condition for the threshold/critical value of spontaneous emission rate.
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41.60.Cr Free-electron lasers
42.50.-p Quantum optics

Curvature aided long range propagation of short laser pulses in the atmosphere

Burak Yedierler

Phys. Plasmas 20, 033107 (2013); http://dx.doi.org/10.1063/1.4795305 (7 pages)

Online Publication Date: 15 March 2013

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The pre-filamentation regime of propagation of a short and intense laser pulse in the atmosphere is considered. Spatiotemporal self-focusing dynamics of the laser beam are investigated by calculating the coupled differential equations for spot size, pulse length, phase, curvature, and chirp functions of a Gaussian laser pulse via a variational technique. The effect of initial curvature parameter on the propagation of the laser pulse is taken into consideration. A method relying on the adjustment of the initial curvature parameter can expand the filamentation distance of a laser beam of given power and chirp is proposed.
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52.38.Hb Self-focussing, channeling, and filamentation in plasmas
02.30.Hq Ordinary differential equations
02.30.Xx Calculus of variations

Passive mode control in the recirculating planar magnetron

Matthew Franzi, Ronald Gilgenbach, Y. Y. Lau, Brad Hoff, Geoff Greening, and Peng Zhang

Phys. Plasmas 20, 033108 (2013); http://dx.doi.org/10.1063/1.4794967 (8 pages)

Online Publication Date: 19 March 2013

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Preliminary experiments of the recirculating planar magnetron microwave source have demonstrated that the device oscillates but is susceptible to intense mode competition due, in part, to poor coupling of RF fields between the two planar oscillators. A novel method of improving the cross-oscillator coupling has been simulated in the periodically slotted mode control cathode (MCC). The MCC, as opposed to a solid conductor, is designed to electromagnetically couple both planar oscillators by allowing for the propagation of RF fields and electrons through resonantly tuned gaps in the cathode. Using the MCC, a 12-cavity anode block with a simulated 1 GHz and 0.26 c phase velocity (where c is the speed of light) was able to achieve in-phase oscillations between the two sides of the device in as little as 30 ns. An analytic study of the modified resonant structure predicts the MCC's ability to direct the RF fields to provide tunable mode separation in the recirculating planar magnetron. The self-consistent solution is presented for both the degenerate even (in phase) and odd (180° out of phase) modes that exist due to the twofold symmetry of the planar magnetrons.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
84.30.Ng Oscillators, pulse generators, and function generators

Enhanced high harmonic generation and the phase effect in double-sided relativistic laser-foil interaction

Yahong Yu, Baifei Shen, Liangliang Ji, Xiaomei Zhang, Wenpeng Wang, Xueyan Zhao, Xiaofeng Wang, Longqing Yi, Yin Shi, Tongjun Xu, Lingang Zhang, and Zhizhan Xu

Phys. Plasmas 20, 033109 (2013); http://dx.doi.org/10.1063/1.4796090 (6 pages)

Online Publication Date: 21 March 2013

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High harmonic generation (HHG) from relativistic laser-foil interaction is investigated analytically and through particle-in-cell simulations. Previous work has shown that when two counter-propagating circularly polarized (CP) laser pulses interact with a thin foil, electrons can be well confined spatially to form a high density layer. The layer electrons oscillate in certain transversal direction and radiate intense high order harmonics. It is demonstrated here that there is a critical foil thickness, only below which can high harmonics be generated efficiently. Furthermore, to enhance the intensity in higher order region, the third linearly polarized (LP) short-wavelength laser pulse with much lower intensity is introduced. Analysis and simulations both show that the enhancement is determined by the relative phase δϕ between the driving CP laser pulses and LP pulse. The enhancement at high order is quite considerable and very sensitive to the relative phase δϕ, thus offering not only a way to efficiently produce HHG but also a new method to measure the phase of intense high-frequency laser pulses.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.65.Rr Particle-in-cell method
52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)

Hybrid proton acceleration scheme using relativistic intense laser light

A. A. Andreev, K. Yu. Platonov, M. Schnürer, R. Prasad, and S. Ter-Avetisyan

Phys. Plasmas 20, 033110 (2013); http://dx.doi.org/10.1063/1.4796053 (8 pages)

Online Publication Date: 22 March 2013

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Ion acceleration phenomena at relativistic intense laser interaction with thin foil targets are studied to find an efficient laser-target interaction concept at the conditions, where neither the ponderomotive pressure of the laser light nor the hot electron pressure is negligible. Particle in cell simulations and the analytical model are allowing to predict optimum laser-target parameters and suggesting a significant increase of proton energy if a hybrid proton acceleration scheme is used. In the proposed scenario, the laser polarisation is changed during the acceleration process: First with circularly polarised laser light the target is accelerated as a whole by the ponderamotive pressure, and then with linearly polarised laser light the electrons are heated which additionally increases the accelerating field. The calculations are in good agreement with experimental findings.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.65.Rr Particle-in-cell method
52.27.Ny Relativistic plasmas
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

Inverse bremsstrahlung heating rate in xenon clusters in the eikonal approximation

R. Dey and A. C. Roy

Phys. Plasmas 20, 033111 (2013); http://dx.doi.org/10.1063/1.4798403 (6 pages)

Online Publication Date: 27 March 2013

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We report inverse bremsstrahlung (IB) heating rates in the eikonal approximation (EA). The present analysis is performed using the plasma-screened Rogers and Debye potentials for Xe clusters with two different charge states (6 and 10). We compare the eikonal results with the first Born approximation (FBA) and classical-simulation (CL-sim) (Moll et al., Phys. Plasmas 19, 033303 (2012)) calculations for clusters in infrared light. Calculations have been performed for the field strength of 2.6 × 108 V/cm. We find that compared to the FBA and CL-sim methods, the IB heating rate in the EA is less sensitive to the choice of the two potentials considered here. The present EA calculation shows that the influence of the inner structure of atomic ion on the heating rate is more prominent for the smaller ion charge (Xe6+). In the case of low laser field approximation based on the elastic transport cross sections, it is seen that in contrast to the FBA and classical methods, the heating rate predicted by the EA does not deviate much all over the range of mean kinetic energy of electrons (20–500 eV) considered here for both the charge states of xenon (Xe6+ and Xe10+). Furthermore, for the Rogers potential, EA is found to be in closer agreement with the classical method than the FBA. We also compare the results of the IB heating rate using the present and low-field approximation approaches to the above three methods and observe that the magnitudes of the IB heating rate calculated in the low field approximation are, in general, higher than the corresponding values predicted by the present approach for both the electron-ion potentials.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.65.-y Plasma simulation
52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Fi Transport properties

Energetic protons from an ultraintense laser interacting with a symmetric parabolic concave target

Muhammad Ali Bake, Bai-Song Xie, Shan-Zhang, and Hong-Yu Wang

Phys. Plasmas 20, 033112 (2013); http://dx.doi.org/10.1063/1.4798528 (6 pages)

Online Publication Date: 27 March 2013

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A scheme of a symmetric parabolic concave target irradiated by an ultraintense laser for efficient proton acceleration is proposed and involved problem is studied by using two-dimensional particle-in-cell (PIC) simulations. Results indicate that on one hand, the laser field is focused by the front parabolic concave surface of target and, on the other hand, more energetic hot electrons will traverse to the rear surface of target due to concave shape. The space-charge-separation field, induced by those hot electrons escaping form parabolic concave rear surface of target, can accelerate protons to relatively high energy with narrow energy spread. The dependence of the efficiency of proton acceleration on the target parameters is examined, and the optimal target parameters are obtained. Particle-in-cell simulations show that the proton peak energy and energy spread are greatly enhanced when the target parameters are chosen optimal, for example, a proton bunch with the maximum energy ∼ 27.5 MeV and energy spread ∼ 7% can be generated. Some implications of our results to experiments and comparisons with the other works are also discussed briefly.
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52.59.-f Intense particle beams and radiation sources
52.65.Rr Particle-in-cell method
41.75.Ak Positive-ion beams
52.25.Fi Transport properties
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
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