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Aug 2009

Volume 16, Issue 8, Articles (08xxxx)

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Phys. Plasmas 16, 082701 (2009); http://dx.doi.org/10.1063/1.3195065 (14 pages)

I. V. Igumenshchev, F. J. Marshall, J. A. Marozas, V. A. Smalyuk, R. Epstein, V. N. Goncharov, T. J. B. Collins, T. C. Sangster, and S. Skupsky
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back to top Lasers, Particle Beams, Accelerators, Radiation Generation

Linear and saturated characteristics of a coaxial-waveguide gyrotron backward-wave oscillator

C. L. Hung

Phys. Plasmas 16, 083101 (2009); http://dx.doi.org/10.1063/1.3192763 (7 pages) | Cited 3 times

Online Publication Date: 3 August 2009

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A coaxial waveguide interaction structure may be suitable for a gyrotron backward-wave oscillator (gyro-BWO) operating in millimeter and submillimeter waves with good mode selectivity, frequency tunability, and high power. This study analyzes the linear and nonlinear behaviors of a coaxial-waveguide gyro-BWO by using a single-mode, self-consistent nonlinear code. Simulation results indicate that the coaxial gyro-BWO exhibits features similar to those of a cylindrical-waveguide gyro-BWO, such as nonlinear field contraction, the relation of start-oscillation current to interaction length, and the difference value of the transit angle between adjacent axial modes. Additionally, the coaxial gyro-BWO has distinctive characteristics due to its transverse geometrical parameter C, i.e., the ratio of the outer radius to the inner radius. The beam-wave coupling strength of the coaxial gyro-BWO is a function of parameter C. As a result, the start-oscillation current of the coaxial gyro-BWO varies as the C value selected varies. The coaxial gyro-BWOs with different C values require different interaction lengths to reach the saturated state for the same beam current. Parameter C also impacts the magnetic tuning bandwidth of the coaxial gyro-BWO. However, maximum efficiency at saturation, which was obtained by tuning the magnetic field, is not highly dependent on the value of parameter C.
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52.75.-d Plasma devices
84.40.Ik Masers; gyrotrons (cyclotron-resonance masers)
84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
84.40.Az Waveguides, transmission lines, striplines

A low-impedance transit-time oscillator without foils

Yibing Cao (曹亦兵), Jiande Zhang (张建德), and Juntao He (贺军涛)

Phys. Plasmas 16, 083102 (2009); http://dx.doi.org/10.1063/1.3195070 (6 pages) | Cited 11 times

Online Publication Date: 7 August 2009

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A low-impedance transit-time oscillator (LITTO) without foils is proposed and studied by simulation. By using a coaxial structure, the space-charge limiting current can be improved significantly, thus allowing higher input and output powers. With no foils to erode, the only factor limiting the repetition rate is the ability to maintain an adequate vacuum. By contrast with conventional transit-time oscillators, the proposed LITTO has the advantages of low diode impedance, rapid saturation time, and a possibility of repetitive operation. As indicated in PIC simulation, the average microwave output power is over 5.0 GW at the main frequency of 1.6 GHz, with an input electron beam of 36.0 kA current and 600 kV voltage, and an external magnetic field of 0.45 Tesla.
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84.30.Ng Oscillators, pulse generators, and function generators
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)

Relativistic laser piston model: Ponderomotive ion acceleration in dense plasmas using ultraintense laser pulses

T. Schlegel, N. Naumova, V. T. Tikhonchuk, C. Labaune, I. V. Sokolov, and G. Mourou

Phys. Plasmas 16, 083103 (2009); http://dx.doi.org/10.1063/1.3196845 (16 pages) | Cited 26 times

Online Publication Date: 13 August 2009

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Laser ponderomotive force at superhigh intensities provides an efficient ion acceleration in bulk dense targets and evacuates a channel enabling further laser beam propagation. The developed quasistationary model of a laser piston—a double layer structure supported by the radiation pressure—predicts the general parameters of the acceleration process in homogeneous and inhomogeneous overdense plasmas. Particle-in-cell simulations confirm the estimated characteristics in a wide range of laser intensities and ion densities and show advantages of circularly polarized laser pulses. Two nonstationary effects are identified in the simulations. First, oscillations of the piston velocity and of the thickness of the ion charge separation layer broaden the energy spectrum of accelerated ions. Second, the electrons accelerated toward the incoming laser wave emit strong high-frequency radiation, enabling a cooling effect, which helps to sustain high charge neutrality in the piston and to maintain an efficient ion acceleration.
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52.27.Ny Relativistic plasmas
52.40.Mj Particle beam interactions in plasmas

Electron acceleration by intense short laser pulse in the preplasma of a target

Ming-Ping Liu, Hai-Cheng Wu, Bai-Song Xie, Xue-Ren Hong, Shan Zhang, and M. Y. Yu

Phys. Plasmas 16, 083104 (2009); http://dx.doi.org/10.1063/1.3206669 (4 pages) | Cited 3 times

Online Publication Date: 17 August 2009

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Electron acceleration in the low-density preplasma of a thin solid target by a short intense laser pulse is investigated by particle-in-cell simulation. Electrons in the preplasma are trapped and accelerated by the laser ponderomotive force as well as the wake bubble field. When the laser pulse is stopped and reflected by the target, the trapped electrons continue to move forward inertially, passing through the target with small energy spread and emittance. The use of the present scheme in practical applications is discussed.
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41.75.Jv Laser-driven acceleration
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Improving the relativistic self-focusing of intense laser beam in plasma using density transition

R. Sadighi-Bonabi, M. Habibi, and E. Yazdani

Phys. Plasmas 16, 083105 (2009); http://dx.doi.org/10.1063/1.3202694 (4 pages) | Cited 12 times

Online Publication Date: 18 August 2009

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The propagation of a Gaussian beam in underdense plasma with upward increasing density ramp is analyzed. In this work are shown that the spot size oscillations of laser beam increases and its amplitude shrinks with proper plasma density ramp. This causes the laser beam to become more focused and penetrations deep into the plasma by reduction of diffraction effect. The related focusing parameters are optimized to get the best possible focusing at the relativistic threshold intensity of Nd-glass laser and the effect of the laser intensity in the self-focusing parameters is also investigated. The analytical calculations are presented and showed more reliable results in comparison to the previous works.
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52.38.Hb Self-focussing, channeling, and filamentation in plasmas
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

The space-charge limiting current of a sheet relativistic electron beam in a rippled rectangular waveguide

Guang-Xing Du and Bao-Liang Qian

Phys. Plasmas 16, 083106 (2009); http://dx.doi.org/10.1063/1.3216457 (4 pages) | Cited 3 times

Online Publication Date: 28 August 2009

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The expression of the space-charge limiting current of a sheet relativistic electron beam propagating in a rippled rectangular waveguide is derived. Using numerical computation, interesting results of the space-charge limiting current are provided. For the same sheet relativistic electron beam, the space-charge limiting current in a rippled rectangular waveguide with an average height of be and a ripple amplitude of h0 is less than that in a smooth-walled rectangular waveguide with a height of be−2h0, and larger than that in a smooth-walled rectangular waveguide of a height of be+2h0, and increases as the ripple period decreases. In addition, the larger the ripple amplitude of the rectangular waveguide is, the more rapidly the space-charge limiting current increases with the decrease in the ripple period. These results should be of interest to the area of propagation of intense sheet relativistic electron beam in a rippled rectangular waveguide for application of generating high-power microwaves.
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84.40.Az Waveguides, transmission lines, striplines

Temporal optimization of neutron generation from the exploding deuterated methane jet of clusters subjected to an intense laser pulse

Haiyang Lu (卢海洋), Jiansheng Liu (刘建胜), Cheng Wang (王成), Wentao Wang (王文涛), Zili Zhou (周子理), Aihua Deng (邓爱华), Changquan Xia (夏长权), Yi Xu (许毅), Yuxin Leng (冷雨欣), Guoquan Ni (倪国权), Ruxin Li (李儒新), and Zhizhan Xu (徐至展)

Phys. Plasmas 16, 083107 (2009); http://dx.doi.org/10.1063/1.3211928 (8 pages) | Cited 7 times

Online Publication Date: 31 August 2009

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An experimental investigation on the interaction of an ultraintense femtosecond laser pulse at the intensity of 2×1017 W/cm2 (60 fs, 120 mJ at 800 nm) with clusters in a supersonic jet of deuterated methane gas has shown the generation of energetic deuterons and nuclear fusion events. The deuteron density and the average size of the clusters in the gas jet, as well as the fusion neutron yields under different backing pressures were measured simultaneously as a function of the time delays of the laser pulses with respect to the puffing of the gas jet. The results demonstrate that during the development of the gas jet expanding through a conical nozzle, the clusters grew up with time, and the average size of the clusters reached the maximum when the molecular density in the jet started to drop. The fusion neutron yields were found to increase with the larger average cluster size and the higher deuteron density, in accordance with the theoretical prediction. Experimental data indicate the existence of a ∼ 1 ms steady region in which the fusion neutron yields have reached the maximum of 2.0×105 per shot at the backing pressure of 74 bars. Consequently, an efficiency of 1.6×106 neutrons per joule of incident laser energy was realized.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
28.52.Cx Fueling, heating and ignition
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