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To investigate the temporal-spatial distribution and evolutions of global Total Electron Content (TEC), we estimate the global TEC data from 1999 to 2013 by processing the GPS data collected by the International Global Navigation Satellite System (GNSS) Service (IGS) stations, and robustly constructed the TEC time series at each of the global 5°×2.5° grids. We found that the spatial distribution of the global TEC has a pattern where the number of TECs diminishes gradually from a low-latitude region to high-latitude region, and anomalies appear in the equatorial crest and Greenland. Temporal variations show that the peak TEC appears in equinoctial months, and this corresponds to the semiannual variation of TEC. Furthermore, the winter anomaly is also observed in the equatorial area of the northern hemisphere and high latitudes of the southern hemisphere. Morlet wavelet analysis is used to determine periods of TEC variations and results indicate that the 1-day, 26.5-day, semi-annual and annual cycles are the major significant periods. The fitting results of a quadratic polynomial show that the effect of solar activity on TEC is stronger in low latitudes than in mid-high latitudes, and stronger in the southern hemisphere than in the northern hemisphere. But the effect in low latitudes in the northern hemisphere is stronger than that in low latitudes in the southern hemisphere. The effect of solar activity on TECs was analyzed with the cross wavelet analysis and the wavelet coherence transformation, and we found that there appears to be a strong coherence in the period of about 27 days. So the sunspot as one index of solar activity seriously affects the TEC variations with the sun's rotation. We fit the TEC data with the least squares spectral analysis to study the periodic variations of TEC. The changing trend of TEC is generally -0.08 TECu per year from 1999 to 2013. So TECs decrease over most areas year by year, but TECs over the Arctic around Greenland maintained a rising trend during these 15 years.

Unlike at any single TEM (n, m) mode laser, there is a subluminous phase velocity region located along the central region of a TEM (1,0)+TEM (0,1) mode laser. In conjunction with the high longitudinal electric field in this region, it forms another acceleration channel, which also locates inside the transverse ponderomotive potential trap. Through simulation, it is found that relativistic electrons injected into this acceleration channel can stand at the acceleration phase for a long time and be synchronously accelerated to high energies. Also, the accelerated electrons can be well confined inside the trap avoiding the transverse scattering problem. [ABSTRACT FROM AUTHOR]

The phase stability in the capture and acceleration scenario (CAS) is studied and compared with that of conventional linear electron accelerators (CLEAs). For the CAS case, it has been found that a slow phase slippage occurs due to the difference between the electron velocity and the phase velocity of the longitudinal accelerating electric field. Thus, CAS electrons cannot remain in a fixed small phase region of the accelerating field to obtain a quasimonoenergy gain in contrast to the stability of phase oscillation in CLEAs. Also, the energy spread of the output electron beam for the CAS case cannot be kept as small as the CLEA because there is no good phase bunching phenomenon generated by phase oscillation. [ABSTRACT FROM AUTHOR]

The interaction of free electrons with intense laser beams in vacuum is studied using a three-dimensional test particle simulation model that solves the relativistic Newton-Lorentz equations of motion in analytically specified laser fields. Recently, a group of solutions was found for very intense laser fields that show interesting and unusual characteristics. In particular, it was found that an electron can be captured within the high-intensity laser region, rather than expelled from it, and the captured electron can be accelerated to GeV energies with acceleration gradients on the order of tens of GeV/cm. This phenomenon is termed the capture and acceleration scenario (CAS) and is studied in detail in this article. The accelerated GeV electron bunch is a macropulse, with duration equal to or less than that of the laser pulse, which is composed of many micropulses that are periodic at the laser frequency. The energy spectrum of the CAS electron bunch is presented. The dependence of the energy exchange in the CAS on various parameters, e.g., α[sub 0] (laser intensity), w[sub 0] (laser radius at focus), τ (laser pulse duration), b[sub 0] (the impact parameter), and θ [sub i] (the injection angle with respect to the laser propagation direction), are explored in detail. A comparison with diverse theoretical models is also presented, including a classical model based on phase velocities and a quantum model based on nonlinear Compton scattering. [ABSTRACT FROM AUTHOR]

Backward terahertz radiation can be produced by a high-intensity laser normally incident upon an underdense plasma. It is found that terahertz radiation is generated by electrons refluxing along the bubble shell. These shell electrons have similar dynamic trajectories and emit similar backward radiations to vacuum. This scheme has been proved through electron dynamic calculations and by using an ionic sphere model. In addition, the bubble shape is found to influence the radiation frequency, and this scheme can be implemented in both uniform and up-ramp density gradient plasma targets. The terahertz radiation may be used for diagnosing the electron bubble shape in the interaction between an intense laser and plasma. All the results are presented via 2.5 dimensional particle-in-cell simulations. [ABSTRACT FROM AUTHOR]

The use of a static magnetic field with a modest intensity in the conventional vacuum laser acceleration scheme has been investigated. It has been found that the applied magnetic field can break the symmetry of the laser acceleration and deceleration phases experienced by the electrons after they leave the focal region, allowing the electrons to be accelerated in the focal region to gain more energy from the combined fields of the laser and the static magnetic field. The later process is the second stage acceleration taking place in the region outside the laser focal area. Explanations of these interaction features based on analytical calculations and simulations are presented. [ABSTRACT FROM AUTHOR]

The dynamic characteristics of an electron accelerated by ultraintense circularly polarized laser pulses in vacuum following the capture and acceleration scenario were studied. Comparing them with that from the use of linearly polarized laser pulses, we found (i) that the acceleration channel is wider, leading to greater acceleration efficiency, and (ii) that the maximum energy gains rise much more slowly as the laser intensity increases. This slow rise is caused by the magnetic-field force, which weakens the longitudinal acceleration force at higher laser intensity. [ABSTRACT FROM AUTHOR]

Electrons acceleration by the vacuum laser acceleration scheme CAS [see, e.g., P. X. Wang et al., Appl. Phys. Lett. 78, 2253 (2001)] (capture and acceleration scenario) is simulated. The general features of the outgoing electrons are examined at different laser intensities. Explanations based on the mechanism behind the CAS scheme of those output characteristics are presented. The results show it is hopeful that CAS becomes a useful scheme for laser accelerators. [ABSTRACT FROM AUTHOR]

In this paper, we studied the characteristics of radiation emitted by electrons accelerated in a laser-plasma interaction by using the Lienard-Wiechert field. In the interaction of a laser pulse with a underdense plasma, electrons are accelerated by two mechanisms: direct laser acceleration (DLA) and laser wakefield acceleration (LWFA). At the beginning of the process, the DLA electrons emit most of the radiation, and the DLA electrons emit a much higher peak photon energy than the LWFA electrons. As the laser-plasma interaction progresses, the LWFA electrons become the major radiation emitter; however, even at this stage, the contribution from DLA electrons is significant, especially to the peak photon energy. [ABSTRACT FROM AUTHOR]

A well-defined near-steady plasma channel is successfully formed in an interaction of an intense short-pulse laser with a near-critical underdense plasma. The laser is well self-focused in the plasma channel. Our 2.5-dimensional particle-in-cell simulations also demonstrate a violent electron acceleration with a effective gradient of several tens of GeV/cm by a strong longitudinal charge-separated field. At the same time, a strong ion acceleration appears at the rear plasma boundary. The results present a regime for the plasma channel formation and the particle acceleration by ultrashort laser-plasma interaction. [ABSTRACT FROM AUTHOR]

The radiative reaction effect of an electron is usually very small and can be neglected in most cases. But for an ultra intensity laser-electron interaction region, the radiation can become large. The influence of the radiative reaction effect of an electron interacting with an ultra intense laser pulses in vacuum on electron dynamics is investigated within the classical relativistic Lorentz-Dirac approach. A predictor-corrector method is proposed to numerically solve the equation of motion with the electron radiative reaction included. We study the counter-propagating case (for Thomson scattering scheme) and the same direction propagating cases (for laser acceleration). Our simulation results show that radiation can have great effect in the counter-propagating case. But in the vacuum laser electron acceleration regime, both the ponderomotive acceleration scenario case and the capture and acceleration scenario, radiative reaction effect can totally be ignored for laser intensity available presently or in the near-future. [ABSTRACT FROM AUTHOR]

The electric field intensity distribution and the phase velocity distribution of a slit in laser beams with different parameters are analyzed. Using three-dimensional test particle simulation, the laser beam with a slit induced acceleration of electrons with different initial momenta is investigated. Contrary to anticipation, the maximum net energy gain is not monotone increasing as the incoming momentum increasing. Based on the field structure and analysis, we gave an explanation for this. [ABSTRACT FROM AUTHOR]

Using three-dimensional test particle simulations, we investigated electrons accelerated by a focused flat-top laser beam at different intensities and flatness levels of the beam profile before focusing in vacuum. The results show that the presence of sidelobes around the main focal spot of the focused flat-top laser beam influences the optimum (as far as electron acceleration is concerned) initial momentum (and incident angle) of electrons for acceleration. The difference of initial conditions between laser beams with and without sidelobes becomes evident when the laser field is strong enough (a0>10, corresponding to intensities I>1×1020 W/cm2 for the laser wavelength λ=1 μm, where a0 is a dimensionless parameter measuring laser intensity). The difference becomes more pronounced at increasing a0. Because of the presence of sidelobes, there exist three typical CAS (capture and acceleration scenario) channels when a0≥30 (corresponding to I>1×1021 W/cm2 for λ=1 μm). The energy spread of the outgoing electrons is also discussed in detail. [ABSTRACT FROM AUTHOR]

When electrons are accelerated by the ponderomotive force of an intense short-pulse laser, the electrons are accelerated at the head of the laser and they lose their energy gained at the tail of the laser. Therefore the electrons cannot finally obtain the laser energy. In this research, an overdense slab plasma separator is introduced in order to separate the accelerated electrons from the laser, just before they enter the deceleration phase. The laser is reflected by the plasma separator, so the electrons pass through the thin plasma separator without a significant influence and are successfully accelerated. [ABSTRACT FROM AUTHOR]

The output properties of electrons accelerated by the vacuum laser acceleration scheme CAS (capture and acceleration scenario) are addressed. The transport process of the electron bunch, the fraction of the CAS electrons of the incident electrons, the correlation of electron energy with position and scattering angle, the energy spectrum and angular distributions as well as the emittance of the outgoing electrons are studied at a laser intensity of a0 = 10. In addition, the effects of the laser intensity, beam width, and pulse duration on the properties of the output electrons are also examined. Physical explanations of those output characteristics are presented based on the mechanism behind the CAS scheme. The feasibility of CAS to become a realistic laser accelerator scheme is explored. [ABSTRACT FROM AUTHOR]

By utilizing a pulsed laser beam of TEM(1,0)+TEM(0,1) mode, it was found numerically for the first time that an electron bunch can be effectively trapped by the transverse ponderomotive force in the transverse direction and at the same time accelerated by the longitudinal ponderomotive force to about 378 MeV at the laser peak intensity of I∼5.48×10[sup 18] W/cm[sup 2]. In addition, the electron bunch size is preferably small: at this laser intensity the electron bunch thickness is ∼10λ in the longitudinal direction and the bunch radius is about 625λ in the transverse direction. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this letter we report the studies of the mechanism which can lead to substantial energy exchange between electrons and intense laser beams interacting in vacuum. It has been found that the wave phase experienced by the electron when it encounters the intense beam is a crucial ingredient for discriminating the two totally different trajectories: capture and violent acceleration scenario (CAS) or conventional inelastic scattering (IS). When an electron is captured, due to the diffraction effect of the optical beam near the focused region, the effective wave-phase velocity along the dynamic trajectory of the captured particle is found to be less than c, the speed of light in vacuum. Thus the captured electron can remain in the acceleration phase of the wave for a long time, and gain considerable energy from the laser field. [ABSTRACT FROM AUTHOR]

By means of 3D test particle simulation programs, the effect caused by the high-order corrections of Gaussian laser fields on the electron dynamics in a stationary ultraintense laser beam is examined. In this letter, special attention is given to the studying of the capture phenomenon (J. X. Wang et al., Phys. Rev. E58, 6575 (1998)), which shows interesting prospect making it to become a viable mechanism for laser-driven GeV electron accelerators without media involved. It was found that as w[sub 0] ≥ 50, where w[sub 0] is the beam width at the focus center, the paraxial approximation field (PAF) is good enough for reproducing all the electron dynamic characteristics, not only in qualitative detail, but also in quantitative detail. [ABSTRACT FROM AUTHOR]

This paper studies the characteristics of GeV electron bunches driven by ultra-intense lasers in vacuum based on the mechanism of capture and violent acceleration scenario [CAS, see, e.g. J. X. Wang et al., Phys. Rev. E58, 6575 (1998)], which shows an interesting prospect of becoming a new principle of laser-driven accelerators. It has been found that the accelerated GeV electron bunch is a macro-pulse composed of a lot of micro-pulses, which is analogous to the structure of the bunches produced by conventional linacs. The macro-pulse corresponds to the duration of the laser pulse while the micro-pulse corresponds to the periodicity of the laser wave. Therefore, provided that the incoming electron bunch with comparable sizes as that of the laser pulse synchronously impinges on the laser pulse, the total fraction of electrons captured and accelerated to GeV energy can reach more than 20%. These results demonstrate that the mechanisms of CAS is a relatively effective accelerator mechanism. [ABSTRACT FROM AUTHOR]

Phase velocity plays a key role in wave-matter interactions where phase matching is essential. Laser acceleration of electrons is a good example. We have derived an exact formula with two deductions of the phase velocity for a monochromatic wave field in homogeneous medium from the fundamental wave equation. The core of these formulae is that the phase velocity is sufficiently determined in terms of the wave amplitude with no explicit reference to the phase. We applied these formulae to make out the phase velocity distribution of a Gaussian laser beam and compared with the traditional method. [ABSTRACT FROM AUTHOR]

A mechanism is proposed to enhance a proton acceleration field in laser plasma interaction. A double-layer plasma with different densities is illuminated by an intense short pulse. Electrons are accelerated to a high energy in the first layer by the wakefield. The electrons accelerated by the laser wakefield induce the enhanced target normal sheath (TNSA) and breakout afterburner (BOA) accelerations through the second layer. The maximum proton energy reaches about 1 GeV, and the total charge with an energy higher than 100 MeV is about several tens of μC/μm. Both the acceleration gradient and laser energy transfer efficiency are higher than those in single-target-based TNSA or BOA. The model has been verified by 2.5D-PIC simulations. [ABSTRACT FROM AUTHOR]

Abstract: The electric field intensity distribution and the phase velocity distribution of high-order Laguerre–Gaussian (LGρℓ) mode laser beams are analyzed. Using three-dimensional test particle simulation, the numerical results of electrons accelerated by LG00, LG40 and LG41 mode laser beams are presented. Compared with the LG00 mode (the fundamental mode) laser beam, low-energy injection electrons can be more favorably accelerated in a high-order LG mode laser beam. Contrary to anticipation, a high-order LG mode laser beam with intense axial electric field distribution is inferior to the LG00 mode in capture acceleration for electrons with high injection energy. [Copyright &y& Elsevier]