Numerical study of 1.1 GeV electron acceleration over a-few-millimeter-long plasma with a tapered density

We present two-dimensional particle-in-cell simulations of laser wakefield electron acceleration up to 1.1 GeV over a-few-millimeter-long plasma with the help of density tapering. We observed that, in a uniform plasma, the electron beam reaches the dephasing state not only by the slow phase velocity of the wakefield but also by the relativistic prolonging of the plasma wavelength. Such a dephasing between the wakefield and beam can be mitigated by an upward density taper. By employing a parabolically increasing plasma density, we obtained a significant enhancement of the beam energy from 850 MeV (uniform) to 1.1 GeV (tapered). However, the similar relativistically promoted dephasing was observed again in the environment of tapered density. Over a few millimeters the driving laser pulse was well self-guided without any externally prepared channel. Thus, this parameter regime is suitable for the gas-jet laser wakefield electron acceleration experiments.open6

Additional focusing of a high-intensity laser beam in a plasma with a density ramp and a magnetic field

Propagation of a high power Gaussian laser beam through a plasma with a density ramp where a magnetic field is present has been investigated. The spot size of the laser beam decreases as the beam penetrates into the plasma due to the role of a plasma density ramp. The studies show that the combined effect of a plasma density ramp and a magnetic field enhances the self-focusing property of the laser beam. Both factors not only reduce the spot size of the laser beam but also maintain it with only a mild ripple over several Rayleight lengths.open161

Realistic laser focusing effect on electron acceleration in the presence of a pulsed magnetic field

As we know, for a significant electron energy gain, a fast electron should be injected into the highest intensity region of the laser focus. Such intensities may be achieved in the laboratory by tight focusing of a laser. For a tight focused laser beam, it is necessary to consider all field components the arise due to the tight focusing of the laser beam, when the waist of the laser beam is of the order of the laser wavelength. By using the accurate field components of a tightly focused laser beam, we investigate the electron acceleration in the presence of a pulsed magnetic field. Our study shows that the electron energy gain during laser acceleration is found to be considerably higher.open9

Study of electron trapping by a transversely ellipsoidal bubble in the laser wake-field acceleration

We present electron trapping in an ellipsoidal bubble which is not well explained by the spherical bubble model by [Kostyukov, Phys. Rev. Lett. 103, 175003 (2009)]. The formation of an ellipsoidal bubble, which is elongated transversely, frequently occurs when the spot size of the laser pulse is large compared to the plasma wavelength. First, we introduce the relation between the bubble size and the field slope inside the bubble in longitudinal and transverse directions. Then, we provide an ellipsoidal model of the bubble potential and investigate the electron trapping condition by numerical integration of the equations of motion. We found that the ellipsoidal model gives a significantly less restrictive trapping condition than that of the spherical bubble model. The trapping condition is compared with three-dimensional particle-in-cell simulations and the electron trajectory in test potential simulations.open1

A universal characterization of nonlinear self-oscillation and chaos in various particle-wave-wall interactions

The comprehensive parameter space of self-oscillation and its period-doubling route to chaos are shown for bounded beam-plasma systems. In this parametrization, it is helpful to use a potentially universal parameter in close analogy with free-electron-laser chaos. A common parameter, which is related to the velocity slippage and the ratio of bounce to oscillation frequencies, is shown to have similar significance for different physical systems. This single parameter replaces the dependences on many input parameters, thus suitable for a simplifying and diagnostic measure of nonlinear dynamical and chaotic phenomena for various systems of particle-wave interactions. The results of independent kinetic simulations verify those of nonlinear fluid simulations.open9

Envelope-kinetic analysis of the electron kinetic effects on Raman backscatter and Raman backward laser amplification

The electron kinetic effects on Raman backscattering and Raman backward laser amplification were analyzed. The analysis is based on the envelope-kinetic equations of a plasma wave, which are composed of the conventional envelope equation of a fluid plasma and the kinetic term. One major goal of this paper is to close the envelope-kinetic model by analyzing the kinetic term, which was not fully covered in the previous work [M. S. Hur et al., Phys. Rev. Lett. 95, 115003 (2005)]. It was found that the closed envelope-kinetic equation in the nontrapping regime takes the same form as the envelope equation of the fluid plasma used in the three-wave model. For the closure in the trapping-dominant regime, the test particle technique is employed to calculate the kinetic term. Results from the full kinetic and test particle simulations agree well with each other, while the latter has a great advantage in computation speed. The frequency shift and resonance breaking by the trapped particles are discussed with the help of a new diagnostic inserted in the full kinetic averaged particle-in-cell code.open5

Effects of the frequency detuning in Raman backscattering of infinitely long laser pulses in plasmas

Raman backscattering (RBS) in an infinite homogeneous laser-plasma system was investigated with the three-wave fluid model and averaged particle-in-cell (aPIC) simulations in the nonrelativistic and low temperature regime. It was found that the periodic boundary condition for the electrostatic potential, which is commonly used in an infinite homogeneous plasma, induces a numerical frequency shift of the plasma wave. The initial frequency detuning between the three waves is modified by the frequency shift, leading to a significantly wrong result in the RBS system. An alternative boundary condition based on the Maxwell equation is presented. The aPIC simulations with the modified boundary condition show that the pump depletion level depends sensitively on the frequency mismatch between the three waves. This sensitivity is closely related with the erroneous RBS: the numerical frequency shift is very minor (a few percent of the plasma frequency or less than that) but RBS can be greatly affected even by such a small frequency change. Analytic formulas for the pump depletion time and level is derived and compared to the aPIC simulations with the modified boundary condition, showing an excellent agreement.open2

Effects of the energy spread of secondary electrons in a dc-biased single-surface multipactor

The effects of the energy spread of secondary electrons are theoretically investigated for a dc-biased single-surface multipactor. In our previous publication [S. G. Jeon et al., Phys. Plasmas 16, 073101 (2009)], we obtained the conditions for the phase lock of an electron bunch, assuming zero velocity spread of the secondary electrons. In this work, we extended our previous theory to derive a quadratic map, by which the stability and bifurcation of the electron bunch can be systematically investigated. For the study of the energy spread of the secondary electrons, a randomized term was added to this map. The modified map then showed significant smearing-out of the bifurcated branches. The theoretical results were verified by particle-in-cell simulations, which showed good agreement in wide parameter ranges for both cases of monoenergetic and energy-spread secondary electrons.open4

Electron acceleration by a short laser beam in the presence of a long-wavelength electromagnetic wave

A scheme for laser-induced acceleration of an electron injected initially at an angle to the direction of a short-wavelength laser is investigated, where an additional long-wavelength electromagnetic wave is introduced to achieve high energy gain. Due to the beating effect of the electromagnetic waves, the electron can gain additional energy. Some computational results are presented to estimate the electron energy gain by the proposed scheme, where the gain increases by increasing the difference of the wavelengths.open9

Measuring the magnetic field of a magnetized plasma using Raman scattering

We studied the Raman scattering in a magnetized plasma by one-dimensional particle-in-cell (PIC) simulations in non-relativistic regime. It is found from the X-mode dispersion relation that the frequency of the backward scattered wave is downshifted by an amount of upper hybrid frequency, while that of the forward scattered wave merely depends on the magnetic field. We propose such a spectral difference be used to measure simultaneously the plasma density and magnetic field of magnetized plasmas. The idea was verified by a series of PIC simulations, where we used the directional field splitting method to obtain accurate peak position of the scattered waves' frequencies. We compared the frequency shift and the growth rate of the scattering from theory and simulations to obtain reasonably good agreement between them for different external magnetic fields.open0