Overdense Plasma Slab Research Articles

Terahertz wave generation by nonlinear mixing of lasers in a finite overdense plasma slab

Terahertz wave generation by nonlinear mixing of lasers in a finite overdense plasma slab

Laser pulse compression by a density gradient plasma for exawatt to zettawatt lasers

We propose a new method of compressing laser pulses to ultrahigh powers based on spatially varying dispersion of an inhomogeneous plasma. Here, compression is achieved when a long, negatively frequency-chirped laser pulse reflects off the density ramp of an over-dense plasma slab. As the density increases longitudinally, high-frequency photons at the leading part of the laser pulse penetrate more deeply into the plasma region than lower-frequency photons, resulting in pulse compression in a similar way to that by a chirped mirror. Proof-of-principle simulations performed using particle-in-cell simulation codes predict compression of a 2.35 ps laser pulse to 10.3 fs—a ratio of 225. As plasma is robust and resistant to damage at high intensities—unlike solid-state gratings commonly used in chirped-pulse amplification—the method could be used as a compressor to reach exawatt or zettawatt peak powers.

Transmission of electromagnetic waves through a nonlinear over-dense plasma slab

A study on the transmission of the electromagnetic waves from a structure consisting of an over-dense plasma layer with nonlinear effects is undertaken. The applied nonlinearity is presented due to the series expansion of the polarization in the medium. The nonlinear plasma layer is supposed to be placed between two linear dielectric layers. The transparency conditions are investigated for the p-polarized obliquely incident waves. It is shown that the formation of the surface waves can eventuate to the transmission of the incident waves. This fact has already be seen in the case of the linear over-dense plasma, but, here, it is examined in the presence of the nonlinear effects. To determine the propitious conditions for the surface wave excitation, the exact solutions of the electromagnetic field equations are used in all regions. The transmission, the reflection, and the dissipation rates of the electromagnetic waves from the entire structure are obtained, and the effects of the main parameters on them are discussed.

Plasmon resonances, anomalous transparency, and reflectionless absorption in overdense plasmas

The structure of the surface and standing wave resonances and their coupling in the configuration of the overdense plasma slab with a single diffraction grating are studied, using impedance matching techniques. Analytical criteria and exact expressions are obtained for plasma and diffraction grating parameters which define resonance conditions for absolute transparency in the ideal plasma and reflectionless absorption in a plasma with dissipation.

Laser second-harmonic generation from an overdense plasma slab

AbstractA s-polarized short-pulse laser impinged obliquely on an overdense plasma slab is shown to produce very significant second harmonic in the direction of specular reflection and transmission. The laser induces a non-linear current on electrons, which is curl free. However, with sharp plasma boundary, it gives rise to electromagnetic radiation at the second harmonic. Our formalism includes multiple reflections of the incident and second-harmonic waves from both the front and rear surfaces. The present work includes finiteness of the slab. The normalized second-harmonic amplitude acquires a sharp peak at some specific angle of incidence for a particular set of parameters dependent on thickness of the slab and plasma density.

Analytical approach to high harmonics spectrum in the nanobunching regime

With high-order harmonic generation (HHG) from plasma surfaces, it is possible to turn a laser pulse into a train of attosecond or even zeptosecond pulses in the backward radiation. These attosecond pulses may be several orders of magnitude higher than that of the laser pulse under appropriate conditions. We study this process in detail, especially the nanobunching of plasma electron density. We derive an analytical expression that describes the electron density profile and obtain a good agreement with particle-in-cell simulation results. We investigate the most efficient case of HHG at moderate laser intensity (I≈2×1020 W/cm2) on the over-dense plasma slab with an exponential profile pre-plasma. Subsequently, we calculate the spectra of a single attosecond pulse from the backward radiation using our expression for density shape in combination with the equation for the spectrum of nanobunch radiation.

Analytical description of attosecond pulse generation on a plasma surface irradiated by high-intense laser pulses

We study theoretically the process of turning a laser pulse into a train of attosecond or even zeptosecond pulses due to high harmonic generation (HHG) upon backreflection of intense laser radiation from a plasma surface. It is shown that under appropriate conditions these attosecond pulses may have an amplitude that is several orders of magnitude larger than that of the laser pulse. We study this process in detail, especially the nanobunching of the plasma electron density. We derive the analytical expression that describes the electron density profile and obtain a good agreement with particle-in-cell simulations. We investigate the most efficient case of HHG at a moderate laser intensity (normalised vector potential ) on the overdense plasma slab with an exponential pre-plasma profile. Subsequently we calculate the spectra of single attosecond pulses from back radiation using our expression for density shape in combination with the equation for spectrum of nanobunch radiation.

Concave pulse shaping of a circularly polarized laser pulse from non-uniform overdense plasmas

Abstract Pulse shaping of circularly polarized laser pulses in nonuniform overdense plasmas are investigated numerically. Specifically we show by two-dimensional particle-in-cell simulations the generation of a concave pulse front of a circularly polarized, a few tens of petawatt laser pulse from a density-tapered, overdense plasma slab. The concept used for the transverse-directional shaping is the differential transmittance depending on the plasma density, and the laser intensity. For suitable selection of the slab parameters for the concave pulse shaping, we studied numerically the pulse transmittance, which can be used for further parameter design of the pulse shaping. The concavely shaped circularly polarized pulse is expected to add more freedom in controlling the ion-beam characteristics in the RPDA regime.

Plasmon resonance coupling in cold overdense dissipative plasma

Abstract It has been shown that the overdense plasma in the resonance conditions can be totally transparent to the incident electromagnetic waves, due to the excitation of the surface modes or plasmons. This scenario requires excitation of a single surface mode which is provided by placing a dielectric layer in front of the incident wave. Here, it is shown that this procedure would be complete and more efficient by arranging the conditions to excite coupled surface modes, namely by placing two dielectrics on both sides of the plasma layer. The results show that in this case, the overdense plasma slab could poses high transmittance in spite of the presence of the dissipative effects. In addition, in this case, the resonance conditions can be achieved more easily and efficiently, and can be used for wider range of the overdense plasma widths.

Isolated Attosecond Pulses from Laser-Driven Synchrotron Radiation

A quantitative theory of attosecond pulse generation in relativistically driven overdense plasma slabs is presented based on an explicit analysis of synchrotron-type electron trajectories. The subcycle, field-controlled release, and subsequent nanometer-scale acceleration of relativistic electron bunches under the combined action of the laser and ionic potentials give rise to coherent radiation with a high-frequency cutoff, intensity, and radiation pattern explained in terms of the basic laws of synchrotron radiation. The emerging radiation is confined to time intervals much shorter than the half-cycle of the driver field. This intuitive approach will be instrumental in analyzing and optimizing few-cycle-laser-driven relativistic sources of intense isolated extreme ultraviolet and x-ray pulses.

Versatile shaping of a relativistic laser pulse from a nonuniform overdense plasma

We studied the versatile shaping of a petawatt laser pulse using its relativistic transparency in a thin overdense plasma slab. The novel concept here is to use the nonuniformity of the plasma slab in its density or thickness in the transverse direction to control the pulse shaping in both the longitudinal and transverse directions. From 2-dimensional particle-in-cell simulations, we succeeded in fabricating a front shape concave to the propagation direction, an extreme case of transverse shaping. A 1-dimensional analytic formula was then applied to predict the transverse shape, which showed good agreement with the simulations.

Cyclotron effects on double layer ion acceleration from laser-irradiated thin foils

The effect of an axial magnetic field on laser driven ion acceleration from a thin overdense plasma slab is investigated. The magnetic field modifies the refractive index of the plasma and the axial ponderomotive force. The latter compresses the electrons until the space charge field thus created offsets it. When the foil thickness is just bigger than the length at which this happens, the compressed electrons and a thin ion layer detach from the foil forming a double layer that gets accelerated by the laser radiation pressure force. The optimum thickness of laser foil, Δs, for maximum acceleration is sensitive to the polarization of the laser pulse. For right circular polarization it increases, while for left circular polarization it decreases with the magnetic field. The ion energy gain is sensitive to a02ω2∕Δsωp2 (where a0 is the laser field strength, ωp is the plasma frequency, and ω is the laser frequency) and can be tuned by varying the magnetic field.

Electromagnetic wave propagation through an overdense magnetized collisional plasma layer

The results of investigations into the feasibility of using a magnetic window to propagate electromagnetic waves through a finite-sized overdense plasma slab are described. We theoretically calculate the transmission coefficients for right- and left-handed circularly polarized plane waves through a uniform magnetized plasma slab. Using reasonable estimates for the plasma properties expected to be found in the ionized shock layer surrounding a hypersonic aircraft traveling in the earth’s upper atmosphere (radio blackout conditions), and assuming a 1 GHz carrier frequency for the radio communications channel, we find that the required magnetic field for propagation of right-handed circularly polarized, or whistler, waves is on the order of a few hundred gauss. Transmission coefficients are calculated as a function of sheath thickness and are shown to be quite sensitive to the electron collision frequency. One-dimensional particle-in-cell simulations are shown to be in good agreement with the theory. These simulations also demonstrate that Ohmic heating of the electrons can be considerable. Two- and three-dimensional particle-in-cell simulations using a simplified waveguide and antenna model illustrate the same general transmission behavior as the theory and one-dimensional simulations. In addition, a net focusing effect due to the plasma is also observed in two and three dimensions. These simulations can be extended to design and analyze more realistic waveguide and antenna models.

Improvement of Proton Beam Quality from the High-intensity Short Pulse Laser Interaction with a Micro-structured Target

Target design study to improve the quality of an accelerated proton beam from the interaction of a high-intensity short pulse laser with an overdense plasma slab has been accomplished by using a two-dimensional, fully electromagnetic and relativistic particle-in-cell (PIC) simulation. The target consists of a thin core part and a thick peripheral part of equivalent plasma densities, while the ratio of the radius of the core part to the laser spot size, and the position of the peripheral part relative to the fixed core part were varied. The positive effects of this core-peripheral target structure could be expected from the knowledge of the typical target normal sheath acceleration (TNSA) mechanism in a laser-plasma interaction, and were apparently evidenced from the comparison with the case of a conventional simple planar target and the case of the transversal size reduction of the simple planar target. Improvements of the beam qualities including the collimation, the forward directionality, and the beam divergence were verified by detailed analysis of relativistic momentum, angular directionality, and the spatial density map of the accelerated protons.

Anomalous transmission of an intense short-pulse laser through a magnetized overdense plasma

A short-pulse laser having Gaussian radial intensity distribution and incident normally on a magnetized overdense plasma slab undergoes anomalous transmission. The laser induces a large oscillatory velocity on electrons, raising their mass and lowering the plasma frequency. The cyclotron resonance enhances this effect. Moreover, the ponderomotive force-driven electron cavitation lowers the plasma frequency further. These effects are most pronounced near the laser axis; hence, the central portion of the beam tunnels through the magnetized plasma slab, while marginal rays suffer reflection. The output beam thus has a smaller spot size. It also possesses a curvature in the wavefront that may help diagnostics.

High-Quality Energetic Proton Beams from an Intense Laser-Plasma Interaction with a Micro-Structured Slab Target

High-quality energetic proton-beam generation from the interaction of a high-intensity short-pulse laser with an overdense plasma slab has been studied by using a two-dimensional, fully electromagnetic and relativistic particle-in-cell simulation. The collimation and the beam divergence of the accelerated protons are expected to strongly depend on the spatial coupling between the laser pulse and the target plasma. In this work, with a proper micro-modification of the target structure, improved beam quality, including the collimation, the forward directionality and the beam divergence, are clearly demonstrated. For a detailed analysis, the relativistic momentum distribution, the spatial density map and the angular distribution of the accelerated protons are considered.

Effect of plasma profile on ion acceleration in the interaction of a short laser pulse with a thin overdense target

Energetic ion generation from the interaction of a short laser pulse with a thin overdense plasma accompanied by a preplasma and a rear side plasma gradient is investigated by particle-in-cell simulations. The dynamics of ion acceleration depending on the maximum density of the preplasma in front of the overdense plasma slab with a smooth density gradient at the rear side are presented and discussed by comparing a sharp rear side boundary case.

A wavelet-MRA-based adaptive semi-Lagrangian method for the relativistic Vlasov–Maxwell system

In this paper we present a new method for the numerical solution of the relativistic Vlasov–Maxwell system on a phase-space grid using an adaptive semi-Lagrangian method. The adaptivity is performed through a wavelet multiresolution analysis, which gives a powerful and natural refinement criterion based on the local measurement of the approximation error and regularity of the distribution function. Therefore, the multiscale expansion of the distribution function allows to get a sparse representation of the data and thus save memory space and CPU time. We apply this numerical scheme to reduced Vlasov–Maxwell systems arising in laser–plasma physics. Interaction of relativistically strong laser pulses with overdense plasma slabs is investigated. These Vlasov simulations revealed a rich variety of phenomena associated with the fast particle dynamics induced by electromagnetic waves as electron trapping, particle acceleration, and electron plasma wavebreaking. However, the wavelet based adaptive method that we developed here, does not yield significant improvements compared to Vlasov solvers on a uniform mesh due to the substantial overhead that the method introduces. Nonetheless they might be a first step towards more efficient adaptive solvers based on different ideas for the grid refinement or on a more efficient implementation. Here the Vlasov simulations are performed in a two-dimensional phase-space where the development of thin filaments, strongly amplified by relativistic effects requires an important increase of the total number of points of the phase-space grid as they get finer as time goes on. The adaptive method could be more useful in cases where these thin filaments that need to be resolved are a very small fraction of the hyper-volume, which arises in higher dimensions because of the surface-to-volume scaling and the essentially one-dimensional structure of the filaments. Moreover, the main way to improve the efficiency of the adaptive method is to increase the local character in phase-space of the numerical scheme, by considering multiscale reconstruction with more compact support and by replacing the semi-Lagrangian method with more local – in space – numerical scheme as compact finite difference schemes, discontinuous-Galerkin method or finite element residual schemes which are well suited for parallel domain decomposition techniques.

Effects of Underdense Preplasma on the Energetic Proton Generation in Ultraintense Short Laser Pulse Interaction with an Overdense Plasma Slab

High-energy proton generation from the interaction of an ultraintense femtosecond laser pulse with an overdense plasma slab combined with an underdense preplasma has been studied by using two-dimensional, fully electromagnetic and relativistic particle-in-cell simulations. With a proper condition of the preplasma, it was observed that the forwardly directed proton acceleration from the rear side of the plasma slab was surely enhanced and the resulting proton beams had a better directionality compared to those in case of no preplasma. For the detailed analysis, the proton momentum distribution, and the angular distribution of the accelerated protons were investigated with the variation of target thickness, laser intensity, and preplasma scale length.

Self-induced transparency scenario revisited via beat-wave heating induced by Doppler shift in overdense plasma layer

Maxwell-fluid simulations on a flat-topped moderately overdense plasma slab (typically n0∕nc=1–2) by Berezhiani et al. [Phys. Plasmas 66, 062308 (2005)] {see also the previous work of Tushentsov et al. [Phys. Rev. Lett. 87, 275002 (2001)]} were seen to lead to dynamic penetration of an ultrahigh intensity laser pulse into an overdense plasma. Two qualitatively different scenarios for the penetration of laser pulse into the overdense plasma were presented depending on the background density. In the first one, the penetration of laser energy occurs by soliton-like structures moving into the plasma. In the last one, electron cavitation occurs and the penetration is possible over a finite length only. A kinetic extension is made in this paper using Vlasov-Maxwell simulations. Vlasov simulations revealed a rich variety of new phenomena associated with the trapped particle dynamics, which cannot be described in fluid models. Most notably is the observation, during the penetration phase of the pump electromagnetic wave, of a beat-wave heating scenario induced by the Doppler shift on the reflected wave at the (moving) wave front. This beat-wave generates low-frequency acoustic-like electron modes characterized by coherent trapping-type structures in phase space leading to an efficient (nonstochastic) heating process.