Excitation Of Plasma Waves Research Articles

Mitigation of stimulated Raman backscattering by elliptical laser beam in collisionless plasma

This paper presents theoretical and numerical study of the propagation of an elliptical laser beam in collisionless plasma with relativistic-ponderomotive nonlinearities and its effect on the excitation of electron plasma wave (EPW) and resulting stimulated Raman backscattering (SRBS) process. Nonlinear interaction between an intense elliptical laser beams with the electron plasma wave leads stimulated Raman backscattering. Following Wentzel Kramers Brillouin (WKB) and paraxial-ray approximations, non-linear differential equations for the beam width parameters of elliptical laser beam, electron plasma wave and back-scattered beam are set up and solved numerically. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on the focusing of elliptical laser beam in plasma and further its effect on the excitation of electron plasma wave and backreflectivity of stimulated Raman scattering (SRS). The focusing of an elliptical laser beam, EPW and scattered wave are reduced at higher values of laser intensity in plasma. The amplitude of excited electron plasma wave, which depends on the focusing of main laser beam and electron plasma wave reduce for higher values of incident laser intensity. The results indicate that the focusing of waves mitigates the backreflectivity of SRS. The results are also compared with only relativistic nonlinearity and the Gaussian profile of laser beam. It is observed that an elliptical laser beam mitigates the backreflectivity of SRS at higher values of laser intensity. This study is relevant to inertial confinement fusion scheme, where SRS plays very important role.

The turbulent plasmasphere boundary layer and the outer radiation belt boundary

We report on observations of enhanced plasma turbulence and hot particle distributions in the plasmasphere boundary layer formed by reconnection-injected hot plasma jets entering the plasmasphere. The data confirm that the electron pressure peak is formed just outward of the plasmapause in the premidnight sector. Free energy for plasma wave excitation comes from diamagnetic ion currents near the inner edge of the boundary layer due to the ion pressure gradient, electron diamagnetic currents in the entry layer near the electron plasma sheet boundary, and anisotropic (sometimes ring-like) ion distributions revealed inside, and further inward of, the inner boundary. We also show that nonlinear parametric coupling between lower oblique resonance and fast magnetosonic waves significantly contributes to the VLF whistler wave spectrum in the plasmasphere boundary layer. These emissions represent a distinctive subset of substorm/storm-related VLF activity in the region devoid of substorm injected tens keV electrons and could be responsible for the alteration of the outer radiation belt boundary during (sub)storms.

Nonlinear plasma waves driven by short ultrarelativistic electron bunches

We advance a theory of quasistatic approximation and investigate the excitation of nonlinear plasma waves by the driving beam of ultrarelativistic electrons using a novel electrostatic-like particle-in-cell code. Assuming that the beam occupies an infinitesimally small volume, we find the radius and the length of the plasma bubble formed in the wake of the driver for varying values of the beam charge. The mechanism of bubble formation is explained by developing simple models of the bubble at large charges. Plasma electrons expelled by the driver charge excite secondary plasma waves, which complicate the plasma electron flow near the bubble boundary.

The effect of positively chirped laser pulse on energy enhancement of proton acceleration in combinational radiation pressure and bubble regime

Proton energy enhancement in a combinational radiation pressure and bubble regime by applying a positively chirped laser pulse has been studied using a series of two-dimensional particle-in-cell simulations. In this regime, the proton injection in the half-first period of an excited plasma wave in an under-dense plasma plays the main role in the acceleration process. Moreover, exciting as high as large-amplitude plasma waves can significantly increase the conversion efficiency of laser energy into kinetic energy of the trapped protons. Here, the utilization of the positively chirped laser pulse is proposed as an effective approach to excite the higher amplitude wake in the combinational regime. Our studies indicate that in the positively chirped combinational regime, the plasma wake with approximately two-fold enhancement is produced that results in the generation of the proton bunch with the narrower energy spread and also the peak enhancement by a factor of two, compared with the un-chirped one. This improvement in proton energy reveals that the chirped laser pulse can be introduced as a tool to tune the energy of generated protons in the combinational radiation pressure and bubble regime.

Multi-proton bunch driven hollow plasma wakefield acceleration in the nonlinear regime

Proton-driven plasma wakefield acceleration has been demonstrated in simulations to be capable of accelerating particles to the energy frontier in a single stage, but its potential is hindered by the fact that currently available proton bunches are orders of magnitude longer than the plasma wavelength. Fortunately, proton micro-bunching allows driving plasma waves resonantly. In this paper, we propose using a hollow plasma channel for multiple proton bunch driven plasma wakefield acceleration and demonstrate that it enables the operation in the nonlinear regime and resonant excitation of strong plasma waves. This new regime also involves beneficial features of hollow channels for the accelerated beam (such as emittance preservation and a uniform accelerating field) and long buckets of stable deceleration for the drive beam. The regime is attained at a proper ratio between plasma skin depth, driver radius, hollow channel radius, and micro-bunch period.

Excitation and Control of Plasma Wakefields by Multiple Laser Pulses.

We demonstrate experimentally the resonant excitation of plasma waves by trains of laser pulses. We also take an important first step to achieving an energy recovery plasma accelerator by showing that a plasma wave can be damped by an out-of-resonance trailing laser pulse. The measured laser wakefields are found to be in excellent agreement with analytical and numerical models of wakefield excitation in the linear regime. Our results indicate a promising direction for achieving highly controlled, GeV-scale laser-plasma accelerators operating at multikilohertz repetition rates.

Gas-filled capillaries for plasma-based accelerators

Plasma Wakefield Accelerators are based on the excitation of large amplitude plasma waves excited by either a laser or a particle driver beam. The amplitude of the waves, as well as their spatial dimensions and the consequent accelerating gradient depend strongly on the background electron density along the path of the accelerated particles. The process needs stable and reliable plasma sources, whose density profile must be controlled and properly engineered to ensure the appropriate accelerating mechanism. Plasma confinement inside gas filled capillaries have been studied in the past since this technique allows to control the evolution of the plasma, ensuring a stable and repeatable plasma density distribution during the interaction with the drivers. Moreover, in a gas filled capillary plasma can be pre-ionized by a current discharge to avoid ionization losses. Different capillary geometries have been studied to allow the proper temporal and spatial evolution of the plasma along the acceleration length. Results of this analysis obtained by varying the length and the number of gas inlets will be presented.

Excitation of plasma waves by nonlinear currents induced by a high-frequency electromagnetic pulse

Excitation of plasma waves by nonlinear currents induced by a high-frequency electromagnetic pulse is analyzed within the kinetic approach. It is shown that the most efficient source of plasma waves is the nonlinear current arising due to the gradient of the energy density of the high-frequency field. Generation of plasma waves by the drag current is usually less efficient but not negligibly small at relatively high frequencies of electron–ion collisions. The influence of electron collisions on the excitation of plasma waves by pulses of different duration is described quantitatively.

Modeling of LC-shunted intrinsic Josephson junctions in high-Tc superconductors

Resonance phenomena in a model of intrinsic Josephson junctions shunted by LC-elements (L-inductance, C-capacitance) are studied. The phase dynamics and IV-characteristics are investigated in detail when the Josephson frequency approaches the frequency of the resonance circuit. A realization of parametric resonance through the excitation of a longitudinal plasma wave, within the bias current interval corresponding to the resonance circuit branch, is demonstrated. It is found that the temporal dependence of the total voltage of the stack, and the voltage measured across the shunt capacitor, reflect the charging of superconducting layers, a phenomenon which might be useful as a means of detecting such charging experimentally. Thus, based on the voltage dynamics, a novel method for the determination of charging in the superconducting layers of coupled Josephson junctions is proposed. A demonstration and discussion of the influence of external electromagnetic radiation on the IV-characteristics and charge-time dependence is given. Over certain parameter ranges the radiation causes an interesting new type of temporal splitting in the charge-time oscillations within the superconducting layers.

Effect of negative gases admixture on the stability of beam-driven discharges

The influence of negative gas SF6 admixture on the stability of low-pressure beam-driven discharge is studied by self-consistent 1D particle-in-cell/Monte Carlo collisions model. We find that the plasma parameters as well as its stability are changed drastically when SF6 is added to argon gas. In an electropositive plasma we obtain the excitation of both fast and slow electrostatic plasma waves which is explained by the propagation of electron beam through the non-homogeneous bounded plasma. These waves lead to the heating of plasma electrons and ions. On the other hand, in electronegative plasma we also obtain a decay of the electron beam. However, since in electronegative beam-generated plasma the electron plasma density is homogeneous the beam is more stable. As a consequence, we obtain only the excitation of two fast electrostatic waves. One wave is excited due to two-stream instability and another wave is excited due to beam decay. The excitation of these fast waves does not influence the plasma homogeneity but influences the heating of plasma electrons.

Collapse of nonlinear electron plasma waves in a plasma layer

The excitation of nonlinear electron plasma waves in the plasma layer is investigated theoretically. This excitation is realized by means of initial oscillatory perturbations of the volume electron concentration or by initial oscillatory distributions of the longitudinal electron velocity. The amplitudes of the initial perturbations are small and the manifestation of the volume nonlinearity is absent. When the amplitudes of the initial perturbations exceed some thresholds, the values of the electron concentration near the plasma boundary increase catastrophically. The maxima of the electron concentration reach extremely high magnitudes, and sharp peaks in the electron concentration occur, which are localized both in the longitudinal and transverse directions. This effect is interpreted as wave collapse near the plasma boundary.

Effect of self-focused cosh Gaussian laser beam on the excitation of electron plasma wave and particle acceleration

AbstractThis work presents an investigation of the self-focusing of a high-power laser beam having cosh Gaussian intensity profile in a collissionless plasma under weak relativistic-ponderomotove (RP) and only relativistic regimes and its effect on the excitation of electron plasma wave (EPW), and particle acceleration process. Nonlinear differential equations have been set up for the beam width and intensity of cosh Gaussian laser beam (CGLB) and EPW using the Wentzel-Kramers-Brillouin and paraxial-ray approximations as well as fluid equations. The numerical results are presented for different values of decentered parameter ‘b’ and intensity parameter ‘a’ of CGLB. Strong self-focusing is observed in RP regime as compared with only relativistic nonlinearity. Numerical analysis shows that these parameters play crucial role on the self-focusing of the CGLB and the excitation of EPW. It is also found that the intensity/amplitude of EPW increases withbanda. Further, nonlinear coupling between the CGLB and EPW leads to the acceleration of electrons. The intensity of EPW and energy gain by electrons is significantly affected by including the ponderomotive nonlinearity. The energy of the accelerated electrons is increased by increasing the value of ‘b’. The results are presented for typical laser and plasma parameters.

Plasma Instability of 2D Electrons in a Field Effect Transistor with a Partly Gated Channel

We predict the instability of plasma waves excited by a DC current in the field-effect transistors (FETs) with a partly gated channel. The excitation of plasma waves is due to amplified reflection from the boundary of the gated and ungated regions. The boundary also supports the turbulent edge modes whose increment strongly depends on the ratio of the carrier densities in the two FET regions.

Spectroscopic measurements of plasma emission light for plasma-based acceleration experiments

Advanced particle accelerators are based on the excitation of large amplitude plasma waves driven by either electron or laser beams. Future experiments scheduled at the SPARC_LAB test facility aim to demonstrate the acceleration of high brightness electron beams through the so-called resonant Plasma Wakefield Acceleration scheme in which a train of electron bunches (drivers) resonantly excites wakefields into a preformed hydrogen plasma; the last bunch (witness) injected at the proper accelerating phase gains energy from the wake. The quality of the accelerated beam depends strongly on plasma density and its distribution along the acceleration length. The measurements of plasma density of the order of 1016–1017 cm−3 can be performed with spectroscopic measurements of the plasma-emitted light. The measured density distribution for hydrogen filled capillary discharge with both Balmer alpha and Balmer beta lines and shot-to-shot variation are here reported.

Harmonic plasma waves excitation and structure evolution of intense ion beams in background plasmas

The long-term dynamic evolutions of intense ion beams in plasmas have been investigated with two-dimensional electromagnetic particle simulations, taking into account the effect of the two-stream instability between beam ions and plasma electrons. Depending on the initial beam radial density profile and velocity distribution, ring structures may be formed in the beam edge regions. At the later stage of beam-plasma interactions, the ion beams are strongly modulated by the two-stream instability and multiple density spikes are formed in the longitudinal direction. The formation of these density spikes is shown to result from the excitation of harmonic plasma waves when the instability gets saturated. Comparisons between the beam cases with initial flat-top and Gaussian radial density profiles are made, and a higher instability growth rate is observed for the flat-top profile case.

Propagation of two intense cosh-Gaussian laser beams in plasma in the relativistic-ponderomotive regime

This work presents an investigation of the propagation of two intense cosh-Gaussian laser beams (CGLBs) in collisionless plasma taking into account the combined effects of relativistic and ponderomotive nonlinearities. Due to mutual interaction of two CGLBs, the self-focusing of one beam is affected by the presence of another beam in plasma, and cross-focusing takes place. A paraxial-like approach has been used to investigate the cross-focusing of CGLBs in plasma. An appropriate expression for the nonlinear dielectric constant of plasma has been evaluated in the presence of relativistic-ponderomotive force and used to study the propagation of CGLBs in plasma. The effect of various well-established laser and plasma parameters on the cross-focusing of two beams in plasma has been explored and the results are compared with only relativistic nonlinearity and the Gaussian profile of laser beams. Numerical results show that the decentered parameter (b) plays a crucial role in the cross-focusing of the CGLBs. It is observed that CGLBs focused earlier than Gaussian beams. This study is useful in beat wave excitation of electron plasma wave and collective laser particle accelerators.

Parametric excitation of surface plasma waves by stimulated Compton scattering of laser beam at metal-free space interface

AbstractAn obliquely incident high-power laser (ω0,k0z) on the metallic surface can resonantly excite a surface plasma wave (SPW) (ω1,k1z) and a quasi-electrostatic plasma wave (ω,kz) inside the skin layer at the phase-matching conditions of frequency ω1= ω − ω0and wave numberk1z=kz−k0z. The oscillating electrons in the skin layer couples with the seed SPW and exert non-linear ponderomotive force on electrons at the frequency of quasi-static mode. Density perturbations due to quasi-static mode and ponderomotive force associate with the motion of electrons (due to incident laser) and give rise to a non-linear current by feedback mechanism. At ω/kz~vF(wherevFis the Fermi velocity of metal) this non-linear current is responsible for the growth of SPW. The maximum growth of the present process (≅1.5 × 1012s−1) is achieved at incident angle θ = 42° for laser frequency ω0= 2 × 1015rad/s. Growth of SPW enhances from 1.62 × 1011to ≅1.5 × 1012s−1as the magnetic field changes from 12 to 24 MG. The excited SPW can be utilized for surface heating and diagnostics purpose.

Second Harmonic Generation of Self‐Focused Cosh‐Gaussian Laser Beam in Thermal Quantum Plasma by Excitation of an Electron Plasma Wave

AbstractThis paper presents a scheme for second harmonic generation (SHG) of an intense Cosh‐Gaussian (ChG) laser beam in thermal quantum plasmas. Moment theory approach in W.K.B approximation has been adopted in deriving the differential equation governing the propagation characteristics of the laser beam with distance of propagation. The effect of relativistic increase in electron mass on propagation dynamics of laser beam has been incorporated. Due to relativistic nonlinearity in the dielectric properties of the plasma, the laser beam gets self‐focused and produces density gradients in the transverse direction. The generated density gradients excite electron plasma wave (EPW) at pump frequency that interacts with the incident laser beam to produce its second harmonics. Numerical simulations have been carried out to investigate the effects of laser parameters on selffocusing of the laser beam and hence on the conversion efficiency of its second harmonics. Simulation results predict that within a specific range of decentered parameter the ChG laser beams show smaller divergence as they propagate and, thus, lead to enhanced conversion efficiency of second harmonics. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of cross-focusing of two q-Gaussian laser beams on excitation of electron plasma wave in collisional plasma

This paper presents theoretical investigation of the effect of cross-focusing of two q-Gaussian laser beams on excitation of electron plasma wave (EPW) in an underdense plasma where collisional nonlinearity is operative. The EPW is generated on account of beating of the two laser beams of slightly different frequencies ω1 and ω2. Starting from Maxwell's equations, the coupled differential equations governing the evolution of spot size of the two laser beams with distance of propagation have been derived with the help of moment theory approach in Wentzel–Kramers–Brillouin (W.K.B.) approximation. The collisional nonlinearity depends not only on the intensity of first laser beam but also on that of second laser beam. Therefore, the dynamics of one laser beam affects that of other and hence, cross-focusing of the two laser beams takes place. Due to nonuniform intensity distribution of the laser beams, the background electron concentration is modified. The amplitude of EPW, that depends on the background electron concentration, is thus nonlinearly coupled with the laser beams. The effects of collisional nonlinearity and cross-focusing of the laser beams on excitation of EPW have been incorporated. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on cross-focusing of the two laser beams and further its effect on EPW excitation.

Optical excitation of surface plasma waves without grating structures

Surface plasma waves (SPWs) are usually discussed in the context of a metal in contact with a dielectric. However, they can also exist between two metals. In this work we study these bimetallic waves. We find that their dispersion curve always cuts the light line, which allows direct optical coupling without surface grating structures. We propose practical schemes to excite them and the excitation efficiency is estimated. We also show that these waves can be much less lossy than conventional SPWs and their losses can be systematically controlled, a highly desirable attribute in applications. Conducting metal oxides seem fit for experimental studies.