Strong Ponderomotive Force Research Articles

Controlling the characteristics of injected and accelerated electron bunch in corrugated plasma channel by temporally asymmetric laser pulses

In laser-driven plasma wakefield accelerators, the accelerating electric field is orders of magnitude stronger than in conventional radio-frequency particle accelerators, but the dephasing between the ultrarelativistic electron bunch and the wakefield traveling at the group velocity of the laser pulse puts a limit on the energy gain. Quasi-phase-matching, enabled by corrugated plasma channels, is a technique for overcoming the dephasing limitation. The attainable energy and the final properties of accelerated electron beams are of utmost importance in laser wakefield acceleration (LWFA). In this work, using two-dimensional particle-in-cell simulations, the effect of the driving pulse duration on the performance of quasi-phase-matched laser wakefield acceleration (QPM-LWFA) is investigated. It is observed that for a pulse duration around half the plasma period, the maximum energy gain of the beam electrons finds its peak value. However, the results show that for a pulse of that duration the collimation of the bunch is much worse, compared to the case where the pulse duration is twice as long. Furthermore, the dynamics of the laser pulse and the evolution of the quality of the externally-injected electron bunch are studied for a symmetric pulse with sine-squared temporal profile, a positive skew pulse (i.e., one with sharp rise and slow fall), and a negative skew pulse (i.e., one with a slow rise and sharp fall). The results indicate that for a laser pulse with an appropriate pulse length compared with the plasma wavelength, the wakefield amplitude can be greatly enhanced by using a positive skew pulse, which leads to higher energy gain. Initially, this results from the stronger ponderomotive force associated with a fast rise time. Later, due to the distinct evolution of the three pulses with different initial profiles, the wakefield excited by the positive skew pulse becomes even stronger. In our simulations, the maximum energy gain for the asymmetric laser pulse with a fast rise time is almost two times larger than for the temporally symmetric laser pulse. Nevertheless, stronger focusing and defocusing fields are generated as well if a positive skew pulse is applied, which degrade the collimation of the bunch. These results should be taken into account in the design of miniature particle accelerators based on QPM-LWFA.

Third harmonic generation of a relativistic self-focusing laser in plasma under exponential density ramp

Abstract Present study focuses on self-focusing and its effect on third harmonic generation (THG) of a Gaussian laser beam in plasma under the influence of exponential density ramp. Relativistic nonlinearity has been taken into account which is aroused due the modification of electron’s mass in the presence of high intensity laser. Under strong ponderomotive force, electrons acquire very high quiver velocity and mass variation takes place. Equations for beam width parameter of incident laser and the amplitude of THG have been derived under WKB and paraxial ray approximation, and solved them numerically. It is found that the presence of exponential plasma density ramp results strong self-focusing of laser which further leads to enhance the efficiency of THG. Wiggler magnetic field adds an additional momentum to the photons of third harmonic due to which appreciable gain is observed in the normalized amplitude of THG. Significant enhancement in the THG amplitude has been reported in the presence of exponential density ramp for optimum values of intensity of incident laser, wiggler magnetic field and plasma frequency.

Relativistic-induced opacity of electron–positron plasmas

The interaction of intense electromagnetic waves with electron–positron () plasmas is studied by particle-in-cell simulations and theoretical analysis. It is found that an initial underdense plasma can become opaque under the irradiation of a relativistically intense laser pulse. The strong ponderomotive force of the relativistic laser pulse and the small mass density of the plasma can combine to induce the efficient pile-up of the electrons and positrons at the front of the laser pulse. Therefore, the local plasma density at the laser pulse front increases dramatically and finally the initial underdense plasma becomes opaque. This relativistic-induced opacity effect of plasmas is opposite to the well-known relativistic-induced transparency effect, in which an initial overdense electron-ion plasma can become transparent to a relativistically intense laser pulse. Further, the significant red shift of reflected lights as well as the efficient generation of energetic positrons are investigated in the relativistic-induced opacity of plasmas. This relativistic-induced opacity effect is a peculiar phenomenon in the plasmas, which may be encountered in the high-energy astrophysical phenomena or in the interactions of intense lasers with matters in the laboratories.

Interfacial ponderomotive force in solids leads to field induced dissolution of materials and formation of non-equilibrium nanocomposites

We report that microwave radiation can decompose continuous solid-solution materials into their constituent phases – a process that is thermodynamically unfavorable at equilibrium. A detailed analysis of the interaction of the electromagnetic wave with the material showed that a strong ponderomotive force preferentially separates the constituent phases via an enhanced mass transport process amplified particularly near the interfaces. The proof of concept experiments showed that the material, whether it is a solid-solution of two elements, e.g. (Si1-xGex), or two compounds, e.g. (Bi2Te3)1-x(Sb2Te3)x, decomposes into the constituent phases when radiated by a polarized microwave field. The dissolution happens in the bulk of the material and even below the melting point. The degree of decomposition can be controlled by radiation parameters to produce structures composed of gradient phases of the solid-solution. This offers a novel and facile method for synthesizing gradient composite and complex structures for application in thermoelectricity as well as fabrication of core-shell structures for catalysts and biomedical applications.

Laser-pulse shape effects on magnetic field generation in underdense plasmas

Laser pulse shape effect has been considered to estimate the self-generated magnetic field in laser-plasma interaction. A ponderomotive force based physical mechanism has been proposed to investigate the self-generated magnetic field for different spatial profiles of the laser pulse in inhomogeneous plasmas. The spatially inhomogeneous electric field of a laser pulse imparts a stronger ponderomotive force on plasma electrons. Thus, the stronger ponderomotive force associated with the asymmetric laser pulse generates a stronger magnetic field in comparison to the case of a symmetric laser pulse. Scaling laws for magnetic field strength with the laser and plasma parameters for different shape of the pulse have been suggested. Present study might be helpful to understand the plasma dynamics relevant to the particle trapping and injection in laser-plasma accelerators.

High-power terahertz emission in magnetized plasma via optical rectification of a super-Gaussian laser beam

The paper discusses the generation of terahertz (THz) wave via the optical rectification of an amplitude-modulated super-Gaussian laser beam in the preformed periodic density plasma when a magnetic field is applied transversely. The non-uniform spatial variation in intensity of the laser beam drives a strong ponderomotive force. Such ponderomotive force generates nonlinear oscillatory drift velocity to plasma electrons that produce a strong transverse nonlinear current at the modulation frequency (the frequency of the radiation lies in the THz domain). Moreover, the presence of the periodic density plasma structure and magnetic field can shift the condition of resonance and can produce the tunable THz frequency radiation. In the current scheme, the THz field amplitude increases with the profile index of the laser, modulation index, ripple parameters, and transversely applied static magnetic field. As a result, the efficiency of the generated THz wave can be achieved of the order of for the exact phase-matching condition.

Relativistic harmonics for turbulent wakefield diagnostics

The propagation properties of relativistic harmonics excited in a plasma with an intense laser pulse is investigated theoretically and numerically. Focusing on the frequency separation, a cold electron fluid model in two spatial dimension is discussed to obtain the harmonic amplitude. The theoretical predictions are verified by performing particle-in-cell simulations in two spatial dimensions. When the laser amplitude is large, the strong ponderomotive force expels the electrons, creating a large amplitude density structures associated with the wakefield. The harmonics propagate obliquely with respect to the laser propagation direction, which is well represented by the structure of the high density layer resulting from the transverse poderomotive force. We also discuss a possible experimental setup to observe the density structures relevant to wakefield.

Terahertz radiation generation by beating of two super Gaussian lasers in plasma having static dc electric field

This paper presents a scheme for the generation of high power terahertz radiation by the beating of two femtosecond super Gaussian lasers in plasma having dc electric field in the transverse direction. In this mechanism, a strong nonlinear ponderomotive force acts on the plasma electrons at the frequency difference of the two lasers (ω1−ω2) that imparts a nonlinear oscillatory velocity to plasma electrons which further result in the generation of a nonlinear current at this difference frequency (lying in THz domain). The dynamical equations governing the generation of THz waves have been solved semi-analytically; the result shows that the amplitude of the generated waves is considerably enhanced in the presence of dc electric field, and the index of super Gaussian beams also plays a keen role in governing the yield of THz waves.

Temporally asymmetric laser pulse for magnetic-field generation in plasmas

Of particular interest in this article, the case study of an asymmetric laser pulse interaction with a plasma for magnetic field enhancement has been investigated. The strong ponderomotive force due to the short leading edge of the propagating laser pulse drives a large nonlinear current, producing a stronger quasistatic magnetic field. An analytical expression for the magnetic field is derived and the strength of the magnetic field is estimated for the current laser-plasma parameters. The theoretical results are validated through the particle-in-cell (PIC) simulations and are in very close agreement with the simulation based estimations. This kind of magnetic field can be useful in the plasma based accelerators as well as in the laser-fusion based experiments.

Generation of terahertz radiation by beating of two circular flat-topped laser beams in collisional plasma

AbstractThis paper presents a scheme of terahertz (THz) radiation generation based on beating of two flat-topped laser beams by different frequencies and the same electric field amplitudes in actual plasma with spatially periodic density that electron–neutral collisions have been taken into account. Flat-topped laser beams have the exclusive features such as steep gradient in distribution of laser intensities, wider cross-section in comparison with other profiles, which make stronger ponderomotive force and lead to stronger nonlinear current and hence, THz radiation of higher field. The effects of laser and plasma parameters on THz radiation generation are investigated analytically. It is shown that by increasing the order of flatness of incident laser beams, because of their steep gradient, good enhancement in emitted THz radiation take place. It can be deduced that by increasing beating frequency, efficiency of THz generation decreases which can be compensated by manipulating in density ripple magnitudes. The intensity of the emitted radiations is found to be highly sensitive to the order of flatness. Based on the results of this paper, optimization of laser and plasma parameters can increase the efficiency of THz radiation generation strongly.

High-intensity terahertz generation by nonlinear frequency-mixing of lasers in plasma with DC magnetic field

AbstractWe propose a mechanism of highly focused, tunable and high-intensity terahertz (THz) radiation generation by frequency-mixing of two super-Gaussian lasers with frequencies ω1, ω2 and wave numbers k1, k2 (laser profile index p > 2) in a corrugated plasma in the presence of external static magnetic field ${B_0}\hat z$. In this process, a strong nonlinear ponderomotive force is offered to the plasma electrons at frequency ω′ = ω1 − ω2 and wave number k′ = k1 − k2 by laser beams. The ponderomotive force results in a strong, controllable nonlinear transverse oscillatory current, which can be optimized by optimizing the external magnetic field, ripple parameters, and laser indexes. This controllable current produces focused and intense THz radiation of tunable frequency and power along with a remarkable efficiency ~25%.

Semianalytical study of the propagation of an ultrastrong femtosecond laser pulse in a plasma with ultrarelativistic electron jitter

The interaction of a multi-petawatt, pancake-shaped laser pulse with an unmagnetized plasma is studied analytically and numerically in a regime with ultrarelativistic electron jitter velocities, in which the plasma electrons are almost completely expelled from the pulse region. The study is applied to a laser wakefield acceleration scheme with specifications that may be available in the next generation of Ti:Sa lasers and with the use of recently developed pulse compression techniques. A set of novel nonlinear equations is derived using a three-timescale description, with an intermediate timescale associated with the nonlinear phase of the electromagnetic wave and with the spatial bending of its wave front. They describe, on an equal footing, both the strong and the moderate laser intensity regimes, pertinent to the core and to the edges of the pulse. These have fundamentally different dispersive properties since in the core the electrons are almost completely expelled by a very strong ponderomotive force, and the electromagnetic wave packet is imbedded in a vacuum channel, thus having (almost) linear properties. Conversely, at the pulse edges, the laser amplitude is smaller, and the wave is weakly nonlinear and dispersive. New nonlinear terms in the wave equation, introduced by the nonlinear phase, describe without the violation of imposed scaling laws a smooth transition to a nondispersive electromagnetic wave at very large intensities and a simultaneous saturation of the (initially cubic) nonlocal nonlinearity. The temporal evolution of the laser pulse is studied both analytically and by numerically solving the model equations in a two-dimensional geometry, with the spot diameter presently used in some laser acceleration experiments. The most stable initial pulse length is estimated to exceed ≳1.5–2 μm. Moderate stretching of the pulse in the direction of propagation is observed, followed by the development of a vacuum channel and of a very large electrostatic wake potential, as well as by the bending of the laser wave front.

Generation of intense ultrashort midinfrared pulses by laser-plasma interaction in the bubble regime

As an intense laser pulse propagates through an underdense plasma, the strong ponderomotive force pushes away the electrons and produces a trailing plasma bubble. In the meantime the pulse itself undergoes extreme nonlinear evolution that results in strong spectral broadening toward the long-wavelength side. By experiment we demonstrate that this process can be utilized to generate ultrashort midinfrared pulses with an energy three orders of magnitude larger than that produced by crystal-based nonlinear optics. The infrared pulse is encapsulated in the bubble before exiting the plasma, hence is not absorbed by the plasma. The process is analyzed experimentally with laser-plasma tomographic measurements and numerically with three-dimensional particle-in-cell simulation. Good agreement is found between theoretical estimation, numerical simulation, and experimental results.

Effects of ponderomotive forces and space-charge field on laser plasma hydrodynamics

Using a two_fluid two_temperature hydrodynamic code, we studied the hydrodynamic s in the interaction of intense (1015W/cm2) ultrashort (150fs) laser pulses and linear density plasmas. The simulation results show the ponderomotiv e force effect on the formation of the electron density ripples in underdense re gion, such ripples increase the reflection of the laser pulse, and on the separa tion of the plasma in critical surface. Quasi_electroneutrality is not suitable in this case because of the different ponderomotive force and the gradient of th ermal_pressure for ions and electrons. Ions are moved by the electrostatic force . Comparing with the simulation results from one_fluid two_temperature code, we find that under strong ponderomotive force and gradient of thermo_pressure, two_ fluid code is more suitable to simulate the hydrodynamics of plasmas.

Observation of the plasma channel dynamics and Coulomb explosion in the interaction of a high-intensity laser pulse with a He gas jet

We report the first interferometric observations of the dynamics of electron-ion cavitation of relativistically self-focused intense 4 TW, 400 fs laser pulse in a He gas jet. The electron density in a channel 1 mm long and 30 μm in diameter drops by a factor of approximately 10 from the maximum value of ∼8×1019 cm−3. A high radial velocity of the plasma expansion, ∼3.8×108 cm/s, corresponding to an ion energy of about 300 keV, is observed. The total energy of fast ions is estimated to be 6% of the laser pulse energy. The high-velocity radial plasma expulsion is explained by a charge separation due to the strong ponderomotive force. This experiment demonstrates a new possibility for direct transmission of a significant portion of the energy of a laser pulse to ions.

Transient evolution of localized ion acoustic waves in plasmas.

We derive an integrable model of coupled equations for the description of the transient evolution of small-amplitude ion acoustic waves interacting with the electromagnetic waves. We obtain then a universal behavior of the acoustic wave driven by a strong ponderomotive force: The asymptotic solution consists of two supersonic localized singular coherent structures following a sonic wave front and asymptotically colliding at the sound speed. The reflected electromagnetic wave presents singularities in the same regions as the acoustic wave.

Nonlinear ion waves driven by the periodic ponderomotive force

The nonlinear properties of ion acoustic waves produced by a strong ponderomotive force have been analyzed in the regime where the amplitude of ponderomotive potential is comparable to the plasma thermal energy density. Analytical solutions of the nonlinear fluid equations predict formation of the periodic sequence of shock waves. These solutions compare well with the results of numerical simulations of the full set of the hydrodynamical and Maxwell equations, which also reveal the oscillatory structure of the shock. These results are applied to the analysis of stimulated Brillouin scattering.

Edge modelling of ICRF heated plasmas on PLT

Theoretical models are presented to explain the edge plasma-antenna interaction that occurs during ICRF heating. The periodic structure of the Faraday shield is found to result in strong ponderomotive force in the vicinity of the antenna. A fluid model, which incorporates the ponderomotive force, predicts an increase in particle transport to the Faraday shield. Kinetic modelling shows that the strong antenna near-fields act to increase the energy of deuterons that strike the shield, thereby increasing the sputtering of shield material. In addition, kinetic modelling shows that E1 induced between adjacent shield elements acts to heat edge electrons that transit close to the antenna. The predictions of the models are shown to be consistent with measurements of enhanced transport on PLT.

Invited paper: ICRF edge modeling studies

Theoretical models have been developed, and are currently being refined, to explain the edge plasma-antenna interaction that occurs during ICRF heating. The periodic structure of a Faraday shielded antenna is found to result in strong ponderomotive force in the vicinity of the antenna. A fluid model, which incorporates the ponderomotive force, shows an increase in transport to the Faraday shield. A kinetic model shows that the strong antenna near fields act to increase the energy of deuterons which strike the shield, thereby increasing the sputtering of shield material. Estimates of edge impurity harmonic heating show no significant heating for either in or out-of-phase antenna operation. Additionally, a particle model for electrons near the shield shows that heating results from the parallel electric field associated with the fast wave. A quasilinear model for edge electron heating is presented and compared to the particle calculations. The models' predictions are shown to be consistent with measurements of enhanced transport.

Demonstration of collisionally enhanced degenerate four-wave mixing in a plasma.

Experimental evidence of degenerate four-wave mixing in a plasma is presented for the first time. The thermal-force contribution to the signal reflectivity, as evidenced by its temporal shape and dependence on plasma density, pump and probe intensities, and polarizations, dominates that due to the ponderomotive force in the collisional plasma. The pump-pump grating, which is driven by a much stronger ponderomotive force, is found not to affect adversely the signal reflectivity.