Interaction Of Ultraintense Laser Pulse Research Articles

Target transverse size and laser polarization effects on pair production during ultra-relativistic-intense laser interaction with solid targets

Pair production from the Breit-Wheeler process in ultra-intense laser pulse interactions with solid targets are studied by particle-in-cell simulations using the EPOCH code including the quantum electrodynamics module. We find that the pair yield depends on both the target transverse size and the laser pulse duration. For a short laser pulse, the highest pair yield is achieved with a target as wide as the laser spot size. For a long laser pulse, however, the optimal target size for the pair production increases with the pulse duration due to a self-generated cone by the hole-boring process. The effect of laser polarization upon the pair production is also studied. It is found that a circularly polarized laser pulse is more efficient in the ion acceleration rather than in the pair production. With the same laser energy, we find that a linearly polarized laser pulse can generate two times more positrons than the circularly polarized laser pulse does. These findings may benefit the future researches on the laser plasma based electron-positron production.

Particle simulation study on anisotropic pressure of electrons in laser-produced plasma interaction

Direct-drive inertial confinement fusion (ICF) requires a symmetric compression of the fuel target to achieve physical conditions for the ignition. The fast ignition scheme reduces the symmetry requirements for the target compression and the necessary driving energy, but symmetrically compressed target will certainly help improve the efficiency of the nuclear fuel burning. In this paper, with the particle-in-cell (PIC) simulation method, characteristics of the anisotropic pressure tensor of hot electrons are reported for the ultra intense laser pulse interaction with over dense plasmas, which mimics the scenario of the last stage when hot electrons are utilized to ignite the compressed fuel core in the ICF fast ignition scheme. A large number of hot electrons can stimulate pressure oscillations in the high density plasma. As the component parallel to the electron velocity dominates the pressure tensor, the electron density distribution perturbation propagates rapidly in this direction. In order to keep those hot electrons in the high density fuel plasma core for a period long enough for them to deposit energy and momentum, a magnetic field perpendicular to the electron velocity is used. The PIC simulation results indicate that the hot electrons can be trapped by the magnetic field, and the components of the anisotropic pressure tensor related to the parallel direction are significantly affected, thereby producing a high peak near the incidence surface. Since it is a relatively long process for the energy transfer from electrons to fuel ions and the nuclear interaction to be completed, the fluid effects take their roles in the fuel target evolution. The anisotropic electron pressure will deteriorate the fuel core symmetry, reduce the density, and achieve a lower efficiency of nuclear fuel burning and a lower gain of nuclear reaction than expected. The effects of the hot electron anisotropic pressure tensor in the fast ignition scheme should be considered as a factor in experiments where the nuclear reaction gain is measured to be much lower than the theoretical prediction.

Nonlinear interaction of ultraintense laser pulse with relativistic thin plasma foil in the radiation pressure-dominant regime

We present a study of the effect of laser pulse temporal profile on the energy /momentum acquired by the ions as a result of the ultraintense laser pulse focussed on a thin plasma layer in the radiation pressure-dominant (RPD) regime. In the RPD regime, the plasma foil is pushed by ultraintense laser pulse when the radiation cannot propagate through the foil, while the electron and ion layers move together. The nonlinear character of laser–matter interaction is exhibited in the relativistic frequency shift, and also change in the wave amplitude as the EM wave gets reflected by the relativistically moving thin dense plasma layer. Relativistic effects in a high-energy plasma provide matching conditions that make it possible to exchange very effectively ordered kinetic energy and momentum between the EM fields and the plasma. When matter moves at relativistic velocities, the efficiency of the energy transfer from the radiation to thin plasma foil is more than 30% and in ultrarelativistic case it approaches one. The momentum /energy transfer to the ions is found to depend on the temporal profile of the laser pulse. Our numerical results show that for the same laser and plasma parameters, a Lorentzian pulse can accelerate ions upto 0.2 GeV within 10 fs which is 1.5 times larger than that a Gaussian pulse can.

Effects of filamentation instability on the divergence of relativistic electrons driven by ultraintense laser pulses

Generation of relativistic electron (RE) beams during ultraintense laser pulse interaction with plasma targets is studied by collisional particle-in-cell simulations. A strong magnetic field with a transverse scale length of several local plasma skin depths, associated with RE current propagation in the target, is generated by filamentation instability in collisional plasmas, inducing a great enhancement of the divergence of REs compared to that of collisionless cases. Such an effect is increased with laser intensity and target charge state, suggesting that the RE divergence might be improved by using low-Z materials under appropriate laser intensities in future fast ignition experiments and in other applications of laser-driven electron beams.

Improvement of proton beam quality by an optimized dragging field generated by the ultraintense laser interactions with a complex double-layer target

AbstractA scheme for the improvement of proton beam quality by the optimized dragging field from the interaction of ultraintense laser pulse with a complex double-layer target is proposed and demonstrated by one-dimensional particle-in-cell (Opic1D) simulations. The complex double-layer target consists of an overdense proton thin foil followed by a mixed hydrocarbon (CH) underdense plasma. Because of the existence of carbon ions, the dragging field in the mixed CH underdense plasma becomes stronger and flatter in the location of the proton beam than that in a pure hydrogen (H) underdense plasma. The optimized dragging field can keep trapping and accelerating protons in the mixed CH underdense target to high quality. Consequently, the energy spread of the proton beam in the mixed CH underdense plasma can be greatly reduced down to 2.6% and average energy of protons can reach to 9 GeV with circularly polarized lasers at intensities 2.74 × 1022 W/cm2.

Electron sheath dynamic in the laser–foil interaction

AbstractIn the interaction of ultra-short and ultra-intense high contrast laser pulse with a dense foil, accelerating electron sheath is formed. The dynamic of this sheath is obtained according to the ponderomotive force of the laser pulse and restoring electrostatic force of the stationary heavy ions. In the transient dynamics, maximum electron sheath displacement is obtained for different interaction parameters. This maximum displacement has an important effect in the explanation of the electron blow out condition. It is shown numerically that the electron sheath maximum displacement increases with increasing laser pulse amplitude or decreasing its rise time, or by decreasing plasma electron density. Recently, backward MeV acceleration of electrons in the interaction of intense laser pulse with solid targets was observed. The ponderomotive force of the compressed reflected laser pulse includes in our formalism and is used for explanation of the electron's backward acceleration. The threshold values of the interaction parameters for the occurrence of this phenomenon are considered. The electron blow out condition and backward acceleration are accompanied with numerical modeling and 1D3V, particle-in-cell simulation code.

Controlled electron bunch generation in the few-cycle ultra-intense laser–solid interaction scenario

The generation of Maxwellian or exponentially decaying spectra in the interaction of ultra-intense ultra-short laser pulses with solid foils is very general observation both in experiments and simulations. Yet, the physical origin of this observation is not well understood. For a very idealized situation of plane wave, plane and cold target interaction, we show that both randomization between individual electron bunches accelerated by the laser through the plasma as well as randomization during a single bunch are not observable in particle-in-cell simulations. Hence they are not accountable for the apparent thermalization (exponential spectrum).

Collimated proton beams by ultra-short, ultra-intense laser pulse interaction with a foil–ramparts target

AbstractA foil–ramparts target interaction with an ultra-short, ultra-intense laser pulse (pulse duration between 10−12 and 10−15 s, intensity above 1018 W cm−2) to produce proton beams with controlled divergence and concentrated energy density in target normal sheath acceleration regime is studied. Two-dimension-in-space and three-dimension-in-velocity particle-in-cell simulations show that the foil–ramparts target helps to reshape the sheath electric field and generate a transverse quasi-static electric field of ~6.7 TV m−1 along the inner wall of the ramparts. The transverse electric field suppresses the transverse expansion of the proton beam effectively, as it tends to force the produced protons to focus inwards to the central axis, resulting in controlled divergence and concentrated energy density compared with that of a single plain target. The dependence of proton beam divergence on length of the rampart h is investigated in detail. A rough estimation of h ranges depending on dimensionless parameter a0 of the incident laser is also given.

Physics of the interaction of ultra intense laser pulses with cold collisional plasma using large scale kinetic simulations

We present a set of 2D collisional particle-in-cell simulations of the interaction of ultra-intense laser pulses with over-dense cold collisional plasmas. The size of these simulations is about 100 times as large as those previously published. This allows studying the transport of energetic particles on time scale of the order of 400 fs without perturbations due to the influence of boundary effects and performing a very detailed analysis of the physics of the transport. We confirm the existence of a threshold in intensity close to the relativistic threshold above which the beam of energetic particles diverges when it penetrates the cold plasma. We also study the applicability of Ohm's law to compute the electric field, which is the method commonly used in hybrid codes. The heating of the cold plasma is then studied and we show that half of the heating is anomalous, i.e., not given by standard Joule effect. We discuss the previously published results in the light of these new simulations.

Multi-MeV quasi-mono-energetic electron beam generation from interaction of ultraintense laser pulse with solid target at grazing incidence

Generation of a quasi-mono-energetic electron beam with peak energy ~3 MeV and full cone divergence angle of ~3° is observed in the interaction of 45 fs, 3 TW Ti:sapphire laser beam incident on a planar carbon target surface at grazing incidence. The collimated and quasi-mono-energetic electron beam was observed when a pre-pulse pedestal of 1-ns duration was used with the main 45-fs laser pulse. The study presented on generation of spatially and spectrally concentrated multi-MeV electron beam along solid target surface could be of significance for current areas of research like fast ignition scheme of inertial confinement fusion.

Transient changes in electric fields induced by interaction of ultraintense laser pulses with insulator and metal foils: Sustainable fields spanning several millimeters.

The temporal evolutions of electromagnetic fields generated by the interaction between ultraintense lasers (1.3×10(18) and 8.2×10(18)W/cm(2)) and solid targets at a distance of several millimeters from the laser-irradiated region have been investigated by electron deflectometry. For three types of foil targets (insulating foil, conductive foil, and insulating foil onto which a metal disk was deposited), transient changes in the fields were observed. We found that the direction, strength, and temporal evolution of the generated fields differ markedly for these three types of targets. The results provide an insight for studying the emission dynamics of laser-accelerated fast electrons.

Enhanced dense attosecond electron bunch generation by irradiating an intense laser on a cone target

By using two-dimensional particle-in-cell simulations, we demonstrate enhanced spatially periodic attosecond electron bunches generation with an average density of about 10nc and cut-off energy up to 380 MeV. These bunches are acquired from the interaction of an ultra-short ultra-intense laser pulse with a cone target. The laser oscillating field pulls out the cone surface electrons periodically and accelerates them forward via laser pondermotive force. The inner cone wall can effectively guide these bunches and lead to their stable propagation in the cone, resulting in overdense energetic attosecond electron generation. We also consider the influence of laser and cone target parameters on the bunch properties. It indicates that the attosecond electron bunch acceleration and propagation could be significantly enhanced without evident divergency by attaching a plasma capillary to the original cone tip.

Non-linear interaction of ultra-intense ultra-short laser pulse with a relativistic flying double-sided dense plasma slab/mirror

AbstractAnalytical and numerical investigation of the reflection and transmission of a counter-propagating relativistically strong laser pulse from a relativistically flying dense plasma double-sided mirror is studied. We assume that the incident laser pulse is short, so that we can neglect the slow ion dynamics and consider the electron motion only. Numerical results of the amplitudes of the reflected/transmitted electric fields from a uniformly moving mirror, accelerated mirror, and oscillating mirror are obtained. Fourier spectrum of the reflected intensity from the moving mirror shows that the intensity decreases with increase in the frequency. The reflected pulse has an up-shifted frequency and increased intensity. It is seen that the first few cycles of the reflected radiation exhibit presence of high harmonics, while the later cycles are compressed together with harmonics in comparison with the earlier cycles. The variation of the reflection coefficient for a uniformly moving mirror as a function of the thin foil plasma-density parameter is numerically studied.

Study on magnetic field generation and electron collimation in overdense plasmas

An analytical fluid model is proposed for artificially collimating fast electron beams produced in interaction of ultraintense laser pulses with specially engineered sandwich structure targets. The theory reveals that in low-density-core structure targets, the magnetic field is generated by the rapid change of the flow velocity of the background electrons in transverse direction (perpendicular to the flow velocity) caused by the density jump. It is found that the spontaneously generated magnetic field reaches as high as 100 MG, which is large enough to collimate fast electron transport in overdense plasmas. This theory is also supported by numerical simulations performed using a two-dimensional particle-in-cell code. It is found that the simulation results agree well with the theoretical analysis.

Large quantity ion beam generation by persistent Coulomb explosion in a near-critical density plasma channel

The mechanism of Coulomb explosion induced by the interactions of ultra-intense laser pulses with near-critical density plasmas was investigated using 2.5D particle-in-cell simulations. While the Coulomb explosion occurred continuously during pulse propagation inside the plasma, a large quantity of charge was generated and acquired in the backward direction. The accelerated ion beam had a peak energy of several tens of MeV, and the maximum energy was over hundreds MeV. A theoretical model has been proposed to estimate the total acquired charge quantity, the maximum ion energy, and their dependence on the initial plasma density.

Ultra-relativistic ion acceleration in the laser-plasma interactions

An analytical relativistic model is proposed to describe the relativistic ion acceleration in the interaction of ultra-intense laser pulses with thin-foil plasmas. It is found that there is a critical value of the ion momentum to make sure that the ions are trapped by the light sail and accelerated in the radiation pressure acceleration (RPA) region. If the initial ion momentum is smaller than the critical value, that is in the classical case of RPA, the potential has a deep well and traps the ions to be accelerated, as the same described before by simulation results [Eliasson et al., New J. Phys. 11, 073006 (2009)]. There is a new ion acceleration region different from RPA, called ultra-relativistic acceleration, if the ion momentum exceeds the critical value. In this case, ions will experience a potential downhill. The dependence of the ion momentum and the self-similar variable at the ion front on the acceleration time has been obtained. In the ultra-relativistic limit, the ion momentum at the ion front is proportional to t4/5, where t is the acceleration time. In our analytical hydrodynamical model, it is naturally predicted that the ion distribution from RPA is not monoenergetic, although the phase-stable acceleration mechanism is effective. The critical conditions of the laser and plasma parameters which identify the two acceleration modes have been achieved.

Optimisation of neutron yield under ultra-intense laser impact on deuterated polyethylene targets

The modelling of interaction of ultra-intense femtosecond laser pulses with deuterated polyethylene targets is carried out with the processes of multiple field ionisation of the target atoms and theemission of neutrons produced in the course of the fusion reaction at high-energy deuteron collisions taken into account. The possibility of essential increase in the neutron yield (by tens of times) when using layered deuterated polyethylene targets with optimal dimensions of the layers and interlayer separations ∼1 μm is demonstrated.

Collimation of laser-driven energetic protons in a capillary

AbstractEnergetic divergent proton beams can be generated in the interaction of ultra-intense laser pulses with solid-density foil targets via target normal sheath acceleration (TNSA). In this paper, a scheme using a capillary to reduce the proton beam divergence is proposed. By two-dimensional particle-in-cell (PIC) simulations, it is shown that strong transverse electric and magnetic fields rapidly grow at the inner surface of the capillary when the laser-driven hot electrons propagate through the target and into the capillary. The spontaneous magnetic field collimates the electron flow, and the ions dragged from the capillary wall by hot electrons neutralize the negative charge and thus restrain the transverse extension of the sheath field set up by electrons. The proton beam divergence, which is mainly determined by the accelerating sheath field, is therefore reduced by the transverse limitation of the sheath field in the capillary.

薄膜靶整形强激光脉冲的理论分析和数值模拟

薄膜靶整形强激光脉冲的理论分析和数值模拟

Magnetic-field generation and electron-collimation analysis for propagating fast electron beams in overdense plasmas

An analytical fluid model is proposed for artificially collimating fast electron beams produced in the interaction of ultraintense laser pulses with specially engineered low-density-core-high-density-cladding structure targets. Since this theory clearly predicts the characteristics of the spontaneously generated magnetic field and its dependence on the plasma parameters of the targets transporting fast electrons, it is of substantial relevance to the target design for fast ignition. The theory also reveals that the rapid changing of the flow velocity of the background electrons in a transverse direction (perpendicular to the flow velocity) caused by the density jump dominates the generation of a spontaneous interface magnetic field for these kinds of targets. It is found that the spontaneously generated magnetic field reaches as high as 100 MG, which is large enough to collimate fast electron transport in overdense plasmas. This theory is also supported by numerical simulations performed using a two-dimensional particle-in-cell code. It is found that the simulation results agree well with the theoretical analysis.