Relativistic Self-induced Transparency Research Articles

Effects of oblique incidence and colliding pulses on laser-driven proton acceleration from relativistically transparent ultrathin targets

The use of ultrathin solid foils offers optimal conditions for accelerating protons to high energies from laser–matter interactions. When the target is thin enough that relativistic self-induced transparency sets in, all of the target electrons get heated to high energies by the laser, which maximizes the accelerating electric field and therefore the final ion energy. In this work, we first investigate how ion acceleration by ultraintense femtosecond laser pulses in transparent CH $_2$ solid foils is modified when turning from normal to oblique ( $45^\circ$ ) incidence. Due to stronger electron heating, we find that higher proton energies can be obtained at oblique incidence but in thinner optimum targets. We then show that proton acceleration can be further improved by splitting the laser pulse into two half-pulses focused at opposite incidence angles. An increase by ${\sim }30\,\%$ in the maximum proton energy and by a factor of ${\sim }4$ in the high-energy proton charge is reported compared to the reference case of a single normally incident pulse.

Self-Referencing Spectral Interferometric Probing of the Onset Time of Relativistic Transparency in Intense Laser-Foil Interactions

Irradiation of an ultrathin foil target by a high intensity laser pulse drives collective electron motion and the generation of strong electrostatic fields, resulting in ultrabright sources of high-order harmonics and energetic ions. The ion energies can be significantly enhanced if the foil undergoes relativistic self-induced transparency during the interaction, with the degree of enhancement depending in part on the onset time of transparency. We report on a simple and effective approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light, providing a route to optically controlling and optimizing ion acceleration and radiation generation. The scheme involves a self-referencing approach to spectral interferometry, in which coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil. The relative timing of the onset of transmission with respect to the transition radiation generation is determined from spectral fringe spacing and compared to simultaneous frequency-resolved optical gating measurements. The results are in excellent agreement, and are discussed with reference to particle-in-cell simulations of the interaction physics and an analytical model for the onset time of transparency in ultrathin foils.

Effect of finite ion mass on relativistic self-induced transparency of plasma layers with a sharp boundary

We consider the effect of ion mobility on the threshold of relativistic self-induced transparency under irradiation of thin plasma layers by circularly polarised laser radiation. An analytical model of the motion of ions during the removal of electrons from the layer surface by the ponderomotive force of laser radiation is constructed. The model is used to analyse the motion of probe electrons in the resulting electromagnetic field. It is shown that the higher the ion mobility and the longer the laser pulse, the more stable the plasma layer to longitudinal perturbations of a finite value and the higher the threshold of self-induced transparency. These conclusions are verified by one-dimensional numerical simulation of a complete system of kinetic equations for plasma and Maxwell’s equations.

High order mode structure of intense light fields generated via a laser-driven relativistic plasma aperture

The spatio-temporal and polarisation properties of intense light is important in wide-ranging topics at the forefront of extreme light-matter interactions, including ultrafast laser-driven particle acceleration, attosecond pulse generation, plasma photonics, high-field physics and laboratory astrophysics. Here, we experimentally demonstrate modifications to the polarisation and temporal properties of intense light measured at the rear of an ultrathin target foil irradiated by a relativistically intense laser pulse. The changes are shown to result from a superposition of coherent radiation, generated by a directly accelerated bipolar electron distribution, and the light transmitted due to the onset of relativistic self-induced transparency. Simulations show that the generated light has a high-order transverse electromagnetic mode structure in both the first and second laser harmonics that can evolve on intra-pulse time-scales. The mode structure and polarisation state vary with the interaction parameters, opening up the possibility of developing this approach to achieve dynamic control of structured light fields at ultrahigh intensities.

Laser pulse dispersion in underdense plasma and associated ion acceleration by relativistic self-induced transparency

The propagation of laser pulses in the underdense plasma is a very crucial aspect of the laser-plasma interaction process. In this work, we explored the two regimes of laser propagation in the plasma, one with a0 < 1 and the other with a0≳10. For the a0 < 1 case, we used a cold relativistic fluid model, wherein apart from immobile ions no further approximations are made. The effects of laser pulse amplitude, pulse duration, and plasma density are studied using the fluid model and compared with the expected scaling laws and also with the particle-in-cell (PIC) simulations. The agreement between the fluid model and the PIC simulations are found to be excellent. Furthermore, for the a0≳10 case, we used the PIC simulations alone. The delicate interplay between the conversion from the electromagnetic field energy to the longitudinal electrostatic fields results in dispersion, and so the redshift of the pump laser pulse. The dispersed pulse is then allowed to be incident on the subwavelength two-layer composite target. The underdense plasma before the target regulates the dispersion of the pulse. We observed an optimum pretarget plasma density which results in the acceleration of the ions from the secondary layer to ∼170 MeV by a ∼8 fs linearly polarized Gaussian laser pulse with ∼8.5 × 1020 W/cm2.

Chirp assisted ion acceleration via relativistic self-induced transparency

We study the effect of the chirped laser pulse on the transmission and associated ion acceleration by the sub-wavelength target. In the chirped laser pulses, the pulse frequency has a temporal variation about its fundamental frequency, which manifests to the temporal dependence of the critical density (nc). In this work, we used a chirp model which is beyond the linear approximation. For negatively (positively) chirped pulses, the high (low) frequency component of the pulse interacts with the target initially followed by the low (high) frequency component. The threshold plasma density for the transmission of the pulse is found to be higher for the negatively chirped laser pulses as compared to the unchirped or positively chirped pulses. The enhanced transmission of the negatively chirped pulses for higher densities (6nc) results in very efficient heating of the target electrons, creating a very stable and persistent longitudinal electrostatic field behind the target. The void of the electrons results in expansion of the target ions in either direction, resulting in the broad energy spectrum. We have introduced a very thin, low density (<nc) secondary layer behind the primary layer. The ions from the secondary layer are then found to be accelerated as a mono-energetic bunch under the influence of the electrostatic field created by the primary layer upon interaction by the negatively chirped pulse. Under the optimum conditions, the maximum energy of the protons is found to be ∼100 MeV for 10 fs (intensity FWHM); Circularly Polarized; Gaussian; negatively chirped laser pulse with peak intensity ∼8.5 × 1020 W/cm2.

Radiation Pressure-Driven Plasma Surface Dynamics in Ultra-Intense Laser Pulse Interactions with Ultra-Thin Foils

The dynamics of the plasma critical density surface in an ultra-thin foil target irradiated by an ultra-intense (∼6 × 10 20 Wcm − 2 ) laser pulse is investigated experimentally and via 2D particle-in-cell simulations. Changes to the surface motion are diagnosed as a function of foil thickness. The experimental and numerical results are compared with hole-boring and light-sail models of radiation pressure acceleration, to identify the foil thickness range for which each model accounts for the measured surface motion. Both the experimental and numerical results show that the onset of relativistic self-induced transparency, in the thinnest targets investigated, limits the velocity of the critical surface, and thus the effectiveness of radiation pressure acceleration.

Kinetic and finite ion mass effects on the transition to relativistic self-induced transparency in laser-driven ion acceleration

We study kinetic effects responsible for the transition to relativistic self-induced transparency in the interaction of a circularly-polarized laser-pulse with an overdense plasma and their relation to hole-boring (HB) and ion acceleration. It is demonstrated using particle-in-cell simulations and an analysis of separatrices in single-electron phase-space, that ion motion can suppress fast electron escape to the vacuum, which would otherwise lead to transition to the relativistic transparency regime. A simple analytical estimate shows that for large laser pulse amplitude a0 the time scale over which ion motion becomes important is much shorter than usually anticipated. As a result of enhanced ion mobility, the threshold density above which HB occurs decreases with the charge-to-mass ratio. Moreover, the transition threshold is seen to depend on the laser temporal profile, due to the effect that the latter has on electron heating. Finally, we report a new regime in which a transition from relativistic transparency to HB occurs dynamically during the course of the interaction. It is shown that, for a fixed laser intensity, this dynamic transition regime allows optimal ion acceleration in terms of both energy and energy spread.

Efficient ion acceleration by relativistic self-induced transparency in subwavelength targets

In this work we have studied the effect of target thickness on relativistic self-induced transparency (RSIT) and found out that for subwavelength targets the corresponding threshold target density (beyond which target is opaque to incident laser pulse of a given intensity) increases. The accelerating longitudinal electrostatic field created by RSIT from subwavelength target is used to accelerate the ion bunch from a thin, low density layer behind the main target to $\sim$ 100 MeV. A suitable scaling law for optimum laser and target conditions is also deduced. The word `via' in the title signifies the fact that we are interested in acceleration of ions from the layer placed behind the target. It is also being observed that as per as energy spectrum is concerned; an extra low density layer is advantageous than relying on target ions alone.

Relativistic self-induced transparency effect during ultraintense laser interaction with overdense plasmas: Why it occurs and its use for ultrashort electron bunch generation

A novel explanation of the relativistic self-induced transparency effect during superintense laser interaction with an overdense plasma is proposed. It was studied analytically and verified with direct modeling by particle-in-cell simulations. Based on this treatment, a method of ultrashort high-energy electron bunch generation with durations on a femtosecond time scale is also proposed and studied via numerical simulation.

A non-periodic 2D semi-Lagrangian Vlasov code for laser–plasma interaction on parallel computer

For the first time, a 2D electromagnetic and relativistic semi-Lagrangian Vlasov model for a multi-computer environment was developed to study the laser–plasma interaction in an open system. Numerical simulations are presented for situations relevant to the penetration of an ultra-intense laser pulse inside a moderately overdense plasma and the relativistic filamentation instability in the case of an underdense plasma. The Vlasov model revealed a rich variety of phenomena associated with the fast particle dynamics induced by the laser pulse as particle trapping, particle acceleration and relativistic self-induced transparency in overdense plasma. Attention was focused on the efficiency and stability properties on the numerical scheme and implementation facilities on massively parallel computers. Success of the semi-Lagrangian Vlasov model is enhanced by the good conservation of the continuity equation and stability of Maxwell system due to the fine description of the electron distribution function and particularly of the charge density and current density.