Ultra-intense Laser Pulse Research Articles

Simulation of relativistically colliding laser-generated electron flows

The plasma dynamics resulting from the simultaneous impact, of two equal, ultra-intense laser pulses, in two spatially separated spots, onto a dense target is studied via particle-in-cell simulations. The simulations show that electrons accelerated to relativistic speeds cross the target and exit at its rear surface. Most energetic electrons are bound to the rear surface by the ambipolar electric field and expand along it. Their current is closed by a return current in the target, and this current configuration generates strong surface magnetic fields. The two electron sheaths collide at the midplane between the laser impact points. The magnetic repulsion between the counter-streaming electron beams separates them along the surface normal direction, before they can thermalize through other beam instabilities. This magnetic repulsion is also the driving mechanism for the beam-Weibel (filamentation) instability, which is thought to be responsible for magnetic field growth close to the internal shocks of gamma-ray burst jets. The relative strength of this repulsion compared to the competing electrostatic interactions, which is evidenced by the simulations, suggests that the filamentation instability can be examined in an experimental setting.

Seeded optically driven avalanche ionization in molecular and noble gases

We report experimental and numerical results on the dual laser-pulse plasma excitation in molecular and noble gases at atmospheric pressure. Dilute plasma channels generated through filamentation of ultraintense femtosecond laser pulses in air, argon, and helium are densified through the application of multijoule nanosecond heater pulses. Plasma densification in molecular gases is always accompanied by the fragmentation of the plasma channels into discrete bubbles, while in atomic gases, under certain conditions, the densified channels remain smooth and continuous. The densification effect in atomic gases persists through considerably longer delays between the femtosecond and nanosecond pulses compared to that in molecular gases. Using rate equations we trace this difference in the temporal dynamics of densification to the different cooling mechanisms operative in atomic and molecular cases.

Laser-induced photo transmutation of 126Sn – A hazardous nuclear waste product-into short-lived nuclear medicine of 125Sn

Relativistic electrons, generated in the interaction of an ultra-intense laser pulse with plasma in front of a high-Z solid target, when passing near the nuclei of the solid target produce several MeV highly collimated Bremsstrahlung gamma beam, which can be used to induce photo-nuclear reactions. In this work the possibility of photo-induced transmutation (γn) of a nuclear waste of 126Sn with a half-life of 100,000years into 125Sn with a half-life of 9.64days was investigated for the first time. Calculations based on the available experimental data show that the Bremsstrahlung γ beam generated by irradiating a 2mm thick tantalum target as a converter with 1020Wcm-2μm2 and 10Hz table-top laser for an hour can produce 293Bq activity in a 1cm thick 126Sn sample placed directly behind it. The remarkable feature of this work is to evaluate the optimal laser intensity to produce maximum activity of 2257Bq which is 1.18×1021Wcm-2μm2. The selective excitation of nuclear resonance states are discussed by studying the rate equations and calculating the Einstein coefficient. According to our calculations, we achieve relative decay rate of 0.0025, for two possible decay channels. Our results show the most probable channel for this mechanism which has a good agreement with direct excitation process.

Hole-boring radiation pressure acceleration as a basis for producing high-energy proton bunches

The production of high-energy protons by the ‘hole-boring’ radiation pressure acceleration (HB-RPA) mechanism of laser-driven ion acceleration is examined in the case where the plasma has a density less than a0nc in 2D. Previously this was examined in 1D (Robinson 2011 Phys. Plasmas 18 056701) and was motivated by previous predictions of the non-linear criterion for an ultra-intense laser pulse to penetrate a dense plasma. By reducing the density well below a0nc the proton energies achieved increases considerably, thus leading to proton energies >100 MeV at laser intensities close to current capabilities. The results show that good quality proton beams with proton energies >100 MeV can be obtained via HB-RPA using targets with densities in the range 12–20nc and laser intensities in the range 5 × 1021–3 × 1022 W cm−2.

Study on the effects of ion motion on laser-induced plasma wakes

A 2D analytical model is presented for the generation of plasma wakes (or bubbles) with an ultra-intense laser pulse by taking into account the response of plasma ions. It is shown that the effect of ion motion becomes significant at the laser intensity exceeding 1021 W/cm2 and plasma background density below 1019 cm−3. In this regime, ion motion tends to suppress the electrostatic field induced by charge separation and makes the electron acceleration less effective. As a result, the assumption of immobile ions overestimates the efficiency of laser wake-field acceleration of electrons. Based on the analytical model, the dynamics of plasma ions in laser-induced wake field is investigated. It is found that only one bubble appears as the plasmas background density exceeds the resonant density and the deposited laser energy is concentrated into the bubble, resulting in the generation of an ion bunch with extremely high energy density.

Regions for Brillouin seed pulse growth in relativistic laser-plasma interaction

Parametric plasma processes received renewed interest in the context of generating ultraintense and ultrashort laser pulses up to the exawatt-zetawatt regime. For Brillouin scattering and seed pulse amplification at high intensities, the strong coupling regime is of special interest. The intensity-driven low-frequency modes depend on the amplitudes of the laser fields. It is investigated here how these modes develop in the relativistic regime. Then, a unified treatment of Raman and Brillouin processes becomes necessary. Assuming circular polarization, it is shown that with increasing intensity an overlap of the originally different Raman, Brillouin, and modulational instability branches occurs. Numerical simulations with a linearized Maxwell-fluid code confirm the analytically predicted behavior.

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.

Target normal sheath acceleration sheath fields for arbitrary electron energy distribution

Relativistic electrons, generated by ultraintense laser pulses, travel through the target and form a space charge sheath at the rear surface which can be used to accelerate ions to high energies. If the laser pulse duration is comparable or shorter than the time needed for the electrons to travel through the target, the electrons will not have the chance to form an equilibrium distribution but must be described by a non-equilibrium distribution. We present a kinetic theory of the rear sheath for arbitrary electron distribution function f(E), where E is the electron energy, and evaluate it for different shapes of f(E). We find that the far field is mainly determined by the high energy tail of the distribution, a steep decay of f(E) for high energies results in a small electric field and vice versa. The model is extended to account for electrons escaping the sheath region thereby allowing a finite potential drop over the sheath. The consequences of the model for the acceleration of ions are discussed.

Theoretical Analysis on Propagation of Electromagnetic Wave in Preformed Narrow Plasma Channel

In the plasma sheath a narrow plasma channel generated by ultraintense laser pulses is simplified as a special cylindrical and hollow plasma waveguide with the infinite thickness of the plasma cladding. The electromagnetic wave (EM) propagation properties of the plasma channel near the cutoff and far from the cutoff are considered. Theoretical analysis shows that TE0m and TM0m and hybrid modes emerge in the plasma channel, which is influenced by the normalized frequency parameter B and numerical aperture N A. The cutoffs of the various modes are approximated. Single-mode operation is possible without a high-frequency limitation in the channel.

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.

X-ray spectroscopy diagnoses of clusters surviving under prepulses of ultra-intense femtosecond laser pulse irradiation

AbstractIt is shown that various spectroscopic methods based on measurements of X-ray spectra radiated from cluster targets can be used for estimation of the destruction degree of clusters by laser prepulses. These methods allow insight to be gained regarding the important issue of preservation of the dense cluster core at the moment of the arrival of the main laser pulse. In addition, they can be used for quantitative estimation of the size of the undestroyed parts of the clusters and also for measuring the temperature and density of the preplasmas produced by the laser prepulses.

Collisional particle-in-cell modelling of the generation and control of relativistic electron beams produced by ultra-intense laser pulses

We present two-dimensional fully kinetic collisional particle-in-cell (PIC) simulations of the interaction of an ultra-intense laser with a solid density target. We find that the angular spread of the electrons is mostly due to the curved geometry of the electron acceleration region. Electron scattering off the magnetic fields caused by Weibel-like instabilities plays a smaller role. The angular spread can be counteracted by structuring the target into regions of different resistivity and we present a novel elliptical geometry of the high-Z core which can keep the electrons collimated for larger distances behind the target compared with cylindrical geometry.

Relativistic channeling of a linearly polarized laser pulse in overdense plasma

AbstractWe investigated a dynamic procedure for relativistic channeling by a linearly polarized ultraintense laser pulse in overdense plasma, subsequently determining a phenomenological formula for the channel-digging speed. Channeling of the linearly polarized pulse usually results in a sharp-cut (non-adiabatic) pulse front, since the pulse is continuously reflected on the transparency-opacity interface during the channeling process. Using the novel formula for the channel-digging speed, it was possible to analytically predict where such a sharp-cut occurs longitudinally.

Measuring hot electron distributions in intense laser interaction with dense matter

Retrieving the characteristics of hot electrons produced in the interaction between solid targets and ultra-intense (I > 1018 W cm−2) laser pulses is essential for achieving progress in our understanding of the interaction physics, which is key for optimizing numerous downstream applications. Until now, various methods have been used, direct or indirect, but no correlation and no assessment of their respective merits were performed. Here we compare results retrieved from four different diagnostics, direct or indirect, as well as local or non-local, i.e. spectrometry of electrons, spectrometry of the protons accelerated by the electrons and optical probing of these beams expanding into vacuum from the targets. We show that measurements obtained locally at the target rear surface are consistent with those far away from the target and that one can use the diagnostics of the co-moving proton beams to retrieve information about electrons.

Ultra-intense laser pulse propagation in plasmas: from classic hole-boring to incomplete hole-boring with relativistic transparency

Relativistic laser pulse propagation into homogeneous plasmas has been investigated as a function of plasma density. At first, the propagation features are compared systematically between relativistic transparency (RT) and hole-boring (HB). Paramountly, a considerably broad intermediate regime, namely the incomplete HB regime, has been found between the RT regime and the HB regime for an extremely intense circularly polarized (CP) pulse. In this regime HB proceeds in collaboration with RT, resulting in a much faster propagation speed and a higher cut-off energy of fast ions than in the classic HB regime. Similarly to the classic HB regime, formulae are presented to model the laser propagation and the ion acceleration according to the modified momentum flux balance in this incomplete HB regime. The simulations give the density boundary between this incomplete HB regime and the classic HB regime for CP pulses, which is crucial for estimating the maximum mean ion energy and the maximum conversion efficiency that can be achieved by the classic HB acceleration at a given laser intensity. For linear polarization (LP) the propagation mechanism apparently undergoes a transition in time between these two regimes. A detailed comparison between LP and circular polarization is made for these phenomena.

Rayleigh-Taylor Instability of an Ultrathin Foil Accelerated by the Radiation Pressure of an Intense Laser

We report experimental evidence for a Rayleigh-Taylor-like instability driven by radiation pressure of an ultraintense (10(21) W/cm(2)) laser pulse. The instability is witnessed by the highly modulated profile of the accelerated proton beam produced when the laser irradiates a 5 nm diamondlike carbon (90% C, 10% H) target. Clear anticorrelation between bubblelike modulations of the proton beam and transmitted laser profile further demonstrate the role of the radiation pressure in modulating the foil. Measurements of the modulation wavelength, and of the acceleration from Doppler-broadening of back-reflected light, agree quantitatively with particle-in-cell simulations performed for our experimental parameters and which confirm the existence of this instability.

Towards Sub-TeV electron beams driven by ultra-short, ultra-intense laser pulses

AbstractEnergetic electron beam generation from a thin foil target by the ponderomotive force of an ultra-intense circularly polarized laser pulse is investigated. Two-dimensional particle-in-cell (PIC) simulations show that laser pulses with intensity of 1022–1023 Wcm−2 generate about 1–10 GeV electron beams, in agreement with the prediction of one-dimensional theory. When the laser intensity is at 1024–1025 Wcm−2, the beam energy obtained from PIC simulations is lower than the values predicted by the theory. The radiation damping effect is considered, which is found to become important for the laser intensity higher than 1025 Wcm−2. The effect of laser focus positions is also discussed.

Electron-positron plasma drop formed by ultra-intense laser pulses

We study the initial properties and positron annihilation within a small electron-positron plasma drop formed by intense laser pulse energy. Such QED cascade generated plasma is, in general, far below the chemical (particle yield) equilibrium. We find that the available electrons and positrons equilibrate kinetically, yet despite relatively high particle density, the electron-positron annihilation is very slow, suggesting a rather long life span of the plasma drop.

Production of high-density high-temperature plasma by collapsing small solid-density plasma shell with two ultra-intense laser pulses

Three-dimensional particle-in-cell simulations show that the anisotropic collapse of a plasma microshell by impact of two oppositely directed intense laser pulses can create at the center of the shell cavity a submicron-sized plasma of high density and temperature suitable for generating fusion neutrons.