O. Klimo

Short pulse laser interaction with micro-structured targets: simulations of laser absorption and ion acceleration

The interaction of an ultrashort intense laser pulse with thin foil targets is accompanied by the acceleration of ions from the target surface. To make this ion source suitable for application, it is of particular importance to increase the efficiency of laser energy transformation into accelerated ions and the maximum ion energy. This can be achieved by using a thin foil target with a microscopic structure on the front, laser-irradiated surface. The influence of the microscopic structure on the target surface on the laser target interaction and subsequent ion acceleration is studied here using numerical simulations. The influence of the shape and size of the microstructure, the density profile and the laser pulse incidence angle is also studied. Based on the simulation results, we propose to construct the target for ion acceleration experiments by depositing a monolayer of polystyrene microspheres of a size similar to the laser wavelength on the front surface of a thin foil.

Evidence of resonant surface-wave excitation in the relativistic regime through measurements of proton acceleration from grating targets.

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, is experimentally investigated. Ultrahigh contrast (~10(12)) pulses allow us to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultrahigh intensity >10(19) W/cm(2). A maximum increase by a factor of 2.5 of the cutoff energy of protons produced by target normal sheath acceleration is observed with respect to plane targets, around the incidence angle expected for the resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.

Quasi-mono-energetic ion acceleration from a homogeneous composite target by an intense laser pulse

The paper presents an analytical model and particle-in-cell simulations of the quasi-mono-energetic ion acceleration by an intense laser pulse in a multispecies target and the corresponding experimental observations. Homogeneous and heterogeneous targets are considered, and it is shown that the formation of the energy spectrum proceeds in three stages: (1) the initial light ion acceleration in the sheath electric field, (2) the ion species separation followed by the electrostatic shock formation, and (3) the interaction of spatially separated ion bunches accompanied by electron cooling. The field ionization of heavy ions and interaction between the heavy and light species play an important role in the formation and preservation of the energy spectrum of light ions. The simulation results are compared with the theoretical predictions and the experiments.

Laser plasma interaction studies in the context of shock ignition--Transition from collisional to collisionless absorption

The shock ignition concept implies laser pulse intensities higher than 1015 W/cm2 (at the wavelength of 351 nm), which is the commonly accepted limit where the inverse bremsstruhlung absorption dominates. The transition from collisional to collisionless absorption in laser plasma interactions at higher intensities is studied in the present paper with the help of large scale one-dimensional particle-in-cell simulations. The initial parameters are defined by the hydrodynamic simulations corresponding to recent experiments. The simulations predict that a quasi-steady regime of laser plasma interaction is attained where the total laser energy absorption stays on the level of ∼65% in the laser intensity range 1015–1016 W/cm2. However, the relation between the collisional and collisionless processes changes significantly. This is manifested in the energy spectrum of electrons transporting the absorbed laser energy and in the spectrum of the reflected laser light.

Electron dynamics and γ and e-e+ production by colliding laser pulses

The dynamics of an electron bunch irradiated by two focused colliding super-intense laser pulses and the resulting γ and e(-)e(+) production are studied. Due to attractors of electron dynamics in a standing wave created by colliding pulses the photon emission and pair production, in general, are more efficient with linearly polarized pulses than with circularly polarized ones. The dependence of the key parameters on the laser intensity and wavelength allows us to identify the conditions for the cascade development and γe(-)e(+) plasma creation.

Lateral hot electron transport and ion acceleration in femtosecond laser pulse interaction with thin foils

Hot electron transport along the target surface out of the laser-irradiated spot plays an important role in such applications as ion acceleration or fast ignition of fusion reactions. In this paper, the lateral electron transport in a thin foil, limited in transverse sizes, is studied by numerical particle-in-cell simulations for two linear polarizations (p and s) of femtosecond laser pulse incident on a foil at various angles. Two mechanisms of the transport are identified: the first one is due to hot electron recirculation across the foil and the second is electron guiding along the foil surface by quasistatic magnetic and electric fields. It is demonstrated that the second mechanism takes place for larger incidence angles, although the recirculation is still important. The ions accelerated from a lateral foil edge, which is out of the laser focal spot, can have higher energies than the ions from the rear foil side.

Fast magnetic-field annihilation in the relativistic collisionless regime driven by two ultrashort high-intensity laser pulses.

The magnetic quadrupole structure formation during the interaction of two ultrashort high power laser pulses with a collisionless plasma is demonstrated with 2.5-dimensional particle-in-cell simulations. The subsequent expansion of the quadrupole is accompanied by magnetic-field annihilation in the ultrarelativistic regime, when the magnetic field cannot be sustained by the plasma current. This results in a dominant contribution of the displacement current exciting a strong large scale electric field. This field leads to the conversion of magnetic energy into kinetic energy of accelerated electrons inside the thin current sheet.

Controllable Laser Ion Acceleration

When an intense laser illuminates a target, temporarily a strong electric field is formed, and the target ions are accelerated. The control of the ion energy spectrum and the ion particle energy, the ion beam collimation and the ion beam bunching are successfully realized by a multi-stage laser-target interaction.

High density ultrashort relativistic positron beam generation by laser-plasma interaction

A mechanism of high energy and high density positron beam creation is proposed in ultra-relativistic laser-plasma interaction. Longitudinal electron self-injection into a strong laser field occurs in order to maintain the balance between the ponderomotive potential and the electrostatic potential. The injected electrons are trapped and form a regular layer structure. The radiation reaction and photon emission provide an additional force to confine the electrons in the laser pulse. The threshold density to initiate the longitudinal electron self-injection is obtained from analytical model and agrees with the kinetic simulations. The injected electrons generate γ-photons which counter-propagate into the laser pulse. Via the Breit–Wheeler process, well collimated positron bunches in the GeV range are generated of the order of the critical plasma density and the total charge is about nano-Coulomb. The above mechanisms are demonstrated by particle-in-cell simulations and single electron dynamics.

Fast magnetic field annihilation driven by two laser pulses in underdense plasma

Fast magnetic annihilation is investigated by using 2.5-dimensional particle-in-cell simulations of two parallel ultra-short petawatt laser pulses co-propagating in underdense plasma. The magnetic field generated by the laser pulses annihilates in a current sheet formed between the pulses. Magnetic field energy is converted to an inductive longitudinal electric field, which efficiently accelerates the electrons of the current sheet. This new regime of collisionless relativistic magnetic field annihilation with a timescale of tens of femtoseconds can be extended to near-critical and overdense plasma with the ultra-high intensity femtosecond laser pulses.