Search

Clusters represent a new class of laser pulse targets which show both the properties of underdense and of overdense plasmas. We present analytical and numerical results (based on 2D- and 3D-PIC simulations) of the Coulomb explosion of the ion cloud that is formed when a cluster is irradiated by a high-intensity laser pulse. For laser pulse intensities in the range of 1021−1022 W/cm2, the laser light can rip electrons from atoms almost instantaneously and can create a cloud made of an electrically nonneutral plasma. Ions can then be accelerated up to high energy during the Coulomb explosion of the cloud.

An optically dense ionization wave (IW) produced by two femtosecond (approximately 10/30 fs) laser pulses focused cylindrically and crossing each other may become an efficient coherent x-ray converter in accordance with the Semenova-Lampe theory. The resulting velocity of a quasiplane IW in the vicinity of pulse intersection changes with the angle between the pulses from the group velocity of ionizing pulses to infinity allowing a tuning of the wavelength of x rays and their bunching. The x-ray spectra after scattering of a lower frequency and long coherent light pulse change from the monochromatic to high order harmoniclike with the duration of the ionizing pulses.

The optical field ionization of a transparent media by two, cylindrically focused femtosecond laser pulses may result in production of an ionization wave (IW). Velocity of such a quasi‐plane IW in the vicinity of pulse intersection can be tuned by changing the intersection angle and can even exceed the speed of light. We study the conversion of a coherent light to x‐rays by means of particle‐in‐cell simulation and by solution of continuous equation with the correct current: j(x,t) = −e∫(dNe/dt0)v(t,t0)dt0.X‐ray spectrum of converted lower frequency light changes from the monochromatic to a high order harmonic‐like with the duration of ionizing pulses. The conversion efficiency can be increased via suppression the energy of the generated magnetic field.

We have studied the ion emission from a thin‐foil target irradiated by p, s and circularly polarized laser pulses in order to clarify the mechanism of ion acceleration using a Titanium Sapphire laser with a peak irradiance of 3 × 1018 W/cm2. A tape target driver provided a fresh surface of a tantalum foil with the thickness of 3 micro‐meters during the measurements. The laser polarization was changed from p to s‐ and to circular polarization by using the 1/2 and 1/4 wave plates made of mica. To obtain the ion energy spectra and to resolve the ion species, a Thomson parabola ion analyzer was placed behind the target along the target normal direction. The energy spectra close to 1MeV were obtained. Possible explanation of the experimental result is described. Finally, we describe the phase rotation technique to obtain a monochromatic proton beam.

An intense laser-plasma interaction regime of the generation of high density ultrashort relativistic ion beams is suggested. When the radiation pressure is dominant, the laser energy is transformed efficiently into the energy of fast ions.

With detailed experimental studies and hydrodynamics and particle-in-cell simulations we investigate the role of the prepulse in laser proton acceleration. The prepulse or pedestal (amplified spontaneous emission) can completely evaporate the irradiated region of a sufficiently thin foil; therefore, the main part of the laser pulse interacts with an underdense plasma. The multiparametric particle-in-cell simulations demonstrate that the main pulse generates the quasistatic magnetic field, which in its turn produces the long-lived charge separation electrostatic field, accelerating the ions.

The ion acceleration is studied with 3D and 2D PIC simulations of the intense laser pulse interaction with the underdense plasma slab. The simulations reveal the forward acceleration of ions at the rear plasma-vacuum interface. A novel mechanism of the ion acceleration is found. Efficient ion acceleration occurs when an intense quasistatic magnetic field is generated at the rear side of the plasma slab. The magnetic field pressure pushes electrons inside the slab shifting them with respect to the plasma ions. As a result large-scale regions of non-compensated positive electric charge are formed. These regions act as an accelerator and a collimator for the ions from the dense plasma filament formed inside the self-focusing channel.

Under optimal interaction conditions ions can be accelerated up to relativistic energies by a petawatt laser pulse in both underdense and overdense plasmas. Two-dimensional particle in cell simulations show that the laser pulse drills a channel through an underdense plasma slab due to relativistic self-focusing. Both ions and electrons are accelerated in the head region of the channel. However, ion acceleration is more effective at the end of the slab. Here electrons from the channel expand in vacuum and are followed by the ions dragged by the Coulomb force arising from charge separation. A similar mechanism of ion acceleration occurs when a superintense laser pulse interacts with a thin slab of overdense plasma and the pulse ponderomotive pressure moves all the electrons away from a finite-diameter spot.

Efficient and fast self-injection of plasma electrons into the wake-field acceleration phase can be procured during the transverse wake-wave breaking when the wake-wave is generated by the high-intensity laser pulse propagating in a narrow plasma channel. In the strong focusing regime, when the laser pulse power exceeds critical for the self-focusing power threshold, the injected electron bunch length becomes comparable with the plasma wavelength and the bunch has the femtosecond duration. The total charge of self-injected electrons depends strongly on the laser pulse amplitude.

A study of proton emission from a 3‐μm-thick Ta foil target irradiated by p-, s-, and circularly polarized laser pulses with respect to the target plane has been carried out. Protons with energies up to 880keV were observed in the target normal direction under the irradiation by the p-polarized laser pulse, which yielded the highest efficiency for proton emission. In contrast, s- and circularly polarized laser pulses gave the maximum energies of 610 and 680keV, respectively. The difference in the maximum energy between the p- and s-polarized cases was associated with the difference between the sheath fields estimated from electron spectra.

Coulomb explosion of fullerene, benzene and proton cluster induced in an interaction with an intense femtosecond laser pulse has been investigated with the use of MD and PIC simulations. Simulations reveal different ionization processes in the large molecules and an acceleration mechanism of GeV protons in the cluster.