K. Yu. Platonov

Spatial resolving power of laser plasma ionography

Various biological objects have been studied by means of the proton radiography using laser-generated plasma as a source of ions. It is established that the best quality of imaging can be achieved using double laser targets comprising a hydrocarbon plastic film covered with a thin layer of heavy metal. The laminar character of the ion flux produced by such sources makes possible the imaging of objects with dimensions (∼2 μm) smaller than the ion beam spot size. The minimum size of an object that can be resolved by laser plasma ion-ography with proton beams is estimated.

Generation of energetic protons from thin foil targets irradiated by a high-intensity ultrashort laser pulse

Abstract The results of investigations of fast proton generation from thin foil targets irradiated by 1-ps laser pulses of intensities up to 1.5×10 17 xa0W/cm 2 are reported. The characteristics of forward-emitted proton beams produced from both single- and double-layer targets have been determined by the time-of-flight method for various thicknesses, atomic numbers ( Z ) and structure of the target. It is found that using a double-layer target, containing high- Z front layer and low- Z hydrogen-rich back layer, allows to obtain significantly higher energies and a current of protons as well as shorter proton pulse duration than in the case of a commonly used single-layer target. Both the maximum and the mean proton energies as well as the proton current are correlated with the yield of hard X-rays emitted from the target and they increase with the increase in the Z number of the front layer. For maximizing the energies and/or the current of protons and for minimizing the proton pulse duration both total target thickness and high- Z layer thickness must be carefully selected regarding particularly the hot electron range in the target and a possible overheating of the back target surface by the electron heat wave generated by the prepulse and the leading edge of a laser pulse. If the target thickness is smaller than the characteristic path length of the heat wave, the proton energies can be a decreasing function of laser energy. The angular divergence of a proton beam emitted from a properly prepared double-layer target are rather low (⩽30°) which results in the high proton current density in a far expansion zone (∼0.4xa0mA/cm 2 at 1xa0m from Au/polystyrene target) in spite of relatively low energy (

Effect of foil target thickness on fast proton generation driven by ultrashort-pulse laser

The influence of the target foil thickness on the characteristics of a proton beam produced by the interaction of a 1-ps laser pulse with a plastic foil target at intensities near 1017u200aW/cm2 has been investigated. It is shown that, for maximizing the energies and/or the current of forward-emitted protons, the target thickness must be properly selected. It should be smaller than the hot electron range in the target but greater than the characteristic path length of the electron heat wave generated by the prepulse and the leading edge of the laser pulse. If the target thickness is smaller than the heat wave path length, both the maximum and the mean proton energies can be a decreasing function of laser energy.

Hybrid model of ion acceleration in laser plasma of flat heterogeneous target

We have constructed a two-phase analytical model of acceleration of ions in a two-layer laser target. The first phase of acceleration is isothermic and covers the action time of the laser pulse, while the second phase is adiabatic and occurs after the end of the laser pulse. The maximal ion energy is obtained as a function of parameters of the laser pulse and target. We compare analytical results with PIC calculations and show that the theory is adequate.

Narrow-directed fast-ion flow generation from targets irradiated by a picosecond laser pulse

The proton and deuteron yields from thin targets irradiated by a picosecond laser pulse with an average radiation intensity of ≤4×1018 W/cm2 at the target were measured in the megaelectron-volt energy range. A ring structure was observed for the outgoing ions, and the angular ion-beam divergence was found to be extremely small (0.5°). The fast-ion generation mechanism allowing for the appearance of ring structure is discussed, and the characteristic energies and spatioangular ion-beam distribution are estimated.

Determination of the radiation cross sections of low-energy transitions of isomeric nuclei from observation of laser-induced γ fluorescence

Theoretical estimates are given along with the first experimental results on the observation of resonance fluorescence in nuclei of rubidium isomer under conditions of laser plasma X-ray pumping of the contiguous transition with an energy of 3.4 keV. The laser plasma is prepared by irradiating a silver target by a powerful radiation of a Nd laser with a pulse duration of 600 ps. It is demonstrated how one can use the recorded number of emitted γ quanta to determine the probability of low-energy nuclear transition excited by laser plasma X rays.

Broadening of electron cyclotron maser emission lines in a nonuniform magnetic field

The formation of cyclotron maser emission lines in a non-uniform (regular or random) magnetic field is studied. In the presence of sufficiently small inhomogeneity, the line shape can be described by a broadened Gaussian profile. In the case of stronger inhomogeneity, the initial Gaussian profile splits into two Gaussian components, which could be observationally perceived as “harmonics.” A relation between the distribution of local magnetic trap sizes and the distribution of the spectral widths of solar radio spikes is derived. Possible applications of the results to the interpretation of solar radio spikes and related problems are discussed.

Interaction of a high-intensity ultrashort laser pulse with extended nanofilaments of dense plasma

Generation and propagation of fast electrons in laser targets consisting of thin nanofilaments are studied numerically and analytically. Such targets completely absorb laser radiation and exhibit a large coefficient of laser-energy conversion to kinetic energy of a flow of fast electrons. Analytical estimates show that the optimal thickness of the filament is on the order of the skin depth of the laser plasma, while an optimal distance between filaments is on the order of the Debye radius of hot electrons. A bunch of relativistic electrons can propagate as far as several hundred micrometers in such targets, while the fastest electrons can propagate several millimeters. Upon bending of filaments, the flow of electrons propagates along the filaments and can be focused by bringing the filaments together. Laser targets of the discussed composition are used as sources of dense bunches of relativistic electrons and subsequent generation of high-intensity X-ray radiation with their help.

Interaction of intense ultrashort laser pulse with shell target

Based on numerical PIC modeling and an analytical model, we analyze the interaction of a short high-intensity laser pulse with homo- and heterogeneous (two-layer) shell targets. We show that the shell target is capable of focusing ions in a narrow region. In this case, the ion energy exceeds the ion energy for a flat infinite target of the same thickness. We propose to use the effect of focusing and cumulation of ions to implement the collision of ionic beams that counterpropagate from opposite sides of the sphere and to increase the yield of ion-ion reactions. The constructed model makes it possible to calculate the energy of the ion and the number of ions in the region of focusing.

Key methods for intensifying soft X-ray emission from a laser plasma for lithography

A two-pulse two-wave (Nd and CO2 lasers) scheme is proposed for irradiating a laser target, which ensures the highest factor of laser radiation conversion to the X-ray range (13.5 nm ± 1%). Analytic estimates are obtained for parameters of pulses and of the target made of Xe or Li. Numerical optimization is performed for X-ray emission from a spherical Xe target exposed to a CO2 laser pulse. The maximal factor of conversion of laser radiation to X rays is ∼1%. Angular and spectral characteristics of X-ray radiation are obtained. The flux of fast Xe ions ejected from the target and damaging the Mo/Si coating of X-ray mirrors is estimated.