I. M. Lachko

Generation of High-Energy Negative Hydrogen Ions upon the Interaction of Superintense Femtosecond Laser Radiation with a Solid Target

The formation of a high-energy (∼35 keV) beam of negative hydrogen ions was observed in the expanding femtosecond laser plasma produced at the surface of a solid target by radiation with an intensity of up to 2× 1016 W/cm2. The energy spectra of the H+ and H−-ions show a high degree of correlation.

Formation of fast multicharged heavy ions under the action of a superintense femtosecond laser pulse on the cleaned surface of a target

It is found that the mean charge of tungsten ions in a solid tungsten target cleaned from the surface layer of hydrocarbon and oxide compounds and exposed to femtosecond laser radiation with an intensity exceeding 1016 W/cm2 attains 22+, while the maximum charge is 29+. The maximum energy of such ions approaches 1 MeV. The corresponding values obtained on a dirty target with the same laser pulse parameters constitute 3+, 5+, and 150 keV. The results of numerical simulation show that such a large maximum charge of ions can be attained owing to the emergence of an electrostatic ambipolar field at the sharp boundary between the plasma and vacuum. The main mechanism of ionization of ions with maximum charges is apparently impact ionization in the presence of an external quasi-static field. In addition, direct above-threshold ionization by this field can also play a significant role. It is also shown that heavy ions in a clean target are accelerated by hot electrons. This leads to the formation of high-energy ions. The effect of recombination on the charge of the ions being detected is analyzed in detail.

Laser plasma acceleration of quasi-monoenergetic beams of light multicharged ions

Experimental results on the spectra of residual impurity ions from a plasma created on the surface of a tungsten target that is irradiated by a femtosecond laser pulse and is subjected to preliminary pulsed laser cleaning are reported. It is shown that deep modulation corresponding to the formation of a quasi-monoenergetic ion beam appears in the energy spectra of protons and light multicharged carbon ions in this case. The position of a “dip” at the scale of velocities closely correlates with the position of the front of heavier ions.

Acceleration of heavy multicharged ions up to 1 MeV from a cleaned solid target irradiated by a femtosecond laser pulse with an intensity of 1016 W/cm2

A noticeable increase in the charge and energy of ions accelerated from a solid tungsten target irradiated by a femtosecond laser pulse with an intensity higher than 1016W/cm2 has been found when the target surface is precleaned by a nanosecond laser pulse with an energy density of 3 J/cm2. Tungsten ions with charges up to +29 and energies up to 1 MeV were detected in this case, while the charge and energy of tungsten ions from a target with an uncleaned surface do not exceed +3 and 12 keV, respectively.

Enhanced ionization of W ions at a plasma-vacuum boundary in femtosecond laser plasma at moderate intensities

Experimental and numerical investigations of femtosecond laser plasma at the cleaned surface of W target are carried out. The constructed picture of ionization, acceleration and recombination of ions in plasma well explains main features of atomic, charge and energy spectra of fast and slow ions. It was shown that experimentally measured maximum charge of W ions exceed charge predicted by impact ionization and can be reached due to ambipolar field ionization.

Production of high-energy multi-charged mono-atomic ion bunches from FLP: the role of pulsed laser pre-cleaning

It is shown that the presence of surface contaminant layer exerts influence upon the parameters of ionic bunches from laser plasma formed under interaction of superintense femtosecond laser pulses with solid targets. The usage of pulsed laser pre-cleaning of the targets leads not only to reducing Contaminant ions but also to emerging of additional high-energy component in spectra of bulk ions. We showed that one can control the parameters of ionic bunches from plasma such as maximum and mean ions charge, maximum energy of bulk substance ions by adjusting of the preceding time of the cleaning laser pulse. Thus, using the silicon target and pulses of moderate intensity (2⋅1016 W/cm2) it is possible to produce the ionic bunches with maximum ions energy of about 400 keV and with a charge state of up to +12.

On the origin of fast multi-charged ions from femtosecond laser plasma at moderate intensities

Experimental and numerical investigations of femtosecond laser plasma at the cleaned surface of crystalline Si and w targets are carried out. The constructed picture of ionization, acceleration and recombination of ions in plasma well explains main features of atomic, charge and energy spectra of fast and slow ions.

Control of femtosecond laser plasma parameters by surface contaminants cleaning with preceding pulse laser

Making use of time-of-flight and mass-spectroscopic methods we investigated influence of the film on the target surface at vacuum of 10-5 Torr onto ions emission from plasma created by femtosecond laser pulse with itensity of 2 • 1016 W/cm2. It was shown that the highest energy per charge (8.5 keV) acquire protons, while basic target ions (Si, Ti) gain less energy. Heating by laser nanosecond pulse advancing femtosecond pulse by 0.1 - 100 ms with energy density up to 20 J/cm2 allows for effective surface cleaning due to removal of molecules containing hydrogen, carbon and oxygen. By the contrast to the instantaneous resistive heating the pulsed laser cleaning provides for higher heating temperatures and can be used for any solid targets in the regimes of thermal and plasma cleaning.

Self-channeling of femtosecond visible laser pulse with microjoule energy and micromodification in transparent target

We have observed self-guiding of a single femtosecond visible laser pulse in the bulk of fused silica. The filament length and diameter versus pulse energy have been measured. Velocity of ions flying out from the plasma channel was determined with the use of time-of-flight measurements. The laser-induced micromodification in the bulk of the fused silica has discrete structure and is close to the filament form.

Internal electronic conversion decay of low-energy nuclear levels excited in hot dense femtosecond laser plasma

The properties of internal electronic conversion of nucleus excited in hot dense laser plasma are considered. The important role of dynamics of plasma charge state in residual gas is shown.