We present the results of our experiments directed to the suppression of atmospheric gases influence on delivering of superintense femtosecond laser radiation to solid target at atmospheric conditions.The suppression of atmospheric gases effects is attained by expositing the target to a double-pulse radiation and caused by reducing gas density near the target surface). In our scheme the first pulse (nanosecond UV pulse from XeCl excimer laser) acts as a vacuum pump. The second laser pulse (tightly focused femtosecond pulse) delaying up to several microseconds respect to the first laser pulse delivers the basic energy necessary for switching a hot plasma on the target surface. We have observed the maximum X-ray yield increasing up to 17 times under double pulses exposure compare with the single femtosecond pulse regime.
On-line diagnostics of hot plasma induced by superintense femtosecond laser pulses in a cavity during the deep
hole drilling of solid target has been developed. Such a plasma is characterized by higher values of hard X-ray yield and
hot electron temperature in comparison with the same parameters measured for flat nearsurface plasma. Dynamics of X-rays
yield in the cavity during the deep hole drilling demonstrates several repetitive stages. The yield and energy of X-rays
from plasma ignited in a cavity depend on the beam waist position respect to the target surface plane, i.e. focusing
regime. We have detected strong second harmonic signal that correlated with hard X-Ray yield in the time of cavity
formation. Depth resolved elemental analysis of compound targets, based on this diagnostics technique, has been
Femtosecond laser radiation self-channeling and second harmonic generation under plasma formation conditions were investigated. Laser pulses with sub- and microjoule energy were tightly focused into the volume of a KDP crystal, Estimated laser pulse intensity was of the order 10<sup>13</sup> W/cm<sup>2</sup> in the focal spot exceeding damage and plasma ignition thresholds in the crystal. Threshold of plasma channel formation measured in the experiment corresponds to critical power of self-focusing 1.5f0.1 MW for KDP crystal. Plasma channels with length up to 150 μm were observed in the volume of the KDP crystal. We developed an algorithm allowing estimating electron density and temperature in the plasma channel using experimental dependence of laser energy transmittance through the crystal on incident laser energy. For laser pulse energy 1 μJ estimated electronic density in the plasma channel is of the order of l0<sup>20</sup> cm<sup>-3</sup> (that is about one tenth of the critical plasma density value) and mean electronic temperature is about 3 eV. Free plasma electrons leaded to refraction index increasing by 5 %, extinction coefficient was -25 cm<sup>-1</sup>. The maximum measured efficiency of second harmonic generation was 1.2 %.
Experiments on single pulse plasma channel formation in non-linear KDP crystal by tightly focused (NA = 0.47) fundamental and doubled Cr:forsterite laser radiation with energy of 0.1 ÷ 10 μJ and 100 fs pulse duration were carried out. We propose the simple model of non-linear absorption of femtosecond laser radiation in plasma channel. This model allows estimate laser intensity in the channel and plasma parameters.
We have observed self-guiding of a single femtosecond visible laser pulse in the bulk of transparent nonlinear media (SiO<sub>2</sub>, KDP) and in the water. The dependence of filament length on laser pulse energy was measured. Continuous open-ended channels and frozen modifications of the matter were observed in transparent two-component condensed medium (thin glass plate placed in water).