Daria S. Uryupina

Focused femtosecond beam filamentation after amplitude masks has been studied experimentally and numerically. We deduced conditions (energy per hole, diameter and geometrical composition of holes, focal length) providing for the formation of the regularized bundle of filaments or single on-axis filament at the given pulse duration and beam diameter. We showed that a light channel with small diameter (∼200 μm) and overcritical peak power may be formed well before both the focal distance and the Marburger length, and this channel collapses due to self-focusing and forms the filament. The start position of such a filament can be predicted based on the linear propagation equation, while a more sophisticated non-linear approach that takes into account the Kerr nonlinearity, plasma effects, etc., helps to describe the temporal structure of a filament, its frequency, and its angular spectrum.

Postfilamentation channel resulting from filamentation of freely propagating 744-nm, 5-mJ, 110-fs pulse in the corridor air is examined experimentally and in simulations. The longitudinal extension of postfilament was determined to be 55–95 m from the compressor output. Using single-shot angle-wavelength spectra measurements, we observed a series of red-shifted maxima in the spectrum, localized on the beam axis with the divergence below 0.5 mrad. In the range 55–70 m, the number of maxima and their red-shift increase with the distance reaching 1 μm, while the pulse duration measured by the autocorrelation technique is approximately constant. Further on, for distances larger than 70 m and up to 95 m, the propagation is characterized by the suppressed beam divergence and unchanged pulse spectrum. The pulse duration increases due to the normal air dispersion.

Experimentally it was shown that liquid gallium can be used as a target for 10 Hz femtosecond laser plasma x-ray source. X-ray measurements demonstrated that temperature of hot electrons in gallium plasma does not differ from plasma of solid target and equal to 6.2±1.OkeV.

It was shown that liquid gallium heated up to 270°C can be used as a target for 10 Hz femtosecond laser plasma highly stable x-ray source. The decreasing of hard x-ray yield during 50000 laser shots is less then 25% and can be easy compensated by additional focusing of objective or temperature tuning.

In this paper, we present atomic energy and charge spectra of ions accelerated at the front of the silicon and tungsten targets irradiated by the high-contrast femtosecond laser pulse with the intensity of 3/spl middot/10/sup 16/ W/cm/sup 2/, which is delayed with respect to the cleaning nanosecond laser pulse of the 3-J/cm/sup 2/ energy density. The tremendous increase in the number of fast primary ions (Si or W) and significant growth of their maximum charge in the case of the cleaned target has been observed. The main specific features of the energy and charge spectra have been analyzed by means of one-dimensional hydrodynamic transient-ionization modeling. We are also discussing the possible impact from above threshold ionization by ambipolar field on the generation of deeply ionized fast ions.

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.