D. S. Uryupina

Femtosecond laser-plasma interaction with prepulse-generated liquid metal microjets

Ultrashort laser pulse interaction with a microstructured surface of a melted metal is a promising source of hard x-ray radiation. Microstructuring is achieved by a weak prepulse that produces narrow high-density microjets. As an x-ray source, the interaction of the main laser pulse with such jets is shown to be nearly two orders of magnitude more efficient than the interaction with ordinary metal targets. This paper presents the results of optical and x-ray studies of laser-plasma interaction physics under such conditions supported by numerical simulations of microjet formation and fast-electron generation.

Few-cycle optical pulse production from collimated femtosecond laser beam filamentation

A scheme for stable shot-to-shot few-cycle pulse production has been realized by launching an initially collimated laser beam into a gas tube. We found that the optimum parameters for the sevenfold compression of 55 fs, 5 mJ, Ti:Sapphire laser pulses are the following: 0.8-0.9 atm argon gas pressure and the registration aperture with a diameter of 300-700 μm. A technique for the efficient extraction of the self-compressed pulse from the desired position along the filament has been provided by moving the registration aperture along the tube. With this technique, pulses as short as 8 fs were detected at a distance of 1 m from the filament starting position, in agreement with numerical simulations performed using a 3D+time axially symmetric code.

Angular distribution of the terahertz radiation intensity from the plasma channel of a femtosecond filament

A phenomenological model of the terahertz radiation of the plasma channel of a femtosecond filament has been elaborated that satisfactorily describes the experimental results of the detection of low-frequency radiation in air. The angular distributions of the terahertz radiation intensity in the absence and presence of an external electrostatic field have been obtained. The dependence of the divergence angle of the terahertz radiation on the filament parameters has been determined.

Acceleration of Heavy Multicharged Ions in the Interaction of a Subrelativistic Femtosecond Laser Pulse with a Melted Metal Surface

Results are presented from experimental studies of ion acceleration under the action of femtosecond laser pulses with an intensity of 1017 W/cm2, incident onto the free surfaces of melted gallium and indium. The effect of the polarization direction of a linearly polarized laser pulse and the amplitude of a short prepulse, which precedes the main pulse by several nanoseconds, on the parameters of accelerated ions is investigated. It is found that, even for such a moderate laser intensity, the characteristic velocity of fast ions ejected along the reflected beam is a factor of 1.5 higher than that of ions ejected along the normal to the target surface. It is shown that, as the prepulse energy increases, the hard X-ray yield and the mean energy of hot electrons increase substantially, whereas the velocity of both fast and slow ions decreases appreciably regard-less of laser polarization.

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.

Peculiar properties of hot plasma formation under the influence of intense femtosecond pulses on the surface of molten metal

The peculiar properties of plasma formation on the surface of different liquid metals by femtosecond laser radiation have been studied. It is shown that plasma formation and generation of hard X-radiation on the surface of molten metal depends substantially on the contrast and weakly on the polarization of laser radiation, which clearly distinguishes the plasma produced in our experiments from the plasma generated on the surface of a solid target.

Laser-induced plasma influence onto intrapulse four-wave mixing under femtosecond filamentation in air

We experimentally investigate four-wave mixing under femtosecond filamentation in air of a pre-collimated/loosely focused beam with a single pulse. We show that its angular-frequency spectrum depends on preliminary focusing conditions and attribute this to the plasma density change. A simple model of the phase matched four-wave mixing gives a good agreement with the experimental results only if the plasma dispersion is taken into account.

Laser optoacoustic diagnostics of femtosecond filaments in air using wideband piezoelectric transducers

New opportunities in ultrasound diagnostics of femtosecond laser filaments with wideband piezoelectric transducers are considered. Transverse spatial resolution better than 100 microns is demonstrated in the single and regular multiple filamentation regime making path toward 3D filament tomography. The simple analytical model of the cylindrical acoustic source fitted well with the experimental data.

Microjet formation and hard x-ray production from a liquid metal target irradiated by intense femtosecond laser pulses

By using a liquid metal as a target one may significantly enhance the yield of hard x-rays with a sequence of two intense femtosecond laser pulses. The influence of the time delay between the two pulses is studied experimentally and interpreted with numerical simulations. It was suggested that the first arbitrary weak pulse produces microjets from the target surface, while the second intense pulse provides an efficient electron heating and acceleration along the jet surface. These energetic electrons are the source of x-ray emission while striking the target surface. The microjet formation is explained based on the results given by both optical diagnostics and hydrodynamic modeling by a collision of shocks originated from two distinct zones of laser energy deposition.

Investigation of the transformation of the spectrum of femtosecond laser radiation on filamentation in gas medium

The dependence of the parameters of femtosecond laser radiation (55 fs, 3.7 mJ, 800 nm) in a filament on the distance along the filament propagation axis, the diaphragm diameter, and the type and pressure of gas used for filamentation has been studied experimentally.