Vladimir D. Zvorykin

The process of microwave pulse propagation (λ = 8 mm) in a hollow plasma waveguide, created in air by UV radiation from a high-power KrF laser, is investigated by numerical simulation. It is shown that an effective confinement of microwave radiation in the waveguide is possible when the electron density in plasma walls of the waveguide is ≥1013 cm−3. The possibility of controlling the structure of the microwave beam by changing the aperture angle of the plasma cone of the waveguide is demonstrated. In particular, it is shown that the waveguide can be used as a concentrator of microwave radiation energy at specific distances from the waveguide output. An analogy between radiation propagation in a plasma waveguide and the quantum mechanical evolution of a particle's state in a potential well is discussed.

The influence of a laser’s wavelength on the laser ablation of natural silicates by CO2 laser (λ ∼ 10 μm) and CO laser (λ ∼ 5 μm) irradiation was investigated. High-speed photography was used. For a CO laser, a short and wide-angle conical plume was formed. For a CO2 laser, two types of plumes were observed. For a CO2 laser by lower laser fluency (0.15 kJ/cm2), a short and wide-angle conical plume was formed. At higher fluency Q/S (>0.30 kJ/cm2), one of the two distinct types of plumes was observed during the action of a single laser pulse. The plume of the first type was long (∼20 mm) and narrow with the onset close to the start of the laser pulse. The second type was a conical plume with the onset delayed by a few microseconds from the laser pulse. The first type of plume expands along the laser beam, whereas the second one is directed normally to the surface. Comparing the results for both lasers at the wavelengths of ∼10 and ∼5 μm, we conclude that a long narrow plume is formed due to the intensive resonant absorption of CO2 laser radiation, which leads to breaking of Si-O bonds and to plasma creation in a thin layer of the target surface. The plume expansion is due to laser-induced acceleration of the particles in the direction of a laser beam and is connected with the ponderomotive forces acting on the plasma particles. The wide conical plume is due to thermal heating of the surface.

The dynamics of strong plane shock waves (SW) was studied being generated in a hypersonic laser-driven shock tube (LDST) during the acceleration of thin polymer CH films by the ablation pressure of plasma produced by UV pulses of KrF laser (100 J and 100 ns). The LDST has a square cross-section of 7 × 7 mm and a length of 50 mm. At an incident radiation energy density of 70 J/cm ² and intensity of 0.7 GW/cm ² , the SW velocity in air at atmospheric pressure of 2.6 km/s (Mach number M = 8.2) was constant while it propagated in the LDST, and it increased by an additional 30−50% when the laser plasma was confined by a transparent quartz plate. It was shown that the time of quasi-stationary propagation of the SW in the LDST is ~20 μs, being determined by the energy density of the laser pulse imparted to the plasma piston. For comparison, the SW velocity in free space quickly attenuated due to the lateral expansion of the gas, while the SW front acquired the hemispherical shape.

The nonlinear compression of narrowband (Δν ≈ 0.2 cm⁻¹) 20 ns KrF laser pulses in SF₆ at 10 atm and in CH₄ at 50 atm pressure was studied. Both SBS and SRS optically phase-conjugated backward-reflected radiation was registered with an energy reflectivity of 10–14% in SF₆ and CH₄. In SF₆, the SBS pulses gradually shortened from 10 ns to 2–3 ns with a decrease in pumping to the SBS threshold of ~10 mJ, while the SRS pulse had the shortest length of 30–60 ps for the maximal pumping of 120 mJ and broadened near the SRS threshold of ~30 mJ. For the SRS pulse energy, the ~2 mJ peak power 5 × 10⁷ W was tenfold higher than the pump power. The theoretical model predicted a soliton-like SRS pulse compression to a temporal length of the order of the vibrational relaxation time. There was no pulse compression of backward SBS and SRS radiation in CH₄, while, in the forward direction, SRS pulses shortened to 3–4 ns at reduced pumping.

Nonlinear compression of narrow-band (Δ ≈ 0.2 cm-1) 20 ns KrF laser pulses in SF6 at 10 atm and CH4 at 50 atm pressure was studied. Both SBS and SRS optically phase conjugated backward reflected radiation was registered with energy reflectivity 10−14% in SF6 and CH4. In SF6 SBS pulses gradually shortened from 10 ns to 2−3 ns with pumping decrease to the SBS threshold ~ 10 mJ, while the SRS pulse had the shortest length of 30−60 ps for the maximal pumping of 120 mJ and broadened near the SRS threshold of ~30 mJ. For the SRS pulse energy ~ 2 mJ peak power 5×107 W was tenfold higher than the pump power. The theoretical model predicted a soliton-like SRS pulse compression to a temporal length of the order of vibrational relaxation time. There was no pulse compression of backward SBS and SRS radiation in CH4, while in the forward direction SRS pulses shortened to 3−4 ns at reduced pumping.

The hydrodynamics of plasma formed in the interaction of 100 ns UV KrF laser pulses with foam targets with volume densities from 5 to 500 mg/cm³ was studied. Initial and dynamic transmittance at 248 nm wavelength were measured. At intensities of about 10¹² W/cm², the propagation rates of radiation through foam targets reached 80 km/s, while plasma stream velocities from both the front and rear sides of targets were approximately the same, ~ 75 km/s, which confirms a volumetric absorption of radiation within the target thickness and the explosive nature of the plasma formation and expansion.

The hydrodynamics of plasma formed in the interaction of 100-ns UV KrF laser pulses with foam targets with volume densities from 5 to 500 mg/cm3 was studied. Initial and dynamic transmittance at 248-nm wavelength have been measured. At intensities about 1012 W/cm2, the propagation rates of radiation through foam targets reached 80 km/s, while plasma stream velocities from both front and rear sides of targets were approximately the same ~ 75 km/s, which confirms a volumetric absorption of radiation within the target thickness and the explosive nature of the plasma formation and expansion.