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We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 10 11 W cm −2 and within the 710-740 nm wavelength range. The Sr + spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p 2 1 S 0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr 2+ ion. A weakly evident but reproducible inflection point ('knee' structure) appears in the intensity dependence of the Sr 2+ yield at the location of the 5p 2 1 S 0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p 1/2 , 3/2 , 6s 1/2 and 5d 3/2 , 5/2 excited Sr + states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr + spectrum. The population accumulation in the 5p 1/2 , 3/2 ionic states unambiguously proves the absorption of two photons above the first 5s 1/2 atomic threshold while that of the 5d 3/2 , 5/2 and 6s 1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near-resonant doubly excited states, their role in the dynamics of laser-atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.

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The magnetic field and temperature dependences of critical current densities J c (B , T ) in ceramic high-T c superconductors are described using the model of force induced fluxon motion. Fluxons start moving when the local Lorentz force F L (r) = φ 0 j (r) exceeds the local pinning force F p (r) =F 0 B 2 c (T )/B (r), j (r) and B (r) are the local current density and the local magnetic induction. The model describes the change of curvature on plots of critical current against temperature at various applied magnetic fields. Results of the calculation are in good qualitative agreement with experiments. Using the same model the irreversibility line separating regions of pinned and depinned vortices in the temperature-magnetic field diagram for transport currents is derived.

The calculations of energy characteristics, operating regimes and dimensions of the discharge zone of the supersonic ebeam sustained CO laser are presented. Calculations have been carried out for the laser modules with a power up to 40 megawatts.