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In this paper optimal solutions to limit the subsidence of underground oil pipeline in insular permafrost are proposed.

The vibrational wavepacket revival of a basic quantum system is demonstrated experimentally. Using few-cycle laser pulse technology, pump and probe imaging of the vibrational motion of D+2 molecules is conducted, and together with a quantum-mechanical simulation of the excited wavepacket motion, the vibrational revival phenomenon has been characterised. The simulation shows good correlation with the temporal motion and structural features obtained from the data, relaying fundamental information on this diatomic system.

Control of the reflection coefficient of a 'nonlinear' mirror by an external train of ultrashort pulses from a YAG : Nd laser was investigated. This mirror was part of a synchronously pumped dye laser and it was based on generation of the sum and difference frequencies. The mirror reduced the spectral width of the emission line of the dye laser to 0.2 nm and made tuning possible over the whole gain band. The kinetics of formation of spectral and time parameters of lasing was studied. Time sampling of the reflection coefficient of the mirror by external radiation reduced by a factor of 3–4 the duration of ultrashort pulses emitted by the dye laser.

An investigation was made of stimulated emission from a dye laser pumped synchronously by fairly short rectangular growing and decaying trains of ultrashort pulses. The profile of the pump train envelope influenced the optimal tuning of the resonator length. The shortest output pulses with the highest peak energy were formed when the intensity of the ultrashort pulses in the pump train increased smoothly.

Deexcitation of dye molecules by the stimulated emission was used to reduce the duration of the ultrashort pulses from a synchronously pumped dye laser. This accelerated the formation of the output pulses and stabilized their profile. Mathematical modeling showed that the pulse duration could be more than halved by selecting a suitable delay and deexciting pulse energy.

The aim of this project is to establish a 10 PW facility on the Vulcan laser system capable of being focussed to intensities of at least 10 Wcm and integrate this into a flexible and unique user facility This paper will present progress made in Phase one developing the 10PW Front End as well as the concept for the new Vulcan 10 PW facility. The new facility will be configured in a unique way to maximise the scientific opportunities presented through a combination with the existing capabilities already established on Vulcan. This ground breaking development will open up a range of new scientific opportunities. © 2010 IOP Publishing Ltd.

The role of plasma channels as waveguides for laser-wakefield accelerators is discussed in terms of the results of experiments performed with the Astra-Gemini laser, numerical simulations using the code WAKE, and the theory of self-focusing and self-guiding of intense laser beams. It is found that at a given electron density, electron beams can be accelerated using lower laser powers in a waveguide structure than in a gas-jet or cell. The transition between relativistically self-guided and channel-assisted guiding is seen in the simulations and in the behaviour of the production of electron beams. We also show that by improving the quality of the driving laser beam the threshold laser energy required to produce electron beams can be reduced by a factor of almost 2. The use of an aperture allows the production of a quasi-monoenergetic electron beam of energy 520 MeV with an input laser power of only 30 TW. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

The modulational instability that occurs during the interaction of a long laser pulse and its own wakefield in an underdense plasma has been studied experimentally and theoretically. Recent experiments using laser pulses that are several times longer than the wakefield period have yielded transmission spectra that exhibit a series of secondary peaks flanking the main laser peak. These peaks are too closely spaced to be the result of Raman instabilities; their origin was found to be photon acceleration of the laser's photons in the wakefield instead. In the experiments described in this paper, a laser pulse of 50-200 fs containing 300-600 mJ was focused on the edge of a helium gas jet on a 25 νm focal spot. The observed transmission spectra show evidence of both ionization blueshift and modulation by the pulse's wakefield. The transmission spectra have also been modelled using a dedicated photon-kinetic numerical code. The modelling has revealed a direct correlation between the spectral modulations and the amplitude of the excited wakefield. By comparing the measured and simulating spectra, the origin of various spectral characteristics has been explained in terms of photon acceleration. The feasibility of using this effect as a wakefield diagnostic will be discussed. © 2009 IOP Publishing Ltd.

Intracavity stimulated Raman scattering (STRS) in a cell containing compressed hydrogen placed inside a ruby laser resonator with active mode locking was investigated using an additional high-Q resonator locked to the frequency of the first Stokes component. A theoretical analysis of the dynamics of intracavity STRS and of its influence on mode locking in a master laser was made on the basis of a spectral–mode description of the field in the resonator. It was found that, depending on the nonlinear interaction between the modes governed by the hydrogen pressure and the length of a laser cell, it is possible to establish weak, optimal, or strong energy exchange between the laser and Stokes waves. An experimental study of STRS at various hydrogen pressures confirmed the calculations and showed that feedback at the Stokes frequency reduced the STRS threshold and increased the conversion efficiency.

The spectral mode representation of fields in the resonator is used to calculate temporal and spectral characteristics of pulses from an actively mode-locked laser with intracavity frequency doubling and feedback at the harmonic frequency. The calculation results agree qualitatively with experimental data. It is shown experimentally that phase effects due to mismatch of the waves interacting in a system of coupled resonators are clearly observed only for one of the directions of propagation of the radiation in the resonator.

An investigation was made of the energy and time characteristics of the radiation under conditions of intracavity stimulated Raman scattering (STRS) conversion in hydrogen using ultrashort pulses emitted by an actively mode-locked ruby laser. Dependences of the efficiency of conversion of the laser radiation to the first Stokes component on the hydrogen pressure and on the laser pumping rate were obtained. Both these dependences had a maximum. Time measurements demonstrated some narrowing of a single pulse of the first Stokes component and a decrease in the number of pulses in a train, compared with the laser radiation pulses.

Experimental and theoretical studies were made of the time and energy characteristics of a rhodamine 6G dye ring laser with optically induced distributed feedback, pumped synchronously by a train of ultrashort pulses from an actively mode-locked YAG:Nd laser with intracavity second harmonic generation. Narrow-band (~ 0.08 nm) generation of ~ 10 ps ultrashort pulses, tunable in the 568–602 nm range, and with a divergence of ~ 0.4 mrad was achieved. The energy efficiency was ~ 4% for each of the two output beams at a pump energy of 25 μJ. Under optimal conditions the duration of the output pulse was approximately twenty times shorter than that of the pump pulse. The results of the calculations using the proposed model show a good agreement with the experiments.

Tunable generation of ultrashort pulses was achieved using a dynamic distributed-feedback (DFB) rhodamine 6G dye laser under conditions of two-photon optical pumping by single fundamental-frequency pulses from an actively mode-locked YAG:Nd laser. At a dye concentration of 1.6 × 10 −2 mol/liter, the lasing threshold of the DFB laser was reached at a pump energy of ~ 6 mJ which corresponded to an intensity of ~ 7.7 GW/cm2 in the active zone. The laser operated stably in the 580–606 nm wavelength range. The width of the emission line was 0.1–0.5 nm. A positive chirp of the lasing frequency was observed due to nonlinear modulation of the active medium refractive index in the high-intensity two-photon pump field.