Комбинированные окна для газовых лазеров высокой мощности

Based on the previously developed mathematical model of the behavior of the multi-kilowatt laser window with an unstable cavity, the case of a two-component output window is considered. The two-component window consists of a transparent polycrystalline diamond ring and a central opaque area separated by a plastic vacuum gasket. The central opaque area is equipped with a cryoaccumulator to reduce heat load. Numerical calculations of thermomechanical processes are performed for such windows used in high-power CO2 lasers. Mathematical model used for the calculations consists of three parts - thermophysical, mechanical and optical. The advantages of using a two-component design with a cryoaccumulator under the conditions of a gas laser operating in the multi-kilowatt power range are demonstrated. The dependences of the maximum output power, temperature distribution and mechanical stresses versus the thickness of the window are obtained. The optimal conditions providing maximum radiation strength and minimum beam divergence are considered.

Stabilization of Transverse Modes for a High Finesse Near-Unstable Cavity

We develop a method to lock a high-finesse near-unstable Fabry–Perot (FP) cavity (F = 7330) to a frequency stable dye laser operating at 605.78 nm using the Pound–Drever–Hall technique. The experimental results show the feasibility of locking this cavity to different transverse modes. This method links the external FP cavity to the dye laser cavity, and a 379 kHz final linewidth of the FP cavity is achieved. Such a near-unstable cavity is potentially useful for cavity-enhanced spontaneous parametric down-conversion to generate narrow-band single photon or photon pairs in different transverse modes.

Rotational stabilisation of the Rayleigh–Taylor instability at the inner surface of an imploding liquid shell

A number of applications utilise the energy focussing potential of imploding shells to dynamically compress matter or magnetic fields, including magnetised target fusion schemes in which a plasma is compressed by the collapse of a liquid metal surface. This paper examines the effect of fluid rotation on the Rayleigh–Taylor (RT) driven growth of perturbations at the inner surface of an imploding cylindrical liquid shell which compresses a gas-filled cavity. The shell was formed by rotating water such that it was in solid body rotation prior to the piston-driven implosion, which was propelled by a modest external gas pressure. The fast rise in pressure in the gas-filled cavity at the point of maximum convergence results in an RT unstable configuration where the cavity surface accelerates in the direction of the density gradient at the gas–liquid interface. The experimental arrangement allowed for visualisation of the cavity surface during the implosion using high-speed videography, while offering the possibility to provide geometrically similar implosions over a wide range of initial angular velocities such that the effect of rotation on the interface stability could be quantified. A model developed for the growth of perturbations on the inner surface of a rotating shell indicated that the RT instability may be suppressed by rotating the liquid shell at a sufficient angular velocity so that the net surface acceleration remains opposite to the interface density gradient throughout the implosion. Rotational stabilisation of high-mode-number perturbation growth was examined by collapsing nominally smooth cavities and demonstrating the suppression of small spray-like perturbations that otherwise appear on RT unstable cavity surfaces. Experiments observing the evolution of low-mode-number perturbations, prescribed using a mode-6 obstacle plate, showed that the RT-driven growth was suppressed by rotation, while geometric growth remained present along with significant nonlinear distortion of the perturbations near final convergence.