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We report on an experimental investigation of the incubation effect during irradiation of stainless steel with bursts of ultrashort laser pulses. A series of birefringent crystals was used to split the pristine 650-fs pulses into bursts of up to 32 sub-pulses with time separations of 1.5 ps and 3 ps, respectively. The number of selected bursts was varied between 50 and 1600. The threshold fluence was measured in case of Burst Mode (BM) processing depending on the burst features, i.e. the number of sub-pulses and their separation time, and on the number of bursts. We found as many values of threshold fluence as the combinations of the number of bursts and of sub-pulses constituting the bursts set to give the same total number of impinging sub-pulses. However, existing incubation models developed for Normal Pulse Mode (NPM) return, for a given number of impinging pulses, a constant value of threshold fluence. Therefore, a dependence of the incubation coefficient with the burst features was hypothesized and experimentally investigated. Numerical solutions of the Two Temperature Model (TTM) in case of irradiation with single bursts of up to 4 sub-pulses have been performed to interpret the experimental results.

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Purpose -The purpose of this paper is to describe an experience of R&D in the field of new technologies for solar energy exploitation within the Italian context. Concentrated solar power systems operating in the field of medium temperatures are the main research objectives, directed towards the development of a new and low-cost technology to concentrate the direct radiation and efficiently convert solar energy into high-temperature heat. Design/methodology/approach -A multi-tank sensible-heat storage system is proposed for storing thermal energy, with a two-tanks molten salt system. In the present paper, the typology of a below-grade cone shape storage is taken up, in combination with nitrate molten salts at 5658C maximum temperature, using an innovative high-performance concrete for structures absolving functions of containment and foundation. Findings -Concrete durability in terms of prolonged thermal loads is assessed. The interaction between the hot tank and the surrounding environment (ground) is considered. The developed FE model simulates the whole domain, and a fixed heat source of 1008C is assigned to the internal concrete surface. The development of the thermal and hygral fronts within the tank thickness are analysed and results discussed for long-term scenarios. Originality/value -Within the medium temperature field, an innovative approach is here presented for the conceptual design of liquid salts concrete storage systems. The adopted numerical model accounts for the strong coupling among moisture and heat transfer and the mechanical field. The basic mathematical model is a single fluid phase non-linear diffusion one based on the theory by Bažant; appropriate thermodynamic and constitutive relationships are supplemented to enhance the approach and catch the effects of different fluid phases (liquid plus gas).