The direct conversion of broadband solar radiation into highly coherent laser radiation has attracted scientific and practical interest in recent years. [1] After the first demonstration of a solar-pumped laser (SPL) in 1963, [2] subsequent studies were focused on improving the laser medium and the solar collector design, thereby achieving state-of-the-art high-power SPLs with solar collecting efficiency of up to 32.5 W m À2 (laser output power per unit area of the solar collector optics). [3,4] Such state-of-the-art SPLs comprised laser media, such as Nd 3þ -doped single-crystal yttrium aluminum garnet (Y 3 Al 5 O 12 ) rods, which were pumped by concentrated sunlight (the concentration factors were in the order of thousands). [5][6][7][8][9] SPLs exhibit many potential applications, including solar hydrogen generation, photovoltaic energy conversion, space solar power systems, space propulsion, and remote area telecommunications. [1,[10][11][12][13][14][15][16][17] In addition, SPLs with 20 kW output powers could be achieved by exploiting megawatt-scale solar furnaces; these SPLs could be employed in a fossil-fuel-free energy cycle in which magnesium is chemical energy storage for sunlight. [1,[18][19][20] Despite the range of potential applications, the utilization of highly concentrated sunlight has severely restricted the practical applications of SPLs owing to their dependence on an accurate solar tracking system. [21] Further, only the direct components of sunlight can be harvested by concentrating optical elements, such as lenses or mirrors; they do not function efficiently under cloudy conditions when the diffuse component of sunlight is high. Regarding locations, including Susono (Japan) and Karlsruhe (Germany), diffuse horizontal solar radiation represents %49% of the total terrestrial sunlight averagely received annually. [22] Furthermore, removing the reliance of SPLs on concentrating optics would further render the technology comparable with that of photovoltaic panels, thus availing new applications for SPLs since the modularity of such flat-plate devices would also favor small-scale applications. [23][24][25] Although SPLs, which do not require a mirror/lens-based concentrator system, have been studied for a long time since their introduction in 1983, [26] the technology was only demonstrated T. Masuda Carbon Neutral development division Toyota Motor Corporation

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Experimental visualization for ionic wind motion originated from DC corona discharges in a needle-plate electrode system has been investigated. A vapor-phase biacetyl tracer with laser-induced phosphorescence emission is used for optically characterizing the ionic wind profile. The ionic wind blows the excited biacetyl molecules away in continuing the visible phosphorescence emission for a long radiative lifetime. The captured image with elapsing time from the excitation presents the shifting location of radiative tracer along the ionic wind direction. The experimental results show the ionic wind profile enhanced in the electric field direction corresponding to the corona discharge progress. Especially, the ionic wind near an initiating point of corona discharges is focused as an advantage of this optical technique. The ionic wind velocity along the electrode axis can be obtained at the location close enough to the corona discharge initiation point, and the velocity at 0.5 mm from the discharge point is figured out as 9.3 to 19.2 m/s under the condition of the EHD Reynolds number of 0.95 × 10 3 to 2.1 × 10 3 .

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