Photoniques Magazine No. 131 | Page 40

LARGE SCIENTIFIC PROJECT
CONTROLLING lightning with lasers
Uchida et al. in Fukui, Japan. The researchers twice observed a possible laser effect on lightning over a distance of 1 to 2 m, using a combination of several kilojoule nanosecond lasers focused above a tall metallic tower [ 1 ]. A major breakthrough came with the development of amplified femtosecond lasers [ 2 ]. When these ultrashort pulsed lasers are focused in the air, they generate very long and uniform plasma filaments that can reach hundreds of meters with relatively moderate laser energies. This process known as filamentation is due to a combination of Kerr self-focusing and plasma defocusing that saturate the laser intensity inside the filament. This discovery, at the end of the 90s, quickly triggered the launch of lightning control projects in Quebec with a project piloted by INRS and Hydroquebec, and in Europe with the Franco-German Teramobile project. Numerous laboratory studies were carried out with discharges of several meters produced by Marx generators( see example in Figure 1). In 2005, the first real-life campaign was organized in New Mexico by researchers from the Teramobile project [ 3 ]. The experiment consisted in producing plasma filaments at several hundred meters altitude
Figure 1. Photograph of a 2 m-long electric discharge guided by laser filamentation( left) and without laser filament( right). using a TW laser operating at a repetition rate of 10 Hz. In the presence of thunderclouds, the laser was expected to guide or trigger lightning. Unfortunately, very few thunderstorms occurred during the campaign, which nevertheless enabled them to observe the emission of RF signals due to the formation of filament induced corona discharges during the thunderstorms.
The laser lightning Rod project
The Laser Lightning Rod( LLR) project was launched 10 years later, when new laser technologies based on amplification in Yterbium: Yag crystals allowed to increase the laser repetition rate by a factor of 100. These new ultrashort lasers, developed by Trumpf scientific in Germany, were capable of maintaining a permanent hot-air channel by generating plasma filaments at high repetition rate. This greatly increased the chances of intercepting and guiding a lightning bolt. Under the impetus of the Applied Physics group at the University of Geneva and the Laboratory of Applied Optics in Ecole polytechnique( which were already the main players in the Teramobile project) a consortium was then formed with the German company Trumpf scientific, Ariane group, interested as an end user, and lightning experts from EPFL and from the University of Applied Sciences and Arts Western Switzerland. The project officially kicks off in January 2017 with a funding of € 4 million from the European commission. The site chosen for the experiment was the other key to the project ' s success. Selected by EPFL, it is a 120 m high radio transmission tower on the summit of Mont Saentis( Appenzell, Switzerland, 2502 m), operated by Swisscom. The Saentis tower is one of the most frequently struck by lightning in Europe, but above all, due to the presence of the mountain
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