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Laser Filamentation for Probing and Controlling Atmospheric Processes

Final Report Summary - FILATMO (Laser Filamentation for Probing and Controlling Atmospheric Processes)

Filamentation of high intensity lasers opened new perspectives in atmospheric research. Laser filaments are self-sustained light structures of typically 0.1 mm diameter and up to hundreds of meters in length, widely extending the traditional linear diffraction limit.

While propagating in air, these ultra-intense laser filaments generate a white light supercontinuum (from ultraviolet to mid infrared). The resulting "white light laser" has been shown as an ideal source for Lidar (Light Detection and Ranging) remote sensing of atmospheric pollutants. In the present ERC project, we found that laser filaments are not only able to observe atmospheric processes, they are also able to control them, at least to a certain extent. Four characteristic examples are highlighted in the project: lightning control, laser induced water vapour condensation, transmission of optical data through fog, and modulation of the radiative forcing properties of cirrus clouds.

A broad audience overview of these groundbreaking discoveries can be found in a recent TV broadcast from CNN: http://edition.cnn.com/2015/04/24/tech/laser-cloud-seeding-mci/index.html.

The capability of filaments to trigger Megavolt discharges in the laboratory was already demonstrated in the early 2000’s. Real scale experiments were then carried out, where discharges triggered by the laser within thunderclouds could be clearly identified. However, no lightning strike could be guided towards the Earth. In FILATMO, we identified the optimal laser requirements for a successful “laser lightning rod” , namely high intensity laser systems with repetition rates higher than 1000 shots per second. A new project has been launched to this end in H2020: the laser lightning rod project (www.llr-fet.eu).

Based on field experiments in various atmospheric conditions, we also showed that laser filaments can induce water condensation and fast droplet growth up to several µm in diameter in the atmosphere as soon as the relative humidity (RH) exceeds 70%. Lasers can thus be used to generate clouds and harvest water from the gas phase in the atmosphere. It could represent in the future a much more environmental friendly method to seed clouds than the usual and widely spread method of chemical seeding with, e.g. silver iodide.

Conversely, we recently found that the optical transmission through clouds could be enhanced by filaments. For instance, cirrus clouds are constituted of large (20-100 um) ice crystals, which both reflect the solar visible radiation and the heat emitted from the Earth, contributing to global warming. We found that by shining an intense laser on these ice crystals, the particles were shattered and evaporated, which, in turn produced a large number of smaller (micrometer sized) particles. This modulation of the ice crystal size by filaments would invert the radiative forcing properties of cirrus clouds, as they would transmit the IR emission from the Earth, but still reflect the visible solar radiation.

Finally, clearing of fogs and clouds can be obtained by using high average power (>100 W, >kHz) ultrashort lasers. In this case, the shock waves induced by the filaments radially expel the droplets from the beam center in a quasi-continuous way, so that a cleared channel appears. The applications of such fog and cloud clearing are of paramount importance for recent programs on laser based earth to satellite or earth to drone telecommunication (like, e.g. the LCRD program from NASA or the Facebook Connectivity lab).