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With a view to better predicting the effects of thermal defocusing within the atmosphere, an experimental simulation set-up has been designed at ONERA. This consists essentially of a vertical airtight cell containing a gas or gas mixture sufficiently absorbing to induce "blooming" of a CO2 laser beam over a distance of about 3 m. A return wind tunnel, integrated within the cell, creates a uniform wind on the beam propagation path ; the wind velocity may be precisely adjusted between 0.1 and 2 m/sec. A rotating drum photometric analyser, associated with a data acquisition and processing unit, provides an automatic sequential exploration of the irradiance model observed at the beam focus. Some characteristic results are presented to illustrate the possibilities offered by the set-up

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