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Flying faster with a new laser

Supersonic planes, such as Concorde, were eventually scrapped due to their excessive fuel consumption. However, researchers at the Applied Optics Laboratory are working on new technology that could reduce the fuel consumption of supersonic aircraft, by effectively halving the braking force exerted by their drag.

Supersonic aircraft, such as planes or missiles, present two disadvantages that have reduced their use in industry. Firstly is the noise produced by the sonic boom when they break the sound barrier, that is, when they reach the speed of sound at 340 m/s (1,224 km/h). The greatest disadvantage, however, is the amount of fuel consumed by these machines. When an airplane breaks the sound barrier, its engines must overcome a considerable braking force in order to continue moving forward. A shockwave is generated around the nose and all the pointed parts located on the front of the aircraft, increasing drag and thus requiring more fuel.

To tackle this major problem, researchers from the team led by Aurélien Houard at the Applied Optics Laboratory (LOA, an École Polytechnique/ENSTA ParisTech/CNRS joint research unit), in collaboration with Paul Quentin Elias from ONERA, came up with the idea of using a femtosecond laser to deform the shockwave. With the support of the company Phasics, they were recently able to demonstrate that by using laser filamentation to produce a thin column of plasma at the front of the supersonic aircraft, its drag may be reduced by 50%. These results, published in the journal Science Advances, make way for new possibilities of using intense femtosecond lasers as onboard actuators for drag reduction, trajectory control or sonic boom reduction.

Laser filamentation for drag reduction

This technology is based on the principle of laser filamentation, whereby a very intense laser beam propagates through a medium like air, creating a nonlinear effect called the Kerr effect, which results in extreme self-focusing of the laser beam. The concentrated beam then "breaks up" air molecules, creating a plasma, a mix of ions and electrons (shown in purple in the figure above). The plasma generated by the laser heats up the air and can change the shape of the shockwave, making it more pointed, which in turn allows the plane to penetrate the air better, reducing its drag (as shown in the figures below). This filamentation is only possible with femtosecond lasers. It is an original application of the CPA technology that won the 2018 Nobel Prize in Physics.

Several necessary optimizations

This research, part of an ASTRID project funded by the French Directorate General of Armaments (DGA) and implemented by the French National Research Agency (ANR), still has some areas for improvement before it can be used. For now, the drag deformation only occurs for a short moment, and therefore requires the use of high-rate laser fire. Moreover, the lasers used to produce this filament cannot yet fit on an aircraft (10 m3 per ton of material), so must be miniaturized.

These improvements, alongside research into other applications, are being studied by the laser filamentation research group led by Aurélien Houard. Their projects include work on laser lightning rods and laser sonar, which will also require improved performance of the instruments used at the LOA.