Absorption waves produced by CO₂ laser ablation

Kurzfassung

A requirement for efficient pulsed laser propulsion from ground to LEO is the achievement of a specific impulse of up to 800 s at a jet efficiency of at least 50%. With CO₂ laser radiation at pulse lengths in the range of 10 microseconds and polymers as propellant these numbers cannot be attained by classical laser ablation because the impulse formation by laser ablation is limited by the premature absorption of the incident laser radiation in the initially produced cloud of ablation products. The power fraction of a CO₂ laser pulse transmitted through a small hole in a POM sample has been compared with the incident power. It was found that the transmitted power fraction is directly proportional to the inverse of the pulse energy. The plasma formation in vacuum and in air of 3500 Pa and the spread of the shock wave with velocities of 1.6 to 2.4 km/s in the low pressure air was observed by Schlieren photography. A sharp edged dark zone with a maximum extension of 10 to 12 mm away from the target surface develops within 5 μs independently of the pressure and is assumed to be a plasma. In order to find out, if this is also the zone where the majority of the incident laser radiation is absorbed, a CO₂ probe laser beam was directed through the expansion cloud parallel to and at various distances from the sample surface. The front of the absorption zone is found to move rapidly away from the target surface with increasing speed. The absorption lasts twice as long as the laser pulse. It is not associated with a pressure rise that would increase the mechanical impulse. The radial motion of the absorption wave turned out to be faster than the shock wave seen in the Schlieren pictures.