Direct Reduction of Aero-Optical Laser Beam Aberrations by Active Turbulence Control

This work is part of an effort to investigate methodologies for active turbulence control of laser beam propagation through separated flows relevant to airborne laser applications as well as to detect the turbulenceinduced laser aberrations with Shack-Hartmann sensing. Large scale turbulence suppression control in separated compressible flows is investigated as a means to directly reduce aero-optical aberrations in laser propagation for airborne directed energy capabilities. Experiments are conducted on forced and unforced large-Reynolds-number compressible separated shear layers. Flow forcing is realized using a custom-built dielectric barrier discharge pulsed plasma actuator that can operate at elevated pressures. Results from flow control experiments show significant reductions in the root-mean-square optical path difference depending on the pulsed plasma actuator forcing frequency. Shack-Hartmann wavefront sensor laser profiling is conducted to measure directly the aero-optical aberrations. The flow conditions used in this research are Reynolds number of 6 million, based on the visual thickness of the turbulent separated shear layer, a freestream Mach number of 0.9, and an elevated test section pressure of 3 atm. The Shack-Hartmann sensor provides pathintegrated information regarding the turbulent refractive field and interfaces that the laser wavefront propagated through. Experimental comparison of control on vs. off cases indicates evidence showing the effectiveness of pulsed plasma forcing for the direct reduction of the laser aberrations. Since the dominant contributions to the aberrations, in unforced flows, are caused by large-scale organized structures, our findings indicate that the mechanism by which the significant reduction is observed in the present forced experiments is due to large-scale organized structure suppression effected by pulsed plasma forcing.