The intrinsic phase shift and its effect upon the measurement of air flow velocities using LITRA

LITRA is a velocimetry technique being developed to measure flow speeds under hypersonic conditions. The technique uses a high power laser to generate ionization at a point of interest and a microwave system to track the movement of this ionization within the flow. LITRA infers velocity from the phase of the scattered microwave signal. This phase changes as the laser induced spark moves. However, additional phase shifts occur which are not associated with the velocity of the spark. These phase shifts arise from the interaction between microwaves and plasma, and they limit the accuracy of the velocity measurement. Work is being conducted to understand the causes of the intrinsic phase shift. Computational work, which models the gas dynamic evolution of a laser induced spark, is discussed. Microwave scattering measurements are presented and show the evolution of the signal amplitude and phase. Both computational and experimental results are discussed in light of shadowgraph measurements that show the spatial evolution of the laser induced spark.