Optical quality of supersonic jets of various gases

The optical qualities of supersonic jets of four different gases were investigated. The ratios (optical wavelength)/(jet thickness) were approximately 0.8 < 10(-4). Near-field schlieren photographs and interferograms and far-field intensity distributions of a circular beam traversing the jets are presented. The jets tested could be arranged in the following order of improving optical quality: Ar, N2, He, 62% He/38% Ar. Higher optical quality could be qualitatively correlated with lower values of |n(jet) - n (ambient air)| and [n(jet) - 1], where n is the refractive index. The far-field Strehl ratios for circular beams passing just below the nozzle exit ranged from approximately 0.65 for Ar for which the refractive-index functions were 3.4 x 10(-4) and 6.0 10(-4), respectively, to as high as 0.92 for the He-Ar mixture where the jet refractive index matched that of the ambient air and [n(jet) - 1] was 2.7 x 10(-4). Measured far-field Strehl ratios are compared with values calculated from measurements of near-field phase errors. Methods of improving the optical quality of the jets are discussed.