Measurements of the mean velocities
u∞ and the velocity distributions of the molecular beams from a shockheated source have been carried out by a conventional time-of-flight method, and their translational temperatures
T∞// parallel to the flow direction have been determined. The speed ratios,
S∞// = (
mu∞/2
kT∞//)
1/2, of 6-20 have been obtained in the source temperature range of
T0 =1, 400-10, 800K and its pressure range of
P0=1.3-6.8atm for argon. They have been less than the theoretical terminal speed ratios for a free jet. In the present apparatus with a convergentdivergent conical nozzle, the boundary layer from the nozzle wall limits the expansion and results in a decrease of the speed ratio. Although the flow is strongly affected by the background gas in the far downstream from the nozzle exit, higher speed ratios have been obtained.
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