Measurement of the phase velocity of ion acoustic waves in the boundary layer at a conducting sphere

The phase velocity of a diverging spherical ion acoustic wave is measured in the boundary layer at a negatively biased conducting sphere which is immersed in a homogeneous, stationary, weakly ionized argon plasma. The Doppler shift of the phase velocity is compared with results derived from Lam's (1965) analysis of the boundary layer. Time of flight diagrams indicate that the wave originates at a radius which is approximately equal to the sheath radius predicted by Lam.