Landau Damping of Ion Acoustic Waves in Highly Ionized Plasmas

Ion acoustic waves with frequencies between 5 and 100 kc/sec have been excited in highly ionized cesium and potassium plasmas by modulating the potential of a tungsten grid immersed in the plasma. The waves were detected by another grid, which could be moved along the plasma column. Because the plasma is produced at one end of the column, there is a net flow of plasma between the grids. Phase velocities of 1.3\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm/sec and 0.9\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm/sec along and against this drift were measured for cesium and 2.5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm/sec and 1.3\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm/sec for potassium. The damping distance of the waves was found to be independent of ion density in the range between 2\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and 3\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and equal to 0.55 and 0.25 wavelength along and against the flow in cesium and 0.65 and 0.14 in potassium. A comparison between the results and the collisionless theory shows agreement within about 10%. In particular, it is shown how the present experiment provides a quantitative measurement of the Landau damping of ion acoustic waves.