Measurement of ion-rich sheath thickness by ion acoustic wave

Measurements of sheath thickness in a low-pressure plasma are reported. The sheath is ion-rich and contains both ions and electrons; the sheath edge is taken as a boundary for ion acoustic waves. The thickness of the sheath is estimated from a frequency shift of the wave. Despite the actually continuous transition from plasma to sheath, a clear-cut sheath edge is observed. It should be emphasized that even a very small increase in thickness, such as 0·1mm, can be measured by this frequency-analysis method. The experimentally obtained sheath boundary is in agreement with the Tonks–Langmuir value as modified by Self.