Waves and noise in a plasma column

In the positive column of a low-pressure mercury arc measurements of the transmission of electroacoustic waves are made in small and zero axial magnetic fields. Externally excited and self-excited waves are shown to propagate similarly: a low frequency cut-off in transmission appears for both at about 20 kc/s. Over the lower part of the frequency range a simple theory using the measured steady-state parameters of the discharge fits the observations, usually within the combined experimental errors (±20%). Above 40 kc/s the slope of the experimental dispersion curve falls sharply: this is the upper limit at which an external exciter coil is effective in a magnetic field, and at this frequency the exciter is least effective in zero field. The change in wave character is not understood. The wave damping is less than that expected from the rate at which ions strike the wall. The boundary conditions are not clear.