Excitation of drift instabilities in thermionic plasmas

Low-frequency oscillations travelling near the electron pressure gradient drift velocity in Cs or K plasmas have been observed to exist only in the presence of an ion sheath at the cathodes. It is suggested that these waves are self-exciting `universal' instabilities in the presence of finite resistivity and that their excitation is connected with the insulating properties of an ion sheath, which allow the perturbation amplitude to be finite at the sheath edge. This leads to overstable oscillations with λ|| longer than the length of the machine, a strange concept which is explained physically. Although the solution of the problem involves sheath-type boundary conditions, for an isothermal plasma the criterion for onset of instability can be stated rather simply. The predicted critical sheath voltage agrees well with observations. It appears that alkali-metal plasmas with electron sheaths are quiescent only by virtue of stabilization by conducting end-plates; however, this feature makes such plasmas particularly valuable for the study of the onset of long-wavelength drift instabilities.