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(a) Interferometer traces at a constant frequency for different grid voltages. (b) Ion energy vs grid voltage.

Here are some comments to the observed waves at large grid voltages:

1. The wavelength of ion acoustic waves should be rather independent of wave amplitude. However, the measured wavelength of grid-excited waves strongly increases with rf voltage. This discrepancy is explained by the existence of another effect, called ballistic ion modes.
   
   
2. At large grid voltages, the ions are accelerated by the sheath electric field and injected as free-streaming particles into the plasma. Fourier analysis of the periodic ion bursts by the interferometer circuit produces a wave with dispersion (omega)=kv_i, where the phase velocity v_i equals the ion streaming velocity.
   
   
3. The linear relation between ion energy and grid voltage confirms the above model of ion acceleration. Only at small voltages (V_grid<kT_e/e) does a grid excite true ion acoustic waves.
   
   

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