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(a) Interferometer traces at a constant frequency for different discharge currents. (b) Density, obtained from the ion plasma frequency, vs discharge current.

Here are some comments to the properties and use of ion acoustic waves near the ion plasma frequency:

1. At high discharge currents the wavelength is essentially independent of density which is controlled by the discharge current.
   
   
2. As the discharge current is lowered the wavelength decreases, the wave damping increases, and the wave amplitudes become small. When the plasma frequency falls below the signal frequency no sound waves can propagate.
   
   
3. By measuring the wavelength at different discharge currents one can determine the plasma density from the theoretical dispersion relation as follows: The sound speed is determined as the phase velocity at high plasma densities, c_s= 2.4 × 10⁵cm^-3. The ion plasma frequency is given by f_p,i=f[1- (v_phase/c_s) ²]^-1/2. Finally, the density is calculated from the argon ion plasma frequency by n (in cm^-3) = [f_p,i (in Hz)/33.2]².
   
   
4. A plot of density vs discharge current shows the expected linear dependence. Thus, dispersion measurements near the ion plasma frequency are useful for measuring the plasma density.
   
   

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