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Interferometer traces and dispersion relation of ion acoustic waves.

Here are some comments to the dispersion measurements of ion acoustic waves:

1. Interferometer traces are taken at different frequencies, some of which are displayed above. At each frequency the wavelength is measured.
   
   
2. The dispersion relation, shown below, is a plot of frequency versus wavenumber, (omega) vs k or f vs (lambda)^-1. The slope of the straight line fit to the data points gives a phase velocity which corresponds to the sound speed in argon at an electron temperature of 1.8 eV. This is a reasonable value since the Langmuir probe indicated a bulk temperature of 1.5 eV and a tail temperature of 2.5 eV.
   
   
3. Since the phase velocity is essentially constant the ion plasma frequency must be well above the highest wave frequency. The region near the ion plasma frequency will be analyzed in more detail below.
   
   

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