Cut-off Frequency and Critical Density
In an unmagnetized plasma in the high frequency or low density limit, i.e. for or, the wave speed is the speed of light in vacuum. As the density increases, the phase velocity increases and the group velocity decreases until the cut-off frequency where the light frequency is equal to the plasma frequency. This density is known as the critical density for the angular frequency ω of that wave and is given by
- .
If the critical density is exceeded, the plasma is called over-dense.
In a magnetized plasma, except for the O wave, the cut-off relationships are more complex.
Read more about this topic: Electromagnetic Electron Wave
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