Is it possible to generate longitudinal electromagnetic wave in atmospheric pressure He plasma? If yes what kind of method would be the most efficient? How would be possible to detect such kind of longitudinal electromagnetic waves in atmospheric pressure plasma?
The excitation of longitudinal waves in plasma medium is possible when oscillating time or frequency of electron fluid in E field of wave is higher than the collisional time. In such cases, the energy gain by electron gas could propagate with the time. At atmospheric pressure plasma, background pressure is 10^5 Pa, therefore the collision frequency would be higher than the electron plasma frequency, therefore, electrons loss the energy with neutrals and wave will damp at early stage..
Usually, impossible, due to the transverse nature of EM waves. In an especial case, if somehow made possible, it would be acoustic-like waves only.
There are indeed several natural normal acoustic (longitudinal) modes in plasmas.
Details could be found at:
https://physics.stackexchange.com/questions/22170/why-no-longitudinal-electromagnetic-waves
Actually, there are plenty of longitudinal waves in a plasma - you just cannot get longitudinal waves in vacuum since this is 'forbidden' by Maxwell's equations (the ones for vacuum, of course). The characteristic plasma frequency actually stems from a longitudinal wave that arises when charge carriers are 'pulled' away from their equilibrium position.
Other examples for longitudinal waves in a plasma are the electrostatic ion cyclotron wave or waves triggered by beam plasma instabilities.
I think the easiest way to get a longitudinal wave in an atmospheric pressure plasma would be the cyclotron wave but I have never tried this myself.
Thank you very much for the answers. I always learn a lot.
If such kind of longitudinal electromagnetic wave exists in the plasma, what kind of detection method would be the best to measure it?
that depends on the type of wave. If you are looking at the easy case of the plasma frequency, you can just use a probe (there are some designed for atmospheric pressure plasma), measure the plasma parameters and calculate the plasma frequency.
Thank you very much, Johannes.
Would you please specify the type and manufacturer of that special probe you mentioned?
@Gabor: I think Impedans hat such a probe but this was some years ago, you can check out the company here: https://www.impedans.com and this is the probe they advertised: https://www.ccrprocessproducts.com/wp-content/uploads/customfield/Langmuir_AtmospheriX_Probe.pdf
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