I'm currently attempting to excite ion acoustic waves with 27.12MHz Helicon array plasma source which is operating in steady state.
I'm using the mesh grid method by applying an oscillating potential to a mesh grid at 1Vpp while using an RF compensated Langmuir probe to scan for density perturbations. The link provided explains the setup. I am having trouble resolving an ion acoustic wave. I deviate from what is done from the paper by raising the plasma density from 1e8cm^3 to 1e11cm^3
Once I am able to accomplish this, I will to attempt to excite ion acoustic waves during the plasma afterglow period by pulsing the Helicon source.
Because the wavelength is a function of electron and ion temperature, this wavelength should be nearly unresolvable during the first 40us as the electron temperature rapidly decays from ~2.5eV ~0.2 eV.
Then during the next 50us the electron temperature remains around 0.2eV.
The plasma density decay is negligible within this time frame as well. It is during this time that I believe I might be able to excite and resolve an ion acoustic wave.
Does anyone here have any experience with attempting to do something similar to this?
http://plasma.physics.ucla.edu/papers/Laptag-ion-waves.pdf
Hi Nathan, start by repeating their experiment; Low power and low density. I suspect you are achieving W-mode coupling at 1e11 and this is giving large oscillations of Vp in the downstream region. These will go away when you pulse, but for the setup stage go to low power/ low density and check your experimental setup.
While the LAPTAG writeup says they are operating a 'Helicon' plasma source; At 40W and 1e8 density they are probably in E-mode or (magnetically enhance) H-mode. See paper below and my thesis from ANU for a discussion on operational modes.
Note that after you confirm your ion-acoustic wave launching at low density and start increasing the power/density, the downstream plasma will change non-monotonically as you cross the E-H-W boundaries. Not sure what the residual effects in the afterglow would look like and any potential affects on ion-acoustic wave.
Good luck!
-Bert Ellingboe
Article Capacitive, inductive and helicon‐wave modes of operation of...
-Bert
Sure enough at a lower power and density I am seeing the waves, but there was one other contributing factor. Debye length. The first mesh grid I used had a much larger length on either side of the square apertures on the mesh than that of the Debye length of my plasma. Due to this I believe most of the plasma traveling through my mesh couldn't even "see" the potential on the grid.
Using a much smaller mesh size, and using the same parameters as the paper, I was able to see the ion acoustic waves on a Langmuir probe biased negatively.
The lower the pressure of the system. The further the waves propagated before being damped beyond detection. I suspect neutrals play a role in this phenomenon. Under certain conditions, I'm even observing a frequency shift of these waves. The further the propagation, the lower the frequency is shifted. I cannot explain this yet.
I then began to increase the frequency of the mesh and the background magnetic field. So far I am able to observe ion acoustic waves with the mesh grid oscillating up to 180kHz and 200Watts applied to my Helicon. The SS density is on the order of 1e8 to 1e9 cm^-3.
Once I reach around 50 Gauss, I no longer detect ion acoustic waves. The waves arrive immediately, as if another mode is being excited. This mode is likely traveling within a magnitude or two of the speed of light, but this velocity is hard to pin down as my Oscope is measuring on the microsecond scale, and with such a long wavelength it's impossible to see any kind of shift on the nanosecond scale. Furthermore, in the beginning of the afterglow (15us), I also detect this wave immediately arriving. My Langmuir probe is picking up an EM wave?
I'll check into that paper now! Thanks for the direction.
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