Hello.
Let's say we have narrow sapphire tube (~300 um width and few cm length) which is filled with gas and discharged capacitively with high voltage of ~ 30 kV. With sub atm pressure, we can get order of 10^18 cm^(-3) plasma density and plasma temperature (both electron and ion are in equilibrium due to such a high density) is about 1~2 eV. This is actually for plasma waveguide for intense laser.
I've always worrying melt down of dielectric material due to direct contact of plasma although it have been reported as successful waveguide.
Is it possible that temperature is significantly reduced within very thin sheath possibly formed in proximity to the dielectric wall? Is it possible that temperature there can be even lower than material melting point?
Please give me any comments on this.
In our similar experiments (preionization of the channel for a main discharge), the allumina walls get chared (3mm Inner Diameter). Which results in some ablation of the material and such effect adds measurable amount of added pressure and contaminants to the plasma channel.
Thanks for sharing experience.
Could you explain a little bit more of how material charging leads material ablation? Do you mean that charing mechanism such as electron hitting on the wall leads material damage over time?
Yes, something on those lines, when ions and electrons hit a solid surface they recombine. So we can treat the wall as a perfect sink for particles. Given that the electrons in a plasma generally move much faster than the ions, the initial electron flux into the wall greatly exceeds the ion flux, assuming that the wall starts off unbiased with respect to the plasma. Of course, this flux imbalance causes the wall to charge up negatively and generates a potential barrier which repels electrons, reducing the electron flux futher. Debye shielding confines this barrier to a thin layer of plasma with a thickness of a few Debye lengths, coating the inside surface of the wall. This layer is known as a plasma sheath or a Langmuir sheath.
