# Plasma Physics Question - making a protecting shield

Tags:
1. May 23, 2014

### thetao

I need to find the correct voltage to apply to a wire grid in order to repel a plasma of which I know the density (free electrons per volume) and the particle energy in eV. I see a Debye sheath as being analogous to what I will create with such a charge.

The way I see it is that I will have two wire layers at opposite potentials, creating a capacitance in between them, and this will create an outside charge. This charge will repel the plasma on the outside so that it does not enter the system.

Would someone tell me what I need to consider in order to solve this problem? I understand that different voltages would achieve a larger or smaller distance between the outside wire grid and the plasma, more or less debye lengths, but is there a saturation point, a voltage beyond which the distance would not be noticeably increased? How do I go about calculating this voltage?

Thank you!

2. May 23, 2014

### UltrafastPED

The grid voltage must be greater than the voltage part of the electron energy (eV).

It's not guaranteed that every electron will be repelled ... there is a statistical variation of energies inside the plasma.

Not clear on what you are trying to accomplish - diagrams are preferred over verbal descriptions - but here is a patent that discusses some similar issues, and has diagrams:
http://www.google.com/patents/US20030102402

I've used the voltage on a grid to repel electrons that were _below_ a given energy; in this application they would only have contributed to the noise, and so had to be rejected.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook