While reading about non-thermal plasmas, I came across their ionization potentials(~1%), and other capabilities, such as their non Maxwellian energy distributions. At what temperatures, and pressures do they exhibit such properties, and what materials are used to create non thermal plasma?


At what temperatures, and pressures do they exhibit such properties

First, temperature is only rigorously defined when the particle distribution functions are Maxwellian. For a non-thermal plasma, the notion of a "temperature" is ill-defined. If you do see someone talking about temperature for such a case, they've likely ignored a small perturbation that makes the distribution non-Maxwellian.

and what materials are used to create non thermal plasma?

Materials have little to do with this. By this I mean that the species of gas you pick to ionize, for instance, does not by itself determine whether your plasma will be non-thermal. It has more to do with the ionization process, confinement and / or heating scheme that you're using.

For example, I can launch electromagnetic waves into a plasma that damp on electrons in a certain way (see Landau Damping) such that energy is preferentially transferred to some electrons in the tail of the (originally) Maxwellian distribution. If I do this continuously, it will change the steady-state distribution function away from a Maxwellian.

  • $\begingroup$ this helps a bit on temperature definitions for plasma en.wikipedia.org/wiki/Plasma_%28physics%29#Temperatures $\endgroup$ – anna v May 9 '15 at 3:56
  • $\begingroup$ Why should the temperature be only rigorously defined for Maxwellian distributions? In statistical mechanics you usually define it as $1/T =\frac{\partial S}{\partial U}$, for $S$ the entropy and $U$ the internal energy. This works for a large number of non-Maxwellian distributions... $\endgroup$ – Wraith of Seth Jul 13 '15 at 7:02
  • $\begingroup$ @WraithOfSeth: Because in kinetic theory you use Temperature as the second moment of the underlying velocity distribution. Only in the maxwellian case is the system completely described by 0th, 1st and 2nd moment, else you need more of them, or they simply don't have a straightforward interpretation anymore. $\endgroup$ – AtmosphericPrisonEscape Aug 14 '15 at 0:09
  • $\begingroup$ @AtmosphericPrisonEscape I generally agree with this. To be precise, however, even for a Maxwellian it is not fully described by the 0th, 1st and 2nd moments. We have to add an ansatz for heat transfer ($Q \sim \chi \nabla T$) to close the system. Typically, $\chi$ incorporates complex transport mechanisms and is determined empirically for a plasma. $\endgroup$ – user3814483 Aug 17 '15 at 2:07

Temperature is defined by thermodynamic equilibrium, which should be clarified at first. From this definition, we can easily obtain that the so called "non thermal plasma" is a concept which describes a plasma state without thermodynamic equilibrium. Basically speaking, thermodynamic equilibrium is obtained by collisions between particles caused by molecular force in neutral gas. On the other hand, coulomb force takes place of molecular force in highly ionized plasma. Note that these two forces are both working in plasma. So, particle density, initial velocity distribution and ionization rate will dominate in determining whether an isolated plasma system could get a thermodynamic equilibrium or not.


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