文档介绍：Chapter 1
Introduction
What is a Plasma?
An ionized gas
A plasma is a gas in which an important fraction of the atoms is ionized, so that the electrons
and ions are separately free.
When does this ionization occur? When the temperature is hot enough.
Balance between collisional ionization and bination:
Figure : Ionization and bination
Ionization has a threshold energy. bination has not but is much less probable.
Threshold is ionization energy (, H). χi
Integral over Maxwellian distribution gives rate coefficients (reaction rates). Because of
the tail of the Maxwellian distribution, the ionization rate extends below T = χi. And in
equilibrium, when
n < σ v >
ions = i , ()
nneutrals < σr v >
6
Figure : Ionization and radiative bination rate coefficients for atomic hydrogen
>
the percentage of ions is large (∼ 100%) if electron temperature: Te ∼χi/10. . Hydrogen
> ◦
is ionized for Te ∼ 1eV (11,600 k). At room temp r ionization is negligible.
For dissociation and ionization balance figure see . Delcroix Plasma Physics Wiley (1965)
figure , page 25.
Plasmas are Quasi­Neutral
If a gas of electrons and ions (singly charged) has unequal numbers, there will be charge
density, ρ.
ρ= ne(−e) + ni(+e) = e(ni − ne) ()
This will give rise to an electric field via
ρ e
.E = = (ni − ne) ()
0 0
Example: Slab.
dE ρ
= ()
dx 0
x
→ E = ρ()
0
7
Figure : Charged slab
This results in a force on the charges tending to expel whichever species is in excess. That
is, if ni > ne, the E field causes ni to decrease, ne to increase tending to reduce the charge.
This restoring force is enormous!
Example
19 −3
Consider Te = 1eV , ne = 10 m (a modest plasma; . density of atmosphere nmolecules ∼
25 −3
3 × 10 m ). Suppose there is a small difference in ion and electron densities Δn = (ni − ne)
so ρ= Δn e ()
Then the