1.5. CLASSIFICATION OF PLASMAS 5
e collective oscillation of the electrons should not be impeded by collisions with the
heavy particles. If their mean collision angular frequency is denoted by !
c
,
!
pe
>> !
c
(1.13)
can be formulated as a condition for dynamic collective behavior.
1.5 CLASSIFICATION OF PLASMAS
As seen in the previous sections, the main characteristics of the plasma are determined by the
electron temperature, T
e
, and the electron density, n
e
. Correspondingly, different plasma can
also be identified by their coordinates in a plot of kT
e
vs. n
e
. is is shown in Fig. 1.4.
As seen, the electron density of known plasmas may vary over about 30 orders of mag-
nitude, their electron temperature over about 7 orders of magnitude. All shown plasmas can be
treated non-relativistically, as indicated by the green line.
If the mean kinetic energy of electrons falls below the Coulomb potential of the interaction
of electrons at their mean atomic distance,
V
C
D
e
2
4"
0
hr
e
i
D
e
2
n
1=3
e
4"
0
(1.14)
plasmas are, in accordance to gas theorie, called “
non-ideal
.” e red transition line in Fig.
1.4
is given by hEi D V
C
and scales, according to Eq. (1.3), as kT
e
n
1=3
e
.
In “degenerate” plasmas, the mean kinetic energy of electrons is smaller than the Fermi
energy of a free electron gas, which is given by
E
F
D
„
2
2m
e
3
2
n
e
2=3
: (1.15)
e red line in Fig. 1.4 results from hEi D E
F
and scales as kT
e
n
2=3
e
.
6 1. PLASMA DEFINITION AND CLASSIFICATION
10
7
10
5
10
3
10
1
10
-1
10
-3
10
-5
10
0
10
5
10
10
10
15
10
20
10
25
10
30
<E> = m
e
c
2
<E> = E
F
<E> = V
C
n
e
(cm
-3
)
kT
e
(eV)
Fusian Plasmas
Laser Plasmas
Tokamaks
Flames
Aurora
Nebulae
Space
Ionosphere
Solar Wind
Solar
Core
Metal
Electrons
High-Pressure
Discharge
Low-Pressure
Discharge
Figure 1.4: Different kind of plasmas in astrophysics, solar physics and solid state physics, nuclear
fusion, and for technical applications in a plot of electron temperature vs. electron density. e
green line denotes the limitation of nonrelativistic plasmas, degenerate and non-ideal plasmas
are positioned right from the blue and red lines, respectively.
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