Fundamentals of Plasma Physics

1.12 A quantitative perspective 21

The collision frequency isν=σ∗nvso

νee =

n

2 π

(

e^2

3 ε 0 κT

) 2 √

3 κT

me

lnΛ

=

e^5 /^2

2 × 33 /^2 πε^20 m

1 / 2

e

nlnΛ

T

3 / 2

eV

= 4× 10 −^12

nlnΛ

TeV^3 /^2

. (1.38)

TypicallylnΛlies in the range 8-25 for most plasmas.

Table 1.1 lists nominal parameters for several plasmas of interest andshows these plas-
mas have an enormous range of densities, temperatures, scale lengths, mean free paths, and
collision frequencies. The crucial issue is the ratio of the mean free path to the characteris-
tic scale length.

Arc plasmas and magnetoplasmadynamic thrusters are in the category of denselab plas-
mas;these plasmas are very collisional (the mean free path is much smaller than the char-
acteristic scale length). The plasmas used in semiconductor processing and many research
plasmas are in the diffuse lab plasma category;these plasmas are collisionless. It is possi-
ble to make both collisional and collisionless lab plasmas, and in fact ifthere is are large
temperature or density gradients it is possible to have both collisional andcollisionless
behavior in the same device.

n T λD nλD^3 lnΛ νee lmfp L

units m−^3 eV m s−^1 m m

Solar corona 1015 100 10 −^310719102105108

(loops)

Solar wind 107 10 10 109 25 10 −^510111011

(near earth)

Magnetosphere 104 10 102 1011 28 10 −^81014108

(tail lobe)

Ionosphere 1011 0.1 10 −^210414102103105

Mag. fusion 1020 104 10 −^41072010410410

(tokamak)

Inertial fusion 1031 104 10 −^101028101410 −^710 −^5

(imploded)

Lab plasma 1020 5 10 −^6103910810 −^210 −^1

(dense)

Lab plasma 1016 5 10 −^41051410410110 −^1

(diffuse)

Table 1.1: Comparison of parameters for a wide variety of plasmas