Environmental Engineering Reference
In-Depth Information
density in a magnetic mirror, and a charged particle travels between its ends (mag-
netic taps) many times. Let us make some estimations for the magnetic trap of
magnetron discharge. We take for definiteness the helium pressure in magnetron
discharge to be 0.1 Torr, which corresponds to a number density of helium atoms
at room temperature of
N
a
10
15
cm
3
D
3
and leads to the rate of electron-atom
10
8
s
1
. It is small compared with the Larmor frequency
scattering of
ν
D
2
10
9
s
1
, and hence electrons are magnetized, that is, they
are captured by the magnetic trap. But they have a small lifetime in the magnetic
trap. Indeed, the thermal electron velocity is
v
T
for electrons
ω
2
H
10
8
cm/s, and the mean free
path of electrons
0.5 cm is small compared with the ring radius. In addition,
the electron drift velocity in crossed electric and magnetic fields is in this case
v
λ
10
6
cm/s and is lower that the thermal electron velocity. Correspondingly,
its shift under the action of a field is relatively small, approximately 0.01 cm, be-
tween neighboring electron collisions with atoms. Nevertheless, this magnetic trap
increases the number density of electrons by the factor exp(
U
max
/
T
e
)incompari-
son with regions outside the action of the magnetic field. This is of importance for
magnetron discharge, because a heightened plasma density is created in the mag-
netic trap region, and intense bombardment of the cathode by ions starts from this
region.
2
4.5.6
Charge Particles in the Earth's Magnetic Field
The Earth's magnetic field is determined by internal Earth currents [112-114]
whichcreatethemagneticmomentoftheEarth,whosevalueis
M
10
19
Gm
3
.Themagneticfieldstrength
H
at distance
R
from the center of this magnetic
dipole is
D
7.8
3(
MR
)
R
M
R
2
R
5
H
D
,
(4.155)
and Figure 4.28 shows the positions of magnetic lines of force for the Earth. As
is seen, these lines are parallel to the Earth's surface near the equator, and the
magnetic field strength at the equator near the Earth's surface is
H
0
0.31 G. On
the basis of this formula we have for the magnetic field strength
H
at distance
R
from the Earth's center
D
r
5
H
0
R
R
3
3cos2
θ
H
D
.
2
We assume here the Earth to be a ball of radius
R
˚
D
is the
latitude of a given point. Note that (4.155) in terms of components of the magnetic
field vector
H
has the form
6370 km, and
θ
2
M
sin
θ
M
cos
θ
H
R
D
,
H
θ
D
.
R
3
R
3
