Environmental Engineering Reference
In-Depth Information
Figure 4.26
Magnetic mirror for a charged particle that moves along a varying magnetic field
and reflects from a region with a high magnetic field.
the particle reflects from the region of the strong magnetic field and is trapped in
a bounded space. The above principle is the basis of various magnetic traps. It acts
for both positively and negatively charged particles, so this arrangement works for
both electrons and protons.
This principle is used in magnetic traps, and a simple magnetic trap is represent-
ed in Figure 4.26. If a charged particle is located in such a trap, and since (4.151)
holds true in the course of particle motion, the particle reflects from trap ends of
a high magnetic field and is locked in this magnetic trap. Then the lifetime of a
charged particle in this magnetic trap is determined by its collisions with other
particles (or by the action of other fields), and it is large compared with the time of
one passage between trap ends.
Note that in this type of a trap, magnetic walls reflect charged particles. In a
magnetic trap of another type, a charged particle moves along a closed trajectory
under the action of the magnetic field, and the potential well for the particle is cre-
ated by the magnetic field. Figure 4.27 represents the magnetic trap of magnetron
discharge. In this case the magnetic field is created by two coaxial magnets with
different directions of the poles, and the maximum magnetic field has the form
of a ring over these magnets with the same axis. Stationary magnets allow one
to reach a magnetic field strength of 100 G, and we will use this value for sub-
sequent estimations. The Larmor frequency of electrons in this magnetic field is
ω
10
9
s
1
, and this value must exceed the rate of electron collisions with
atomsinthistrap.
Let us make some estimations for the magnetic trap of magnetron discharge.
An electron drifts under the action of the magnetic and electric field moving over
a ring, and the electron drift velocity
w
along the ring is given by (4.138),
w
D
cE
z
/
H
,where
E
z
is the projection of the electric field strength on the
z
-axis of this
system. We have for the depth of the potential well
U
max
D
1.8
H
D
μ
H
, and the magnetic
