Environmental Engineering Reference
In-Depth Information
pends on the ratio between a typical time of ion location in a plasma and a typical
time for transition between atomic and molecular ions.
Let us consider an example where atomic helium ions are formed at the initial
time at the beginning of the drift chamber and pass through it under action of a low
electric field in a time
dr
. In the course of the drift, atomic ions are converted par-
tially into molecular ions. Since molecular ions move faster, we obtain a broadened
signal at the collector. We take the mobilities of atomic and molecular helium ions
in helium to be 10.4 and 16.7 cm
2
/(V s) according to Tables 4.11 and 4.13, so their
ratio is approximately 1.6. Figure 4.15 shows the time dependence for the reduced
ion current
i
(
t
/
τ
τ
dr
) in this case, where the drift time
τ
dr
relates to atomic helium
ions, and the current is normalized to 1:
Z
i
t
τ
dt
τ
dr
D
1.
dr
In addition, we account for the broadening of the current in time as a result of lon-
gitudinal diffusion of ions in a gas. Thus, we have three typical times in this case,
the drift time
τ
dr
of atomic ions through the drift chamber, the time for conver-
sion
τ
con
of atomic ions into molecular ions, and the diffusion time
τ
D
for atomic
ions defined according to (4.33)
L
2
D
,where
D
i
is the longitudinal diffu-
sion coefficient for atomic ions and
L
is the distance traveled by ions in the drift
chamber. Since the drift time is
D
2
D
i
τ
τ
D
L
/
w
,where
w
is the ion drift velocity, we
dr
D
/
2
dr
obtain
D
τ
τ
D
Lw
/2
D
. For definiteness, we use in Figure 4.15
D
100
and
0.3. As is seen, a change of the type of ion leads to a change in the
character of ion drift.
One can see that this effect is stronger, the larger the ratio of the mobilities for
atomic and molecular ions. At low temperatures these values are determined by the
τ
dr
/
τ
D
con
Figure 4.15
Variation in time of the current
i
of helium ions passing through a drift cham-
ber under the action of an electric field if He
C
ions are formed at
atomic ions
τ
dr
to the diffusion time
τ
D
of
atomic ions in helium is
τ
dr
/
τ
D
D
10, and
the time for conversion
τ
con
of atomic ions in
molecular ions is
τ
con
D
3
τ
dr
.
t
D
0 at the beginning
of this chamber. The ratio of the drift time of
