Environmental Engineering Reference
In-Depth Information
deuterons. The free deuterons D
þ
are
accelerated into the ErD
2
target on the right,
providing a high density of deuterons for the
reaction.
Figure 4.1 Sketch [45] of compact field-
ionizing neutron generator operating in D
2
gas.
The inset shows the 100 nm radius W tip which,
when biased to 80 kV by the pyroelectric crystal,
ionizes nearby deuterium gas to produce free
Coulomb barrier, as explained by Gamow, from distances much larger than the
contact spacing. The value for two deuterons is similar, but would formally be given
from the nuclear radius formula
r
1.07
fA
1/3
with
A
10
15
m. Therefore,
¼
¼
2 and
f
¼
2.14
f
2
1/3
k
C
e
2
/2
r
D
the contact spacing is 2
r
D
533 keV.
Naranjo
et al.
[45] achieved D
-
D fusion in a small chamber
filled with D
2
gas, by
using a 100-nm-radius tungsten W tip biased to a positive voltage 80
-
110 kV. The
electric
field near the tip is large enough to break up the deuteriummolecules and to
ionize the resulting deuterium atoms, producing D
þ
ions and electrons. The D ions
are accelerated into an ErD
2
target and fusion neutrons were observed.
The apparatus is shown in Figure 4.1. A LiTaO
3
pyroelectric crystal (center left, in
Figure 4.1) produces a strong electric
field between the crystal and the ground plane.
A pyroelectric crystal, as the temperature is varied, to give a large surface charge
density,
s
(in C/m
2
), as electrons are pushed outward slightly from the crystal, and
this means a large surface electric
eld
E
¼
¼
2.7 f, so that
U
¼
¼
¼s
/2
e
0
. In this apparatus, the voltages
appear with change in temperature of the pyroelectric crystal. Once ions are formed,
they are carried into the target at energy 80 keV. The pyroelectric crystal, similar in
nature to a ferroelectric crystal, is a clever way to achieve a high voltage in a small
laboratory apparatus.
To generate ions, much larger localized ionizing electric elds
E
are generated near
a tip of radius 100 nm (see inset). The electric eld
E
at the surface of ametal sphere of
radius
a
at voltage
V
is
V
/
a
, which in this case gives
E
(
a
)
10
3
/100
10
9
¼
80
10
12
V/ m
Va
/
r
2
¼
0.8
¼
0.8 keV/nm at radius
a
¼
100 nm. The
field falls off as
E
¼
for
r
larger than
a
.
The authors [45] indicate that
E
field ionization,
thus generating electrons and D
þ
ions from D
2
gas. Thus, ionization rapidly occurs
¼
25 V/nm is suf
cient for 100%
