Environmental Engineering Reference
In-Depth Information
onset value of
N
i
10
16
cm
3
s for a deuterium plasma and an onset value of
10
14
cm
3
s for a tritiumplasma. These values are reached in contemporary plasma
fusion devices, where a hot plasma is created by fast beams of hydrogen atoms,
but it is necessary to solve a circle of engineering and technological problems to
construct an effective fusion power station.
Another method to solve the problem of thermonuclear fusion uses pulsed sys-
tems. In this case a deuterium pellet is irradiated over its entire surface by a laser
pulse or a fast ion beam. When the pulse impinges on the pellet, the pellet is heated
and compressed by a factor of 10
2
τ
D
10
3
. The heating and compression is intended
to promote a thermonuclear fusion reaction. Note that the dense hot plasma that
is formed during compression of the pellet is a special state of matter which does
not have an analogue in an ordinary laboratory setting.
Various concepts are used in plasma devices for special use. The magnetohydro-
dynamic (MHD) generator works on the basis of the Hall effect (Section 4.5.2),
where a stream of hot, weakly ionized gas flows in a transverse magnetic field, and
the electric current is induced in a direction perpendicular to the directions of the
stream and magnetic field (Figure 7.2a). As a result, the flow energy is converted
into electric energy. According to thermodynamic laws, the efficiency of this trans-
formation is quite high because a flowing gas is hot.
The principles of action of MHD generators and plasma engines may have an
analogy, as shown in Figure 7.2. In plasma engines, which are used in the cosmos
as rocket engines of space vehicles [4-8], a flux of ions causes the motion of a sys-
tem in the opposite direction according to Newton's third law. There are two types
of ion engines. In Hall thrusters, electrons are magnetized by a strong magnetic
field, and ions leave the plasma region under the action of the electric field. In ion
thrusters, a magnetic field is used to prevent electron attachment to the anode. In
both cases ions leave the plasma region with a typical velocity of 10
7
cm/s, which
Figure 7.2
A magnetohydrodynamic generator (a) and an ion engine (b). In the first case, the
electric field occurs as a result of motion of a plasma in a magnetic field; in the second case,
electrons and ions obtain their velocity owing to crossing electric and magnetic fields.
