In 1951, Lundquist used mercury as the liquid in attempting to
demonstrate the existence of
MHD waves in the laboratory. He placed mercury in a stainless steel
cylinder with a vertical
magnetic field of about 1,000 gauss parallel to its axis. He used a
rotating circular disk
with projecting vanes attached at the bottom of the cylinder to excite
torsional waves. True
standing waves could not be excited because of dissipation.
In 1957, Lehnert repeated the experiment by using liquid sodium instead
of mercury. Because
of reduced ohmic losses he was able to make more refined measurements.
These experiments
sufficed to demonstrate the existence of MHD waves.
When a perfectly conducting fluid moves transverse to the magnetic lines
of force, it carries
them with it. Therefore, the tube of magnetic lines of force behaves
like an elastic string.
This situation is analogous to the propagation of waves along a
stretched elastic string.
Alfven waves are unimportant in liquids, where the relatively high
inertia damps them
strongly, but are common in plasmas. They occur in the Earth's Van Allen
belts and in the
solar wind. They are believed to be responsible for transporting energy
from the Sun's
interior to sunspots and to be a principal mechanism for heating the
solar corona. On Earth
they have been used to heat plasma in experimental plasma containment
devices.
MHD POWER GENERATORS
In ordinary conventional electric generators, electric power is
generated through the motion
of metal conductors in a magnetic field. Similarly, in MHD generators
electric power is
generated as a result of the passing of a gaseous conductor (plasma)
through a magnetic
field. The motion of the ionized gas produces an electric field between
the two electrodes.
An electric current will flow from one electrode through an external
load to the other
electrode, and the system will operate as a generator. In the MHD
generator, the potential
working temperature is much higher (about 2500 K) than the conventional
generator, and by a
suitable arrangement the heat energy of the ionized gas can be directly
converted into flow
energy. The intermediate step of going through the prime mover (steam
turbines) is therefore
avoided, resulting in higher efficiency. MHD generators are divided into
generators with open
and closed cycles. In the former case the products of combustion are
supplied directly to the
generator; in the latter case the same working gas is constantly
recalculated.
PLASMA PROPULSION
The basic principle of electromagnetic pumps and electromagnetic plasma
accelerators is the
same. A conducting fluid in a pipe is forced to move by the Lorentz
force created when
mutually perpendicular magnetic fields and electric currents are applied
perpendicular to the
pipe. So-called electromagnetic pumps are useful for circulating
corrosive substances such as
liquid sodium carrying heat from the core of nuclear reactors to the
heat exchangers outside.
An electromagnetic flow meter consists of an insulating pipe under a
uniform transverse
magnetic field. The voltage induced by the fluid motion between
electrodes placed at the ends
of a diameter of the pipe perpendicular to the field is used to indicate
the flow rate.