Elements of Modern Physics
356
For the confinement of the plasma, one cannot use the walls of any vessel.
Any contact with the wall will not only quickly cool the plasma but also cause
the wall to evaporate. What is usually done is to confine the plasma in a suitable
magnetic field. The nuclei spiral along the magnetic field lines. By a suitable
arrangement of the field, the nuclei are reflected back and for the between bottle
necks provided by the converging lines (the lines tend to converge in regions
where the magnetic field is stronger). Such an arrangement is called a mirror
machine. Alternatively, the plasma may be confined in toroidal region formed
by a solenoid bent in the form of a torus. In this case, the nuclei spiral along the
closed field lines inside the torus. However, there are as yet serious difficulties
in controlling the instabilities of confinement over appreciable time periods.
There are two important methods of heating a plasma. In one method, fast
neutral atoms are injected into the magnetically-confined system and are ionized
by collisions with the plasma. The energetic ions are now trapped by the magnetic
field for long enough to transfer their energy to the plasma by collisions. For
example, a plasma of H
+
may be heated by a beam of energetic H, or a plasma
of D
+
(nucleus of deuterium) and T
+
(nucleus of tritium) by a beam of energetic
D (Deuterium). The beam energies are generally of the order of a few tens of
keV to several hundreds of keV. The energetic beams are usually produced by
accelerating low energy ions in an electrostatic field and then passing the ions
through a target gas where the ions capture electrons and are neutralized. The
other method of heating a plasma is by radio-frequency electromagnetic waves.
When the waves are incident of a plasma, under suitable conditions, their energy
is converted into ordered particle energy which is then thermalized by collisions.
An alternative approach to controlled fusion is through what is known as
inertial confinement. Here, the fusion fuel, e.g. mixture of deuterium and tritium,
in the form of a pellet, is imploded from all sides by energy sources such as
laser beams, high energy electron or ion beams. The intense compression
pressures and the high temperatures produced in the pellet may produce
conditions conductive to fusion (it is the particle interial which provides the
basis for confinement over the required period and hence the term inertial
confinement). The difficulties in this approach are the low efficiencies of laser
or other sources, and the need to produce stable symmetrical implosion.
For controlled fusion to be a meaningful source of energy, the output energy
must be more than the input energy. There are several technical difficulties
which remain in achieving the break-even point, such as instabilities in
confinement, inefficient heating, etc. As such, controlled fusion has not yet
been realized. When realized, it will provided a virtually inexhaustible source
of energy. Deuterium, which is suitable for a fusion reaction (ordinary hydrogen
has a very small cross-section for fusion, and hence is not suitable), is readily
available, 0.03% by mass of hydrogen in water being in the form of deuterium.
Furthermore, the fusion reactions have important advantages over other sources
356
For the confinement of the plasma, one cannot use the walls of any vessel.
Any contact with the wall will not only quickly cool the plasma but also cause
the wall to evaporate. What is usually done is to confine the plasma in a suitable
magnetic field. The nuclei spiral along the magnetic field lines. By a suitable
arrangement of the field, the nuclei are reflected back and for the between bottle
necks provided by the converging lines (the lines tend to converge in regions
where the magnetic field is stronger). Such an arrangement is called a mirror
machine. Alternatively, the plasma may be confined in toroidal region formed
by a solenoid bent in the form of a torus. In this case, the nuclei spiral along the
closed field lines inside the torus. However, there are as yet serious difficulties
in controlling the instabilities of confinement over appreciable time periods.
There are two important methods of heating a plasma. In one method, fast
neutral atoms are injected into the magnetically-confined system and are ionized
by collisions with the plasma. The energetic ions are now trapped by the magnetic
field for long enough to transfer their energy to the plasma by collisions. For
example, a plasma of H
+
may be heated by a beam of energetic H, or a plasma
of D
+
(nucleus of deuterium) and T
+
(nucleus of tritium) by a beam of energetic
D (Deuterium). The beam energies are generally of the order of a few tens of
keV to several hundreds of keV. The energetic beams are usually produced by
accelerating low energy ions in an electrostatic field and then passing the ions
through a target gas where the ions capture electrons and are neutralized. The
other method of heating a plasma is by radio-frequency electromagnetic waves.
When the waves are incident of a plasma, under suitable conditions, their energy
is converted into ordered particle energy which is then thermalized by collisions.
An alternative approach to controlled fusion is through what is known as
inertial confinement. Here, the fusion fuel, e.g. mixture of deuterium and tritium,
in the form of a pellet, is imploded from all sides by energy sources such as
laser beams, high energy electron or ion beams. The intense compression
pressures and the high temperatures produced in the pellet may produce
conditions conductive to fusion (it is the particle interial which provides the
basis for confinement over the required period and hence the term inertial
confinement). The difficulties in this approach are the low efficiencies of laser
or other sources, and the need to produce stable symmetrical implosion.
For controlled fusion to be a meaningful source of energy, the output energy
must be more than the input energy. There are several technical difficulties
which remain in achieving the break-even point, such as instabilities in
confinement, inefficient heating, etc. As such, controlled fusion has not yet
been realized. When realized, it will provided a virtually inexhaustible source
of energy. Deuterium, which is suitable for a fusion reaction (ordinary hydrogen
has a very small cross-section for fusion, and hence is not suitable), is readily
available, 0.03% by mass of hydrogen in water being in the form of deuterium.
Furthermore, the fusion reactions have important advantages over other sources