| The conditions under which a fusion
reactor would deliver more energy than it consumes are expressed by the Lawson criterion.
The value, calculated in 1957 for a D - T mixture at 100 million degrees C, is : n is the ion fuel density contained
in the plasma, tE
is the energy confinement time (time constant for the exponential decrease in the
temperature of a hot environment, with no external energy input).
The ultimate objective of the controlled fusion
research programme is to achieve ´ignitionª, i.e. combustion of the plasma by means of
the kinetic energy of the confined fusion reaction products. In a reactor burning a D - T
mixture, the reactions will be self-sustaining due to the kinetic energy of the helium
(alpha particle), which will be sufficient to maintain the combustion temperature (no
external energy input needed). The state of ignition can be expressed as :
n Ti0 tE >> 6 x 1022
m-3 Million degree K s |
where n and Ti0
(for 100 to 200 Million degrees C) are, respectively, the density and the temperature of
the D and T nuclei in the centre of the plasma and t is the energy confinement time.
 |
 |
Movement
of charged particles in a plasma
a) In the absence of a confining magnetic field, hot plasmas tend to spread and fill the
space available;
b) If a linear magnetic field is applied, the particles move in helical paths, each
encircling a line of force and thus remain radially confined. |
Two experimental approaches to bring
about ignition are being studied:
Fusion
by magnetic confinement,
in which hot plasma is confined by magnetic fields
forming a ´magnetic trapª for the charged particles. In theory, a stationary burn is
possible for as long as the magnetic confinement is maintained. (In this approach, n ~ 1020m-3
and tE ~ 1
to 5 s). |
|
| Diagram
illustrating the principle of magnetic confinement in a torus (in this case a tokamak).
The plasma is ring-shaped and is kept well away from the vessel wall. |
Fusion
by inertial confinement,
in which a minute fuel capsule is highly compressed
(to more than one thousand times its liquid density) until ignition occurs in the centre
and spreads outwards into the surrounding cold fuel. Ignition lasts as long as the fuel
remains confined by its own inertia. A stationary burn is thus impossible with inertial
confinement. (In this approach, n ~ 1031m-3 and tE ~ 10-11s
; tE is
the time during which the fuel freely expands). |
 |
| Diagram
illustrating the illumination of a target using laser beams. The beams compress and heat
the target; after implosion, the explosion carries the energy towards the wall |
|
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