4) - Heating and current
generation (p 1
- 2 - 3 -
5 - 6 - 7 )
|How do we achieve temperatures of
the order of hundred million degrees, required for the operation of a fusion reactor? Easy, just switch on the heating...
Lawson criterion, which expresses the constraints on plasma parameters to produce energy from thermonuclear fusion, requires a temperature of 10 to 20 keV . To achieve these very high temperatures (several hundred million degrees),
it is necessary to heat the plasma.
The first natural heating mechanism is the Joule effect, associated with
the current flowing in the plasma, necessary to create the
tokamak magnetic configuration. Just as the filament of an electric bulb heats up when a current passes through it, the plasma will increase in temperature under the effect of strong current (in Mega Amperes). Unfortunately, this effect, proportional to the
plasma resistance, which tends to collapse when the temperature increases, saturates and only enables limited temperatures to be reached (around 10 million degrees). This "natural" heating
operation is called ohmic operation, in reference to the unit of measurement of electrical resistance, the ohm.
To reach the required temperatures, we thus resort to additional heating systems. These are classified into two main families :
heating by injection of
highly energetic neutral particles, which consists of heating the plasma using the collisions between
the very energetic injected particles and the plasma particles.
heating by radio-frequency waves, which consists of coupling
to the plasma with a wave at a frequency chosen, so as to be in resonance with a category of particles in the plasma (i.e. at the same frequency) and
therefore caoable of communicating energy to them, a bit like in a microwave oven which heats a dish by shaking up
its water molecules.