transport, confinement, enhanced confinement mode

3) - Magnetic confinement (p 1 - 2 - 3 - 4 )

The magnetic trap confining the particles is by no means perfect: despite the magnetic configuration, a slow drift leads particles from the plasma centre to the edge. The greater this transport phenomenon, the worse the confinement.

c) Particles and heat transport

Once a stable magnetic equilibrium has been established, we have seen that the particles, when they are considered individually, follow the magnetic field lines, if the Larmor radius and drift movements are neglected. However, they undergo other phenomena, which will change this simple image and result in rather more complex transport mechanisms, which may be divided into two major categories:

neoclassical transport: the effects of collisions between particles, which will make them deviate from their initial trajectory in a sort of random walk, giving rise to radial diffusion when they "jump" from one magnetic surface to another, because of the shock. This is called neoclassical transport.

We can then modify the simple image of the particle following its field line as on the diagram on the left by the image of a succession of jumps from one field line to another, as illustrated on the right hand diagram.

"anomalous" transport : the effects of turbulence, i.e. magnetic and electric fields fluctuations, give rise to the propagation of waves in the plasma. This results in an increase in the heat and particles transport. This transport called "anoumalous", has resulted in significant theoretical developments If linear theory is now well-established and helps forecast the conditions in which a wave becomes unstable, it is not valid when a wave increases, and we must then turn on to more complex non-linear models to simulate the development of the instability.

Heat transport is a phenomenon quite comparable to particle transport. First of all, diffusing particles carry their own energy : this is the phenomenon of convection. Then collisions allow particles to exchange energy: this is thermal conduction.

This diffusion phenomenon from inside plasma towards the outside tends to " empty " the plasma of its content in particles and energy, and determines the confinement machine performance. The diffusion of particles is characterised by a coefficient of proportionality, called diffusion coefficient, between the particles flow and the density gradient. Similarly, for heat, the diffusion coefficient is defined by the ratio between the heat flow and the temperature gradient. The larger this coefficient, the greater the diffusion, and the worse the confinement.

Experimentally, we observe much greater losses of energy (and therefore a much shorter time of confinement) than those predicted by neoclassical transport alone. Anoumalous transport would seem to be the dominant term. Many studies are underway to refine the comprehension of phenomena likely to explain this transport. In particular, we are trying to establish the dependence of the diffusion coefficient on the machine and plasma parameters . Two types of behaviour have been identified:

Bohm behaviour: the diffusion coefficient does not depend on the size of the machine (and thus nothing is gained by going up to a large scale reactor)
Gyrobohm behaviour: the diffusion coefficient depends favourably on the size of the machine (and therefore a considerable gain is achieved in going on to the reactor)

The trends arising from the latest studies are that behaviour is different according to the specie under consideration (electrons or ions) and confinement mode :

electrons are Gyrobohm-type whatever the mode of confinement
ions are Bohm-type in the normal confinement mode, and become Gyrobhom when put in the enhanced confinement mode.