| |
|
|
|
|
Turbulence phenomena in plasmas
We have seen that the proper working of a thermonuclear reactor requires that the plasma interior be very hot and dense (see Lawson criterion). Strong gradients (the difference between the centre and the edge) of temperature and density are thus developed. We can show theoretically and experimentally that by increasing the gradients, we increase the level of the electromagnetic field fluctuations way beyond usual the values that one would observe if the plasma were in thermal equilibrium. This is a very common phenomenon, which one can observe in fluids in everyday life. If we start to heat water on a stove, we can see the convective motion that appears shortly afterwards. By increasing the heat, this motion becomes gradually more and more violent and irregular. We talk in terms of turbulence. The effect of turbulence, in plasmas as well as in water, is the increase of matter and energy transport. In the operating conditions of the tokamaks, the combined effect of electric and magnetic fields fluctuations is the cause of "anomalous" transport. The fluctuating electric field gives rise to amplification of particle movement across field lines. The magnetic fluctuations cause distortion in the field lines. The energy and matter losses are associated with particles, which move freely along field lines. The films opposite clearly show fluctuations in the electric field obtained by numerically solving the equations governing turbulent dynamics. We can clearly observe in frame (a) the continuous formation and destruction of vortexes. Frame (b) shows the corresponding level of temperature. You can see the plumes of hot plasma coming out and cold plumes going in. This is an example of the mechanism of abnormal transport. The use of very powerful computers to solve plasma dynamics equations is a very active domain of research, with applications in meteorology for example.
|
|
Watch the film "Density
e
