Speaker
Description
For several decades, an enormous effort has been devoted to the gyrokinetic theory of instabilities and turbulence in stellarators and tokamaks. Thousands of papers have been published on this subject, and millions of lines of code have been written for the purpose of numerically solving gyrokinetic equations.
As a result of this effort, a great deal of knowledge about various microinstabilities has accumulated. Ion- and electron-temperature-gradient-driven modes, trapped-electron modes, kinetic ballooning modes and microtearing modes have, for instance, been found to be unstable and cause turbulence in tokamaks and stellarators. However, these instabilities tend to be sensitive to assumptions made about plasma parameters and the magnetic-field geometry. As a result, little is known in general about gyrokinetic microinstaiblities, despite the great effort devoted to their study.
Proceeding from thermodynamic considerations, we derive universal upper bounds on the growth rates of local gyrokinetic instabilities in any magnetised plasma, regardless of the geometry of the magnetic field, the number of particle species, beta, and collisions. A large number of results that have earlier been derived in special cases or observed in numerical simulations are thus brought into a unifying framework. Moreover, these upper bounds hold not only for linear instabilities but also for the nonlinear growth of free energy in a turbulent plasma.
The same theoretical framework can also be used to derive upper bounds on turbulent transport fluxes. Without additional assumptions about the nature of the turbulence, these bounds are unrealistically high, but can be lowered if such assumptions are made. The formalism may thus offer a useful technique for making concepts like critical balance quantitatively precise.