A Virtual Simons' Hour Talk: Upper Bounds on Gyrokinetic Instabilities

Given by: Per Helander
Max-Planck Institute for Plasma Physics

Note the time: 8:00am EDT / 12:00 UTC.  Zoom link will be sent via email.

Abstract: Energy confinement in stellarators is mostly limited by turbulence, in particular when the plasma temperature is low or  the neoclassical transport has been reduced by optimisation of the magnetic field. The turbulence and the underlying microinstabilities are thought to be well described by the gyrokinetic set of equations, which have been the subject of thousands of papers and millions of lines of computer code. Yet, little is known about the general properties of the solutions to these equations. Here, a family of rigorous upper bounds on the growth rate of local gyrokinetic instabilities is derived. These bounds hold for both electrostatic and electromagnetic instabilities, regardless of the number of particle species, their collision frequency, and the geometry of the magnetic field. A large number of results that have earlier been derived in special cases and observed in numerical simulations are thus brought into a unifying framework. These bounds apply not only to linear instabilities but also imply an upper limit to the nonlinear growth of the free energy.