Speaker: Per Helander, Max Plank Institue for Plasma Physics
Abstract: In this talk, the energy budget of a collisionless plasma subject to electrostatic fluctuations is studied. In particular, the excess of thermal energy over the minimum accessible to it under various constraints that limit the possible forms of plasma motion is considered. This excess measures how much thermal energy is “available” for conversion into plasma instabilities, and therefore constitutes a nonlinear measure of plasma stability. The “available energy” defined in this way becomes an interesting and useful quantity in situations where adiabatic invariants impose non-trivial constraints on the plasma motion. For instance, microstability properties of certain stellarators can be inferred directly from the available energy, without the need to solve the gyrokinetic equation. The technique also suggests that an electron-positron plasma confined by a dipole magnetic field could be entirely free from turbulence.