Gyrokinetic modeling: A multi-water-bag approach

Predicting turbulent transport in nearly collisionless fusion plasmas requires one to solve kinetic or, more precisely, gyrokinetic equations. In spite of considerable progress, several pending issues remain; although more accurate, the kinetic calculation of turbulent transport is much more demanding in computer resources than fluid simulations. An alternative approach is based on a water-bag representation of the distribution function that is not an approximation but rather a special class of initial conditions, allowing one to reduce the full kinetic Vlasov equation into a set of hydrodynamic equations while keeping its kinetic character. The main result for the water-bag model is a lower cost in the parallel velocity direction since no differential operator associated with some approximate numerical scheme has to be carried out on this variable v . Indeed, a small bag number is sufficient to correctly describe the ion temperature gradient instability. © 2007 American Institute of Physics. DOI: 10.1063/1.2804079