Fluid moments and spectral diagnostics in global particle-in-cell simulations

The convergence properties of global electrostatic particle-in-cell gyrokinetic simulations have been studied by means of a set of new 3D diagnostics recently implemented in the code ORB5, allowing for measurements of electromagnetic potentials and relevant fluid quantities (density, temperature, vorticity) as well as turbulence spectral analysis. For all the quantities considered, non converged simulations show flatter spectra. However, not all the physically relevant quantities converge with the same rate: density fluctuation spectra can still show an unphysical flattening of the spectrum at high k(theta)rho(s), even when electrostatic potential and heat fluxes appear to be converged. A remarkable results is that resolution in terms of number of markers, required to correctly describe the turbulence density fluctuations, is found to be size-dependent: larger systems (ITER like) require less numerical particles per active mode than smaller systems.