We report on the implementation of diverted magnetic equilibria in GBS and on first simulations in this geometry. GBS is a simulation code used to evolve plasma turbulence in the tokamak periphery by solving the drift-reduced Braginskii's equations. The model equations are written in toroidal coordinates, abandoning flux coordinate systems that are not defined at the X-point. A fourth order finite difference scheme is used for the implementation of the spatial operators on poloidally and toroidally staggered grids. The GBS numerical implementation is verified through the method of manufactured solutions. The code convergence properties are tested on a relatively simple analytical X-point configuration. Finally, the diverted equilibrium from a TCV tokamak discharge is implemented in the new version of GBS. The analysis of the simulation results is focused on blob formation, radial transport, and plasma poloidal rotation mechanisms.
Mechanisms setting the density decay in the scrape-off layer (SOL) at the outer midplane of a tokamak plasma are disentangled using two-fluid numerical simulations in a double-null magnetic configuration and analytical estimates. Typical experimental observations are retrieved, in particular increasing intermittency of the turbulence going from the near to the far SOL, which is reflected in two different density decay lengths. The decay length of the near SOL is well described as the result of transport driven by a nonlinearly saturated ballooning instability, while in the far SOL, the density decay length is described using a model of intermittent transport mediated by blobs. The analytical estimates of the decay lengths agree well with the simulation results and typical experimental values and can therefore be used to guide tokamak design and operation.
The present work uses the results of a fluid full-turbulence 3D simulation of the tokamak periphery to present the first self-consistent analysis of the radial velocity scaling of plasma blobs in a diverted geometry. A diverted double-null configuration is considered, and the blob motion is studied using a pattern recognition algorithm. The velocity obtained from the simulation results is compared to an analytical scaling accounting for the presence of the X-point. Agreement is found between numerical and analytical results.
A methodology to perform a rigorous verification of Particle-in-Cell (PIC) simulations is presented, both for assessing the correct implementation of the model equations (code verification), and evaluating the numerical uncertainty affecting the simulation results (solution verification).The proposed code verification methodology is a generalization of the procedure developed for plasma simulation codes based on finite difference schemes that was described by Riva et al.
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