Finalist EPFL doctorate Award 2018 - Fabio Riva

Understanding the plasma dynamics in tokamaks is of fundamental importance to reliably predict the performances of future fusion devices. Because of the complex phenomena at play, state-of-the-art simulation codes are needed to investigate plasma properties. The methodology used to assess the reliability of numerical codes constitutes the verification and validation (V&V) procedure. V&V is composed by three separate tasks: the code verification, targeted to assess that the physical model is correctly implemented in a simulation code; the solution verification, which evaluates the numerical errors affecting a simulation; and the validation, which determines the consistency of the code results with experiments.

This thesis focuses on developing and applying rigorous V&V procedures to plasma turbulence simulations, ultimately improving our understanding of the plasma dynamics. To perform a code verification, we propose to use the method of manufactured solutions, a methodology that we generalized to particle-in-cell (PIC) codes. The solution verification procedure we put forward is based on the Richardson extrapolation, used as higher order estimate of the exact solution. These procedures are successfully applied to GBS, a three-dimensional fluid code for plasma turbulence, and to a unidimensional PIC code. Finally, to increase the reliability of our plasma modelling, we carry out a set of validation exercises, comparing GBS simulations with experimental measurements taken on several plasma devices.