Towards realistic orbit-following simulations of fast ions in ITER

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Antti Snicker Name of the doctoral dissertation Towards realistic orbit-following simulations of fast ions in ITER Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 199/2014 Field of research Fusion science Manuscript submitted 22 September 2014 Date of the defence 9 December 2014 Permission to publish granted (date) 17 November 2014 Language English Monograph Article dissertation (summary + original articles) Abstract One of the main scientific goals of the international ITER experiment is to provide understanding of burning plasmas, including the behavior of fusion-born alpha particles. These particles form both a potential risk for the first wall and a massive source of free energy in the plasma. Such free energy can drive a multitude of MHD modes, most notably the Alfvenic ones, that can lead to increased transport and even losses of fast ions.One of the main scientific goals of the international ITER experiment is to provide understanding of burning plasmas, including the behavior of fusion-born alpha particles. These particles form both a potential risk for the first wall and a massive source of free energy in the plasma. Such free energy can drive a multitude of MHD modes, most notably the Alfvenic ones, that can lead to increased transport and even losses of fast ions. In this work, the alpha particle physics has been studied using kinetic orbit-following Monte Carlo code ASCOT. The code was enhanced with two new physics models. The first model relaxes the usual guiding center (GC) approximation used to save computation time. In some cases, this approximation is not valid and the full gyro motion (FO) has to be resolved. The second model is for fast ion relevant MHD modes and its implementation allows taking into account electromagnetic fields due to these modes. When the MHD model was used to simulate ITER plasmas, the wall power loads due to fast particles were not found to exceed the design limits of the wall materials even for unrealistically large perturbations. However, redistribution of fast ions was observed to alter the alpha particle heating profile and neutral beam ion (NBI) driven current profile. Fusion alphas were simulated for the ITER 15 MA scenario using different integration methods. Following the full gyro motion gave slightly larger alpha particle wall power loads than the GC method. Since the FO method uses more than 50 times more CPU than GC integration, a third method was introduced as a compromise between the speed and accuracy: the GC method is used in the plasma core and FO integration is activated in the vicinity of the wall. Finally, alpha-driven current and torque in ITER were studied using different magnetic field configurations. It was found that, independent of the magnetic configurations, the alpha-driven current is less than 1% of the total plasma current for both 9 MA and 15 MA baseline scenarios. On the contrary, the alpha-driven torque depends on the magnetic field configuration. While in the axisymmetric case the total torque was found to be close to zero, with realistic 3D effects the alpha particles produced substantial torque, about one tenth of that driven by the NBI particles, but in direction opposite to it.