Browsing by Author "Groth, Mathias, Associate Prof., Aalto University, Department of Applied Physics, Finland"

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  • Modelling of global impurity transport in tokamaks in the presence of non-axisymmetric effects
    (2015) Miettunen, Juho
     School of Science | Doctoral dissertation (article-based)
    Understanding and control of plasma-surf ace interaction together with the subsequent migration of eroded impurity particles form one of the key challenges for reactor-scale magnetic confinement fusion devices such as tokamaks. An aspect of impurity migration that requires further investigation is the influence of non-axisymmetric (3D) features of tokamaks.  In this work, global impurity transport is simulated in the ASDEX Upgrade and JET tokamaks with the Monte Carlo orbit-following code ASCOT. The simulations address experiments conducted on these devices, using tracer impurities injected into the plasma in a gaseous form, and via special wall tiles installed into the torus. The presented simulations are carried out in a realistic tokamak environment with an unrestricted computational domain and employing 3D descriptions of the wall geometry and the magnetic field.  According to the ASCOT predictions, accounting for the non-axisymmetric wall geometry can be crucial in interpreting the experimentally measured tracer deposition profiles. In particular, studies of ASDEX Upgrade indicate that the impurity deposition pattern on plasma-facing surfaces can exhibit substantial asymmetry in the toroidal direction due to the three-dimensional nature of the wall geometry, in contrast to previous assumptions. For 3D features of the tokamak magnetic field, the simulations predict toroidal field ripple and resonant magnetic perturbations to induce local modifications into the deposition pattern.  The presented simulation results highlight the role of plasma conditions in determining the impurity birth profile and the ratio between main chamber and divertor deposition. Besides the divertor configuration, the main mode of operation in modern tokamaks, it is shown that also the limiter configuration can lead to long-range impurity transport within the torus.  The results of this work suggest that increased attention should be directed towards the non-axisymmetric features of tokamaks for a reliable assessment of impurity migration in next-step devices.