Institute for Plasma Physics
Thursday, February 4, 2021
Abstract: Two interesting cases of the impact of specific particle sources on the turbulence and transport in tokamak plasmas are considered with a combination of experiments at ASDEX Upgrade and of dedicated simulations with gyrokinetic and gyrofluid models and linked by the transient development of hollow density profiles.
The first problem  is related to the type of turbulence and consequent transport which can be destabilized by transient plasma profile modifications produced by the ablation of relatively large fueling pellets. It is shown that the transiently strongly hollow electron density profiles, which are measured at ASDEX Upgrade inside the pellet deposition region, can drive a newly identified density gradient driven turbulence in linear and nonlinear gyrokinetic simulations. This is mainly connected with the non-adiabatic response of passing electrons, is destabilized by increasing collisionality and predominantly located on the high field side, producing an electron inward diffusion which is large enough to be experimentally relevant. In contrast, outside the pellet deposition region, the strong density gradients drive usual density gradient driven trapped electron mode turbulence which is destabilized with decreasing collisionality. Implications for a reactor are briefly discussed.
The second problem  is dedicated to the physics of cold pulses, in particular produced by laser ablation of impurities. Experiments at ASDEX Upgrade explore conditions of both dominant electron and ion heating at low to intermediate density. The fast propagation of the peripheral cold pulse producing a central increase of the electron temperature in response to the peripheral cooling is only observed in low density electron heated conditions. Modelling with the quasi-linear turbulent transport model TGLF combined to the ASTRA and STRAHL transport codes shows that, consistent with modelling results obtained on C-Mod  and DIII-D  experiments, the fast increase of the central electron temperature in response to the peripheral cooling is produced by the sudden reduction of the density gradient in the plasma confinement region. This flattening stabilizes the trapped electron mode turbulence, which is present in conditions of electron heating and large electron to ion temperature ratio. The ASTRA-TGLF modelling also predicts the evolution of the density profiles of the ablated impurity and of the main plasma. It shows that an extremely fast flattening of the electron density profile is the consequence of the sudden inward diffusion of the particles, transiently produced by an impurity gradient driven mode, destabilized by the peripheral extremely hollow impurity density profile, caused by the laser ablation. These results show that the claimed non-local electron heat transport effect observed as a consequence of cold pulses can be completely explained by a local model, where the multi-channel nature of the turbulence, directly connecting heat and particle transport, can produce very fast dynamical behaviors of the plasma.
 C. Angioni et al Nucl. Fusion 57, 116053 (2017)
 C. Angioni et al Nucl. Fusion 59, 106007 (2019)
 P. Rodriguez-Fernandez et al Phys. Rev. Letters 120, 075001 (2018)
 P. Rodriguez-Fernandez et al Phys. Plasmas 26, 062503 (2019)
Bio: After a PhD at the SPC (EPFL) in Lausanne (Switzerland), completed at the end of 2001 and dedicated to neoclassical, turbulent and MHD modelling aspects in tokamaks, I moved to IPP Garching (Germany) as EURATOM Marie-Curie fellow for two years, during which I performed studies dedicated to turbulent particle transport. I received a permanent position in the tokamak theory department at IPP in 2004, where I continued the research on transport and confinement in tokamaks, with a particular interest on particle and impurities. Since 2007 I am a group leader, from 2012 with a W2 position of the Max-Planck-Society, after having received my habilitation at the University of Aix and Marseille (France) in 2010. I moved to the Tokamak Scenario Development department at IPP Garching in 2017, where I presently lead the Transport Group. Among other scientific activities and tasks, since 2005 I have actively participated to the ITPA in the Transport and Confinement topical group. I am first author of 32 refereed papers, and co-author of more than 200 papers. I have obtained particularly important results on the role of turbulence in producing peaked plasma density profiles, on the properties of turbulent impurity and toroidal momentum transport, and on the impact that poloidal asymmetries can have in determining central accumulation of heavy impurities.