Federico Stefanelli

Flux pumping was achieved in recent hybrid scenario experiments in the ASDEX Upgrade (AUG) tokamak, which is characterized by a sawtooth-free helical quiescent state and the anomalous radial redistribution of toroidal current density and poloidal magnetic flux. In this article, the self-regulation mechanism of the AUG core plasma during flux pumping is investigated at realistic parameters using the JOREK code based on the two-temperature, nonlinear, full magnetohydrodynamic (MHD) model. A key milestone in AUG flux pumping modelling is achieved by quantitatively reproducing the clamped current density and safety factor profiles in the plasma core, demonstrating the effectiveness of the dynamo effect in sustaining the flux pumping state. The dynamo term, which is of particular interest, is primarily generated by the pressure-gradient-driven m/n = 1/1 quasi-interchange-like MHD instability. The work systematically extrapolates the parameter regimes of flux pumping from the above AUG base case by scanning dissipation coefficients and plasma beta. The simulation results reveal bifurcated plasma behaviours at different Hartmann numbers, including distinct states such as flux pumping (helical core with a flat current density), sawteeth (periodic kink-cycling), single crash (without subsequent cycle), and quasi-stationary magnetic island (peaked current density). Transitions from marginal flux-pumping state to sawteeth in long-term simulations are observed and analyzed. The relationships between system dissipation, plasma beta, and different plasma states are carefully analyzed. For practical purposes, the potential operational window for flux pumping, as determined by plasma density and temperature, is estimated. The modelling efforts advance the understanding of flux pumping and facilitate the development of a fast surrogate model for efficient evaluation of flux pumping.

An integrated model of sawtooth cycles is presented, which includes also the effects of fishbone oscillations in between the sawtooth core collapses. All phases of a sawtooth cycle are modelled, with the onset predicted by the Porcelli model, the post-crash current and q profiles with the Flat Current model, and the recovery phase with TGLF and NCLASS. It will be shown that TGLF significantly overestimates turbulent transport throughout the whole sawtooth period, and an additional turbulence stabilisation mechanism, which could be driven by fast particle effects, has to be invoked to recover the correct fluxes. The collapse of the temperature and density profiles is obtained with MHD-driven anomalous diffusivity applied within the q = 1 surface at the crash time. The redistribution of fast particles and rotation is modelled as well. The fishbone onset criterion and induced radial transport and radial electric field are also modelled. Notably, the fishbone-induced radial electric field predicted by reduced models is found to have little effects on core turbulence. The results are validated on AUG discharges in presence of neutral beam injection and electron cyclotron heating schemes.

The extended MHD code CASTOR3D is used to perform the MHD stability analysis of predictive pedestal scans of standard ASDEX Upgrade and JET-ILW scenarios. Comparing stability calculations considering, respectively, only ideal MHD, resistive MHD, ideal MHD with gyroviscosity and resistive MHD with gyroviscosity highlights the stabilizing and destabilizing mechanisms of the additional physics. Resistivity is found to be generally destabilizing, even to the lowest pedestal top pressure values considered. Stability calculations considering only gyroviscosity in addition to ideal MHD show a strong stabilizing effect in both devices and across a wide density range. The combination of resistivity and gyroviscosity shows a destabilizing trend with increasing plasma density, consistent with experimental observations. The destabilizing effect of resistivity in these scans has been attributed to increased degrees of freedom of the perturbation, particularly close to the separatrix, allowing the instability to minimize the stabilizing effect of shear Alfvén waves. Finally, the destabilizing effects of both an increasing relative shift between the density and temperature pedestals and increasing separatrix density have been attributed to a combination of the increased instability drive due to resistivity and a reduction of gyroviscous stabilization at high density, particularly close to the separatrix.

Accurately predicting the performance of fusion reactors like ITER is challenging due to uncertainties in scaling laws and limited validation of integrated models. This work presents numerical plasma size scans using the Integrated Model based on Engineering Parameters (IMEP) to investigate its capability to correctly capture the effect of the plasma size on pedestal and global confinement. The scans reveal that IMEP predictions closely align with IPED and the IPB98(y,2) scaling law, and suggest more optimistic confinement trends than the ITPA20-IL scaling law. Simulations of the ITER 15 MA baseline are presented, where IMEP predicts a fusion power of 600 MW and a gain of Q = 12, with pedestal pressures comparable to EPED predictions. The analysis highlights the impact of fueling methods, separatrix density, heating power, and rotation on confinement and performance, emphasizing the need for robust models to account for pedestal and core dynamics. These findings enhance confidence in IMEP predictions for ITER and future reactors, demonstrating its efficacy in investigating the impact of various factors on fusion performance, pedestal and global confinement.