arXiv subjects

Stefano Coda

Publications and source records attributed to Stefano Coda.

5 recordsLinked to original sources

arXiv2025

On the interplay between plasma triangularity and micro-tearing turbulence

In this work, we study the interplay between triangularity and micro-tearing turbulence using linear and nonlinear flux tube GENE simulations. We consider scenarios with negative and positive triangularity plasma shaping taken from existing tokamaks (TCV, DIII-D, MAST-U and SMART) and EU-DEMO. The study of all these tokamaks reveals a coherent picture. Negative triangularity geometry is more susceptible to micro-tearing modes (MTM), which, when present, make transport much worse than in positive triangularity. At sufficiently large $\beta$ (the ratio of plasma pressure over magnetic pressure), magnetic shear and ratio of electron to ion temperature gradient, all the scenarios with negative triangularity are dominated by MTM turbulence. In contrast, the corresponding scenarios with positive triangularity remain dominated by electrostatic turbulence and MTMs are subdominant or stable. We observe that conventional tokamaks usually operate in a parameter space far away from the onset of this MTM-dominated regime in negative triangularity, thus preserving the beneficial effect of negative triangularity on turbulent transport. In contrast, spherical tokamaks operate close to this regime and may ultimately exhibit worse transport at negative triangularity than positive triangularity. We find that lowering the magnetic shear in spherical tokamaks can preserve the beneficial effect of negative triangularity on electrostatic turbulence and prevent strong MTM transport. Finally, linear and nonlinear simulations reveal the reason for stronger MTMs: the magnetic drifts are faster in the negative triangularity geometry.

physics.plasm-ph↗

arXiv2025

First experimental demonstration of plasma shape control in a tokamak through Model Predictive Control

In this work, a Model Predictive Controller (MPC) is proposed to control the plasma shape in the Tokamak \`a Configuration Variable (TCV). The proposed controller relies on models obtained by coupling linearized plasma response models, derived from the \texttt{fge} code of the Matlab EQuilibrium toolbox (MEQ) suite, with a state-space description of the core TCV magnetic control system. It optimizes the reference signals fed to this inner control loop in order to achieve the desired plasma shape while also enforcing constraints on the plant outputs. To this end, a suitable Quadratic Programming (QP) problem is formulated and solved in real-time. The effectiveness of the proposed controller is illustrated through a combination of simulations and experimental results. To the best of our knowledge, this is the first time that a plasma shape control solution based on MPC has been experimentally tested on a real tokamak.

physics.plasm-ph↗

arXiv2025

Survey of the Edge Radial Electric Field in L-mode TCV Plasmas using Doppler Backscattering

A Doppler backscattering (DBS) diagnostic has recently been installed on the Tokamak \`a Configuration Variable (TCV) to facilitate the study of edge turbulence and flow shear in a versatile experimental environment. The dual channel V-band DBS system is coupled to TCV's quasi-optical diagnostic launcher, providing access to the upper low-field side region of the plasma cross-section. Verifications of the DBS measurements are presented. The DBS equilibrium $v_\perp$ profiles are found to compare favorably with gas puff imaging (GPI) measurements and to the $E_r$ inferred from the radial force balance of the carbon impurity. The radial structure of the edge $E_r \times B$ equilibrium flow and its dependencies are investigated across a representative set of L-mode TCV discharges, by varying density, auxiliary heating and magnetic configuration.

physics.plasm-ph↗

arXiv2024

On the feasibility of Ohmically heated negative triangularity tokamak power plants

Negative triangularity tokamak plasmas feature naturally enhanced confinement in the so-called L-mode regime, irrespective of the power of external heating. This is in contrast to conventional scenarios, which require exceeding a given heating power threshold to induce a discrete transition to a regime of enhanced confinement called H-mode. H-mode is, however, subject to problematic instabilities and additionally suffers from confinement degradation with increasing external heating. Using simple zero dimensional power balance and standard empirical scaling laws for confinement, we analyze the impact of external heating on several different reactor-relevant devices (i.e. SPARC, MANTA, ITER and DEMO). We compare the nominal externally heated scenarios with corresponding negative triangularity cases without external heating. For devices with sufficiently high magnetic field and/or fusion gain, the internally (Ohmically) heated negative triangularity versions achieve better performance. We conclude that Ohmically heating a negative triangularity power plant is an attractive option meriting further investigation.

physics.plasm-ph↗

arXiv2024

Physical insights from the aspect ratio dependence of turbulence in negative triangularity plasmas

In this work, we study the impact of aspect ratio A = R0 /r (the ratio of major radius R0 to minor radius r) on the confinement benefits of Negative Triangularity (NT) plasma shaping. We use high-fidelity flux tube gyrokinetic GENE simulations and consider several different scenarios: four of them inspired by TCV experimental data, a scenario inspired by DIII-D experimental data and a scenario expected in the new SMART spherical tokamak. The present study reveals a surprising and non-trivial dependence. NT improves confinement at any value of A for ITG turbulence, while for TEM turbulence confinement is improved only in the case of large and conventional aspect ratios. Additionally, through a detailed study of a large aspect ratio case with pure ITG drive, we develop an intuitive physical picture that explains the beneficial effect of NT at large and conventional aspect ratios. This picture does not hold in TEM-dominated regimes, where a complex synergistic effect of many factors is found. Finally, we performed the first linear gyrokinetic simulations of SMART, finding that both NT and PT scenarios are dominated by micro-tearing-mode (MTM) turbulence and that NT is more susceptible to MTMs at tight aspect ratio. However, we found that a regime where ITG dominates in SMART can be found, and in this regime NT is more linearly stable.

physics.plasm-ph↗