arXiv subjects

A. Di Siena

Publications and source records attributed to A. Di Siena.

At least 19 recordsLinked to original sources

arXiv2025

Study of the impact of fast ions on core turbulence at rational surfaces via global gyrokinetic simulations

In this work, the interplay between fast ions and safety factor rational surfaces is studied in a turbulent plasma via global nonlinear gyrokinetic simulations. Initially, the fast particles-induced enhancement of shearing structures from turbulence self-interaction is analyzed. Our study takes into account the competition between this mechanism and other fast ions effects, i.e. thermal profiles dilution and quasi-resonant interaction. We find the fast ions-induced reduction of destabilization threshold for the zonal modes to be a very efficient way to suppress turbulence. Indeed, it leads to the formation of regions where turbulent transport is reduced by 90\% of its original value. Furthermore, an $n=m=1$ fishbone is driven unstable inside the plasma and its interaction with turbulence is studied. We find the beat-driven zonal structure generate by this mode to further reduce turbulence when its presence does not drastically flatten the thermal profiles.

physics.plasm-ph↗

arXiv2025

First global gyrokinetic profile predictions of ITER burning plasma

In this work, we present the first global gyrokinetic simulations of the ITER baseline scenario operating at 15 MA using GENE-Tango electrostatic and electromagnetic simulations. The modeled radial region spans close to the magnetic axis up to rho_tor = 0.6. Our results show a pronounced density peaking, moderated by electromagnetic fluctuations. The predicted fusion gain for this scenario is Q = 12.2, aligning well with ITER's mission objectives. We further characterize the turbulence spectra and find that electromagnetic modes, such as microtearing modes, kinetic ballooning modes, and Alfvenic ion temperature gradient modes at low binormal wave numbers, play a critical role in the core transport of this ITER scenario, necessitating high numerical resolution for accurate modeling. Local flux-tube simulations qualitatively reproduce the key features observed in the global gyrokinetic simulations but exhibit a much higher sensitivity to profile gradients, reflecting increased stiffness, likely due to the linearization of the equilibrium profiles and safety factor. Our study also reveals that the imposed external toroidal rotation profiles have a negligible impact on turbulent transport, as their magnitudes are substantially lower than the dominant linear growth rates. Furthermore, we demonstrate that the safety factor profile is of paramount importance: scenarios featuring flat q profiles with near-zero magnetic shear lead to the destabilization of kinetic ballooning modes in the plasma core, significantly enhancing turbulent transport and potentially degrading confinement. Finally, although electron temperature gradient turbulence initially appears large, sometimes exceeding ion-scale transport levels, it is ultimately quenched over long timescales by secular evolution of zonal flows, which are weakly damped under the very low collisionality conditions expected in ITER.

physics.plasm-ph↗

arXiv2025

Optimizing Particle Transport for Enhanced Confinement in Quasi-Isodynamic Stellarators

Despite significant advances in reducing turbulent heat losses, modern quasi-isodynamic (QI) stellarators -- such as Stellaris -- continue to suffer from poor particle confinement, which fundamentally limits their overall performance. Using gyrokinetic simulations within the GENE--Tango framework, we identify suppressed inward thermodiffusion, caused by unfavorable magnetic geometry, as the primary cause. To overcome this limitation, we design a new configuration with a reduced mirror ratio, which enhances the contribution of passing electrons to the inward particle flux. This facilitates the formation of strongly peaked density profiles, suppresses turbulence, and leads to a substantial improvement in confinement. Our optimized configuration achieves nearly a twofold increase in energy confinement compared to Stellaris, highlighting the crucial role of optimizing particle transport in next-generation stellarator designs.

physics.plasm-ph↗

arXiv2025

Validation of a Comprehensive First-Principles-Based Framework for Predicting the Performance of Future Stellarators

This paper presents the validation of the $\texttt{GENE-KNOSOS-Tango}$ framework for recovering both the steady-state plasma profiles in the considered radial domain and selected turbulence trends in a stellarator. This framework couples the gyrokinetic turbulence code $\texttt{GENE}$, the neoclassical transport code $\texttt{KNOSOS}$, and the transport solver $\texttt{Tango}$ in a multi-timescale simulation feedback loop. Ion-scale kinetic-electron and electron-scale adiabatic-ion flux-tube simulations were performed to evolve the density and temperature profiles for four OP1.2b W7-X scenarios. The simulated density and temperature profiles showed good agreement with the experimental data using a reasonable set of boundary conditions. Equally important was the reproduction of observed trends for several turbulence properties, such as density fluctuations and turbulent heat diffusivities. Key effects were also touched upon, such as electron-scale turbulence and the neoclassical radial electric field shear. The validation of the $\texttt{GENE-KNOSOS-Tango}$ framework enables credible predictions of physical phenomena in stellarators and reactor performance based on a given set of edge parameters.

physics.plasm-ph↗

arXiv2024

Assessing the Impact of Alpha Particles on Thermal Confinement in JET D-T Plasmas through Global GENE-Tango Simulations

The capability of the global, electromagnetic gyrokinetic GENE code interfaced with the transport Tango solver is exploited to address the impact of fusion alpha particles (in their dual role of fast particles and heating source) on plasma profiles and performance at JET in the discharges with the highest quasi-stationary peak fusion power during the DTE2 experimental campaigns. Employing radially global nonlinear electromagnetic GENE-Tango simulations, we compare results with/without alpha particles and alpha heating. Our findings reveal that alpha particles have a negligible impact on turbulent transport, with GENE-Tango converging to similar plasma profiles regardless of their inclusion as a kinetic species in GENE. On the other hand, alpha heating is found to contribute to the peaking of the electron temperature profiles, leading to a 1keV drop on the on-axis electron temperature when alpha heating is neglected in Tango. The minimal impact of alpha particles on turbulent transport in this JET discharge - despite this being the shot with the highest fusion output - is attributed to the low content of fusion alpha in this discharge. To assess the potential impact of alpha particles on turbulent transport in regimes with higher alpha particle density, as expected in ITER and fusion reactors, we artificially increased the alpha particle concentration to levels expected for ITER. By performing global nonlinear GENE standalone simulations, we found that increasing the alpha particle density beyond five times the nominal value lead to significant overall turbulence destabilization.

physics.plasm-ph↗

arXiv2023

Assessing global ion thermal confinement in critical-gradient-optimized stellarators

We investigate the confinement properties of two recently devised quasi-helically symmetric stellarator configurations, HSK and QSTK. Both have been optimized for large critical gradients of the ion temperature gradient mode, which is an important driver of turbulent transport in magnetic confinement fusion devices. To predict the resulting core plasma profiles, we utilize an advanced theoretical framework based on the gyrokinetic codes GENE and GENE-3D, coupled to the transport code TANGO. Compared to the HSX stellarator, both HSK and QSTK achieve significantly higher core-to-edge temperature ratios, partly thanks to their smaller aspect ratios, with the other part due to more detailed shaping of the magnetic geometry achieved during optimization. The computed confinement time, however, is less sensitive to core temperature than edge temperature, simply due to the disproportionate influence the edge has on stored plasma energy. We therefore emphasize the possible benefits of further optimizing turbulence in the outer core region, and the need to include accurate modelling of confinement in the edge region in order to assess overall plasma performance of turbulence optimized stellarators.

physics.plasm-ph↗

arXiv2023

Global Linear and Nonlinear Gyrokinetic Simulations of Tearing Modes

To better understand the interaction of global tearing modes and microturbulence in the Madison Symmetric Torus (MST) reversed-field pinch (RFP), the global gyrokinetic code \textsc{Gene} is modified to describe global tearing mode instability via a shifted Maxwellian distribution consistent with experimental equilibria. The implementation of the shifted Maxwellian is tested and benchmarked by comparisons with different codes and models. Good agreement is obtained in code-code and code-theory comparisons. Linear stability of tearing modes of a non-reversed MST discharge is studied. A collisionality scan is performed to the lowest order unstable modes ($n=5$, $n=6$) and shown to behave consistently with theoretical scaling. The nonlinear evolution is simulated, and saturation is found to arise from mode coupling and transfer of energy from the most unstable tearing mode to small-scale stable modes mediated by the $m=2$ tearing mode. The work described herein lays the foundation for nonlinear simulation and analysis of the interaction of tearing modes and gyroradius-scale instabilities in RFP plasmas.

physics.plasm-ph↗

arXiv2023

How accurate are flux-tube (local) gyrokinetic codes in modeling energetic particle effects on core turbulence?

Flux-tube gyrokinetic codes are widely used to simulate drift-wave turbulence in magnetic confinement devices. While a large number of studies show that flux-tube codes provide an excellent approximation for turbulent transport in medium-large devices, it still needs to be determined whether they are sufficient for modeling supra-thermal particle effects on core turbulence. This is called into question given the large temperature of energetic particles (EPs), which makes them hardly confined on a single flux-surface, but also due to the radially broad mode structure of energetic-particle-driven modes. The primary focus of this manuscript is to assess the range of validity of flux-tube codes in modeling fast ion effects by comparing radially global turbulence simulations with flux-tube results at different radial locations for realistic JET parameters using the gyrokinetic code GENE. To extend our study to a broad range of different plasma scenarios, this comparison is made for four different plasma regimes, which differ only by the profile of the ratio between the plasma kinetic and magnetic pressure. The latter is artificially rescaled to address the electrostatic limit and regimes with marginally stable, weakly unstable and strongly unstable fast ion modes. These energetic-particle-driven modes is identified as an AITG/KBAE via linear ORB5 and LIGKA simulations. It is found that the local flux-tube simulations can recover well the global results only in the electrostatic and marginally stable cases. When the AITG/KBAE becomes linearly unstable, the local approximation fails to correctly model the radially broad fast ion mode structure and the consequent global zonal patterns. According to this study, global turbulence simulations are likely required in regimes with linearly unstable AITG/KBAEs. In conditions with different fast ion-driven modes, these results might change.

physics.plasm-ph↗

arXiv2023

First-principles based plasma profile predictions for optimized stellarators

In the present Letter, first-of-its-kind computer simulations predicting plasma profiles for modern optimized stellarators -- while self-consistently retaining neoclassical transport, turbulent transport with 3D effects, and external physical sources -- are presented. These simulations exploit a newly developed coupling framework involving the global gyrokinetic turbulence code GENE-3D, the neoclassical transport code KNOSOS, and the 1D transport solver TANGO. This framework is used to analyze the recently observed degradation of energy confinement in electron-heated plasmas in the Wendelstein 7-X stellarator, where the central ion temperature was "clamped" to $T_i \approx 1.5$ keV regardless of the external heating power. By performing first-principles based simulations, we provide key evidence to understand this effect, namely the inefficient thermal coupling between electrons and ions in a turbulence-dominated regime, which is exacerbated by the large $T_e/T_i$ ratios, and show that a more efficient ion heat source, such as direct ion heating, will increase the on-axis ion temperature. This work paves the way towards the use of high-fidelity models for the development of the next generation of stellarators, in which neoclassical and turbulent transport are optimized simultaneously.

physics.plasm-ph↗

arXiv2022

Predictions of improved confinement in SPARC via energetic particle turbulence stabilization

The recent progress in high-temperature superconductor technologies has led to the design and construction of SPARC, a compact tokamak device expected to reach plasma breakeven with up to $25$MW of external ion cyclotron resonant heating (ICRH) power. This manuscript presents local (flux-tube) and radially global gyrokinetic GENE (Jenko et al 2000 Phys. Plasmas {\bf 7} 1904) simulations for a reduced-field and current H-mode SPARC scenario showing that supra-thermal particles - generated via ICRH - strongly suppress ion-scale turbulent transport by triggering a fast ion-induced anomalous transport barrier (F-ATB). The trigger mechanism is identified as a wave-particle resonant interaction between the fast particle population and plasma micro-instabilities (Di Siena et al 2021 Phys. Rev. Lett. {\bf 125} 025002). By performing a series of global simulations employing different profiles for the thermal ions, we show that the fusion gain of this SPARC scenario could be substantially enhanced up to $\sim 80\%$ by exploiting this fast ion stabilizing mechanism. A study is also presented to further optimize the energetic particle profiles, thus possibly leading experimentally to an even more significant fusion gain.

physics.plasm-ph↗

arXiv2022

Global gyrokinetic simulations of ASDEX Upgrade up to the transport time-scale with GENE-Tango

An accurate description of turbulence up to the transport time scale is essential for predicting core plasma profiles and enabling reliable calculations for designing advanced scenarios and future devices. Here, we exploit the gap separation between turbulence and transport time scales and couple the global gyrokinetic code GENE to the transport-solver Tango, including kinetic electrons, collisions, realistic geometries, toroidal rotation and electromagnetic effects for the first time. This approach overcomes gyrokinetic codes' limitations and enables high-fidelity profile calculations in experimentally relevant plasma conditions, significantly reducing the computational cost. We present numerical results of GENE-Tango for two ASDEX Upgrade discharges, one of which exhibits a pronounced peaking of the ion temperature profile not reproduced by TGLF-ASTRA. We show that GENE-Tango can correctly capture the ion temperature peaking observed in the experiment. By retaining different physical effects in the GENE simulations, e.g., collisions, toroidal rotation and electromagnetic effects, we demonstrate that the ion temperature profile's peaking is due to electromagnetic effects of submarginal MHD instability. Based on these results, the expected GENE-Tango speedup for the ITER standard scenario is larger than two orders of magnitude compared to a single gyrokinetic simulation up to the transport time scale, possibly making first-principles ITER simulations feasible on current computing resources.

physics.plasm-ph↗

arXiv2021

Core transport barriers induced by fast ions in global gyrokinetic GENE simulations

A novel type of internal transport barrier (ITB) called F-ATB (fast ion-induced anomalous transport barrier) has been recently observed in state-of-the-art global gyrokinetic simulations on a properly optimized ASDEX Upgrade experiment [A. Di Siena et al. Phys. Rev. Lett. {\bf 127} 025002 (2021)]. Unlike the transport barriers previously reported in literature, the trigger mechanism for the F-ATB is a basically electrostatic wave-particle resonant interaction between supra-thermal particles - generated via ion cyclotron resonance heating (ICRH) - and ion scale plasma turbulence. This resonant effect strongly depends on the particular shape of the fast ion temperature and density profiles. Therefore, to further improve our theoretical understanding of this transport barrier, we present results exploring the parameter space and physical conditions for the F-ATB generation by performing a systematic study with global GENE simulations. Particular emphasis is given to the transport barrier width and its localization by scanning over different energetic particle temperature profiles. The latter are varied in amplitude, half-width, and radial localization of an ad-hoc Gaussian-like energetic particle logarithmic temperature gradient profile. For the reference parameters at hand, a threshold in the amplitude of the fast ion logarithmic temperature gradient is identified to trigger the transport barrier effectively.

physics.plasm-ph↗

arXiv2021

New high-confinement regime with fast ions in the core of fusion plasmas

The key result of the present work is the theoretical prediction and observation of the formation of a new type of transport barrier in fusion plasmas, called F-ATB (fast ion-induced anomalous transport barrier). As demonstrated through state-of-the-art global electrostatic and electromagnetic simulations, the F-ATB is characterized by a full suppression of the turbulent transport - caused by strongly sheared, axisymmetric $E \times B$ flows - and an increase of the neoclassical counterpart, albeit keeping the overall fluxes at significantly reduced levels. The trigger mechanism is shown to be a mainly electrostatic resonant interaction between supra-thermal particles, generated via ion-cyclotron-resonance heating, and plasma micro-turbulence. These findings are obtained by realistic simulations of the ASDEX Upgrade discharge $\#36637$ - properly designed to maximized the beneficial role of the wave-particle resonance interaction - which exhibits the expected properties of improved confinement produced by energetic particles.

physics.plasm-ph↗

arXiv2020

Gyrokinetic investigation of Alfvén instabilities in the presence of turbulence

The nonlinear dynamics of beta-induced Alfvén Eigenmodes (BAE) driven by energetic particles (EP) in the presence of ion-temperature-gradient (ITG) turbulence is investigated, by means of selfconsistent global gyrokinetic simulations and analytical theory. A tokamak magnetic equilibrium with large aspect ratio and reversed shear is considered. A previous study of this configuration has shown that the electron species plays an important role in determining the nonlinear saturation level of a BAE in the absence of turbulence [A. Biancalani, et al., J. Plasma Phys. (2020)]. Here, we extend the study to a turbulent plasma. The EPs are found modify the heat fluxes by introducing energy at the large spatial scales, mainly at the toroidal mode number of the dominant BAE and its harmonics. In this regime, BAEs are found to carry a strong electron heat flux. The feed-back of the global relaxation of the temperature profiles induced by the BAE, and on the turbulence dynamics, is also discussed.

physics.plasm-ph↗

arXiv2020

Global gyrokinetic simulations of ITG turbulence in the configuration space of the Wendelstein 7-X stellarator

We study the effect of turbulent transport in different magnetic configurations of the Weldenstein 7-X stellarator. In particular, we performed direct numerical simulations with the global gyrokinetic code GENE-3D, modeling the behavior of Ion Temperature Gradient turbulence in the Standard, High-Mirror, and Low-Mirror configurations of W7-X. We found that the Low-Mirror configuration produces more transport than both the High-Mirror and the Standard configurations. By comparison with radially local simulations, we have demonstrated the importance of performing global nonlinear simulations to predict the turbulent fluxes quantitatively.

physics.plasm-ph↗

arXiv2020

Nonlinear dynamics of energetic-particle driven geodesic acoustic modes in ASDEX Upgrade

Turbulence in tokamaks generates radially sheared zonal flows. Their oscillatory counterparts, geodesic acoustic modes (GAMs), appear due to the action of the magnetic field curvature. The GAMs can be driven unstable by an anisotropic energetic particle (EP) population leading to the formation of global radial structures, called EGAMs. The EGAMs can redistribute EP energy to the bulk plasma through collisionless wave-particle interaction. In such a way, the EGAMs might contribute to the plasma heating. Thus, investigation of EGAM properties, especially in the velocity space, is necessary for precise understanding of the transport phenomena in tokamak plasmas. In this work, the nonlinear dynamics of EGAMs without considering the mode interaction with the turbulence is investigated with the help of a Mode-Particle-Resonance (MPR) diagnostic implemented in the global gyrokinetic particle-in-cell code ORB5. An ASDEX Upgrade discharge is chosen as a reference case for this investigation due to its rich EP nonlinear dynamics. An experimentally relevant magnetic field configuration, thermal species profiles and an EP density profile are taken for EGAM chirping modelling and its comparison with available empirical data. The same magnetic configuration is used to explore energy transfer by the mode from the energetic particles to the thermal plasma including kinetic electron effects. For a given EGAM level the plasma heating by the mode can be significantly enhanced by varying the EP parameters. Electron dynamics decreases the EGAM saturation amplitude and consequently reduces the plasma heating, even though the mode transfers its energy to thermal ions much more than to electrons.

physics.plasm-ph↗

arXiv2019

Gyrokinetic investigation of Alfvén instabilities in the presence of turbulence

The global and electromagnetic gyrokinetic particle-in-cell code ORB5 is employed to investigate the self-consistent interactions between Alfvén modes (AM) and ion temperature gradient (ITG) turbulence in a magnetically confined plasma. Here, an axisymmetric magnetic equilibrium with reversed shear and large aspect ratio is considered. An AM with toroidal mode number n=5 is driven unstable by introducing a population of suprathermal ions. Once the AM saturates in the presence of the fully developed turbulence, the ion heat flux is dominated by the AM and its main harmonics. ITG-induced transport is found to also be enhanced in the presence of the unstable AM.

physics.plasm-ph↗

arXiv2018

Electromagnetic turbulence suppression by energetic particle driven modes

In recent years, a strong reduction of plasma turbulence in the presence of energetic particles has been reported in a number of magnetic confinement experiments and corresponding gyrokinetic simulations. While highly relevant to performance predictions for burning plasmas, an explanation for this primarily nonlinear effect has remained elusive so far. A thorough analysis finds that linearly marginally stable energetic particle driven modes are excited nonlinearly, depleting the energy content of the turbulence and acting as an additional catalyst for energy transfer to zonal modes (the dominant turbulence saturation channel). Respective signatures are found in a number of simulations for different JET and ASDEX Upgrade discharges with reduced transport levels attributed to energetic ion effects.

physics.plasm-ph↗