The Alfvén instability nonlinearly excited the energetic-particle-driven geodesic acoustic mode on the ASDEX-Upgrade tokamak, as demonstrated experimentally. The mechanism of the energetic-particle-driven geodesic acoustic mode excitation and the mode nonlinear evolution is not yet fully understood. In the present work, a first-principles simulation using the MEGA code investigated the mode properties in both the linear growth and nonlinear saturated phases. Here we show that the simulation successfully reproduced the excitation and coexistence of these two modes, and agreed with the experimental results well. Conclusive evidence showed that the resonance overlap is the excitation mechanism of the energetic-particle-driven geodesic acoustic mode. In the linear growth phase, energetic particles that satisfied different resonance conditions excited the Alfvén instability, which then caused energetic particle redistribution in phase space. These redistributed energetic particles caused resonance overlap, exciting the energetic-particle-driven geodesic acoustic mode in the nonlinear phase.

ASDEX Upgrade

10.1038/s41598-024-82577-3

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Model-based design of multi-mode feedback control in the RFX-mod experiment

Nuclear Fusion (2010)

L. Piron L. Marrelli Paolo Piovesan P. Zanca

Interest in real-time control of magnetic field errors and magnetohydrodynamic (MHD) instabilities has been growing in the last decades due to the demanding stability requirements of high-performance scenarios in fusion devices. In this framework, the RFX-mod experiment (Sonato et al 2003 Fusion Eng. Des. 66 161) plays an important role. One of the main goals of RFX-mod is to explore high-plasma current regimes up to 2 MA for the first time in a reversed-field pinch. To this aim, RFX-mod is equipped with an advanced active coil system for the control of error fields and MHD modes, such as tearing and resistive-wall modes. As far as tearing modes are concerned, both controlling their edge radial magnetic field and maintaining them into slow (∼10–100 Hz) rotation are crucial to reduce both the plasma–wall interaction and the core magnetic stochasticity. In this paper, a model-based optimization of the RFX-mod feedback control is presented. The aim is to find an optimal gain set for a spectrum of multiple tearing modes, which produces the lowest possible value of the edge radial magnetic field, maintaining at the same time the modes into slow rotation and avoiding coil current saturations. These optimal gains have first been calculated offline by simulating the non-linear dynamics of a spectrum of tearing modes interacting through viscous and electromagnetic torques, using an adaptation to the RFX-mod multiple-shell layout of the model described in Zanca ( 2009 Plasma Phys. Control. Fusion 51 015006 ). This gain set has been obtained by scanning the proportional and derivative gains and has been tested in an extensive experimental campaign, showing good agreement with the model. With this approach, a reduction in the edge radial magnetic field up to 15%, with respect to discharges in which an empirical optimization was used, has been obtained. The above model proved to be a powerful tool to tune a multi-mode controller offline, which allowed us to save a large amount of experimental time.

Reversed field pinch

Tearing

Resistive touchscreen

10.1088/0029-5515/50/11/115011

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Citations (15)

Trapped particles in the RFP single helicity States

IEEE Transactions on Plasma Science (2005)

I. Predebon L. Marrelli R. B. White P. Martin

Images of the trapped particle fraction in single helicity states of the reversed field pinch configuration are presented. This study is carried out with the Hamiltonian Monte Carlo code ORBIT. The various kinds of trajectories strongly influence radial particle transport in the core, and are determined by field shape and perturbation strength.

Helicity

Reversed field pinch

Hamiltonian (control theory)

10.1109/tps.2005.845333

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Observation of magnetic islands in tokamak plasmas during the suppression of edge-localized modes

Nature Physics (2024)

M. Willensdorfer V. Mitterauer M. Hoelzl W. Suttrop M. Cianciosa

Abstract In tokamaks, a leading platform for fusion energy, periodic filamentary plasma eruptions known as edge-localized modes occur in plasmas with high-energy confinement and steep pressure profiles at the plasma edge. These edge-localized modes could damage the tokamak wall but can be suppressed using small three-dimensional magnetic perturbations. Here we demonstrate that these magnetic perturbations can change the magnetic topology just inside the steep gradient region of the plasma edge. We identify signatures of a magnetic island, and their observation is linked to the suppression of edge-localized modes. We compare high-resolution measurements of perturbed magnetic surfaces with predictions from ideal magnetohydrodynamic theory where the magnetic topology is preserved. Although ideal magnetohydrodynamics adequately describes the measurements in plasmas exhibiting edge-localized modes, it proves insufficient for plasmas where these modes are suppressed. Nonlinear resistive magnetohydrodynamic modelling supports this observation. Our study experimentally confirms the predicted role of magnetic islands in inhibiting the occurrence of edge-localized modes. This will be beneficial for physics-based predictions in future fusion devices to control these modes.

10.1038/s41567-024-02666-y

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High current regimes in RFX-mod

Plasma Physics and Controlled Fusion (2008)

M. Valisa T. Bolzonella P. Buratti L. Carraro R. Cavazzana

Optimization of machine operation, including plasma position control, density control and especially feedback control on multiple magnetohydrodynamic modes, has led RFX-mod to operate reliably at 1.5 MA, the highest current ever achieved on a reversed field pinch (RFP). At high current and low density the magnetic topology spontaneously self-organizes in an Ohmical helical symmetry, with the new magnetic axis helically twisting around the geometrical axis of the torus. The separatrix of the island disappears leaving a wide and symmetric thermal structure with large gradients in the electron temperature profile. The new topology still displays an intermittent nature but its overall presence has reached 85% of the current flat-top period. The large gradients in the electron temperature profile appear to be marginal for the destabilization of ion temperature gradient modes on the assumption that ions and electrons have the same gradients. There are indications that higher currents could provide the conditions under which to prove the existence of a true helical equilibrium as the standard RFP configuration.

Reversed field pinch

Electron temperature

10.1088/0741-3335/50/12/124031

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Citations (46)

UV imaging of a plasma for space applications

HAL (Le Centre pour la Communication Scientifique Directe) (2004)

F. Bonomo P. Franz G. Spizzo L. Marrelli P. Martin

Astrophysical plasma

Source

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A 3D approach to equilibrium, stability and transport studies in RFX-mod improved regimes

Plasma Physics and Controlled Fusion (2010)

D. Terranova D. Bonfiglio Allen H. Boozer W.A. Cooper M. Gobbin

The full three-dimensional (3D) approach is now becoming an important issue for all magnetic confinement configurations. It is a necessary condition for the stellarator but also the tokamak and the reversed field pinch (RFP) now cannot be completely described in an axisymmetric framework. For the RFP the observation of self-sustained helical configurations with improved plasma performances require a better description in order to assess a new view on this configuration. In this new framework plasma configuration studies for RFX-mod have been considered both with tools developed for the RFP as well as considering codes originally developed for the stellarator and adapted to the RFP. These helical states are reached through a transition to a very low/reversed shear configuration leading to internal electron transport barriers. These states are interrupted by MHD reconnection events and the large Te gradients at the barriers indicate that both current and pressure driven modes are to be considered. Furthermore the typically flat Te profiles in the helical core have raised the issue of the role of electrostatic and electromagnetic turbulence in these reduced chaos regions, so that a stability analysis in the correct 3D geometry is required to address an optimization of the plasma setup. In this view the VMEC code proved to be an effective way to obtain helical equilibria to be studied in terms of stability and transport with a suite of well tested codes. In this work, the equilibrium reconstruction technique as well as the experimental evidence of 3D effects and their first interpretation in terms of stability and transport are presented using both RFP and stellarator tools.

Reversed field pinch

Plasma stability

10.1088/0741-3335/52/12/124023

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Citations (36)

MHD limits and plasma response in high-beta hybrid operations in ASDEX Upgrade

Nuclear Fusion (2017)

V. Igochine P. Piovesan I. G. J. Classen M. Dunne A. Gude

ASDEX Upgrade

BETA (programming language)

Upgrade

10.1088/1741-4326/aa7bf9

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Citations (11)

Feedback Control of (2,1) Mode in q$_edge\sim$2 Tokamak Plasma in RFX-MOD

Bulletin of the American Physical Society (2010)

Piero Martín R. Cavazzana L. Marrelli M. Baruzzo T. Bolzonella

Mod

Alcator C-Mod

Mode (computer interface)

Source

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Resonance between passing fast ions and MHD instabilities both in the tokamak and the RFP configurations

Nuclear Fusion (2008)

M. Gobbin R. B. White L. Marrelli P. Martin

Suprathermal ions in fusion plasmas have different confinement properties in tokamak and reversed field pinch (RFP) configurations. These differences are explained in terms of guiding centre drifts by an analytical approach. Magnetic field perturbations and passing fast ions can satisfy resonance conditions which do not hold for thermal or low energy particles. Such a resonance is controlled by a generalization for passing fast ions of the safety factor parameter. Applications to experimental situations show good agreement in predicting the radial position of the resonances in the fast ion phase space. Numerical simulations in RFP devices are qualitatively consistent with the longer confinement times of fast ions, especially in helical regimes.

10.1088/0029-5515/48/7/075002

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Citations (25)