arXiv subjects

P. Pons-Villalonga

Publications and source records attributed to P. Pons-Villalonga.

2 recordsLinked to original sources

arXiv2025

Characterization of NBI-driven shear Alfvén waves in the TJ-II stellarator using Mirnov probes and electrostatic potential fluctuation measurements

We present the first experimental measurements of the toroidal mode number of shear Alfvén waves in the TJ-II stellarator. A series of experiments were carried out in three different magnetic configurations to investigate counter-NBI driven modes. Co- and counter- electron-cyclotron current drive was used to modify the rotational transform (\iotabar) profile leading to the destabilization of a varied set of Alfvén eigenmodes with different frequencies and mode numbers. To characterize the spatial structure of the modes we have used two Mirnov probe arrays, one dedicated to the measurement of the poloidal mode number and the other, a recently commissioned helical tri-axial array, dedicated to the measurement of the toroidal mode number. A heavy ion beam probe, operated in radial sweep mode, was employed to characterize the radial location of the modes. We show that the induced changes in \iotabar, that are fundamental when it comes to validation studies, cannot be measured experimentally with motional Stark effect so, instead, the shielding current diffusion equation is solved in cylindrical geometry to estimate these changes. We calculate the incompressible shear Alfvén continuum for selected cases using \texttt{STELLGAP} and find reasonable consistency with observations. A database with the observed modes has been created, so that it can be used in future work for theory validation purposes.

physics.plasm-ph↗

arXiv2025

Exploring the operational limits of poloidal and helical arrays of Mirnov coils in stellarators by means of a synthetic diagnostic

A synthetic Mirnov coils diagnostic for non-axisymmetric magnetic configurations is presented and used to study the capabilities of the poloidal array of single-axis coils and the two helical arrays of tri-axial coils installed in the TJ-II stellarator. This tool integrates the plasma currents induced by Alfvén-like perturbations of the electric potential inside the plasma and provides the induced magnetic field oscillations anywhere outside of it. The simulated signals can then be analyzed in the same manner as the experimental ones, and a scan on the radial position and width of the potential perturbation is conducted to find the limiting values that produce identifiable signals. We find, not surprisingly, that core-localized modes are indistinguishable from one another; and that the identification of low-n, low-m modes is often subject to off-by-one errors. We also determine the optimal polarization basis in which to analyze the tri-axial coils signals and address the diagnostic performance when resolving components of gap modes. Additionally, selected cases have been analyzed with a simplified plasma response model, showing that plasma shielding of the mode currents may further deteriorate the accuracy of the mode identification method. We conclude with the analysis of an experimental case taken from the TJ-II database to illustrate the usefulness of the diagnostic.

physics.plasm-ph↗