Speaker
Description
Shear Alfvén waves destabilized by neutral beam injection are quite common in TJ-II plasmas [1] and provide a good testing ground for model validation purposes in non-axisymmetric configurations. In order to obtain a set of well-characterized Alfvén perturbations, we have carried out a series of experiments on plasmas with combined ECRH and NBI heating using different magnetic configurations. In these experiments,
changes in the plasma current of only a few kA, which can be produced for instance using ECCD, allows us to investigate different spectra of waves while keeping thermal plasma parameters and fast ion pressure almost constant [2]. To characterize the spatial structure of Alfvén instabilities, three arrays of Mirnov coils, poloidally and helically
distributed, which give information on mode periodicity and polarization [3], and one heavy ion beam probe (HIBP), which measures the radial profile of the perturbed potential [4], are used. The interpretation of mode number measurements is performed with the aid of a synthetic Mirnov coils diagnostic [5]. Using current density profiles due to both neutral beam and microwave beam injection, we calculate the evolution of the rotational transform using a cylindrical model to estimate the evolution of the toroidal current. We also show that MSE measurements cannot distinguish changes in iota due to the effect of currents induced by heating systems. With the estimated iota profile and the thermal plasma profiles we can calculate the Alfvén wave continuum using the STELLGAP code [6] and perform a preliminary comparison of the experimental results with the candidate modes that can be destabilized in the system.
[1] R. Jimenez-Gomez et al, Alfven eigenmodes measured in the TJ-II stellarator, Nucl.
Fusion 51, 3 (2011) 033001.
[2] A. Cappa et al, Stability analysis of TJ-II stellarator NBI driven Alfven eigenmodes
in ECRH and ECCD experiments, 2021 Nucl. Fusion 61 066019.
[3] E. Ascasibar et al., Measurements of magnetic field fluctuations in TJ-II plasmas
with new in-vessel helical arrays of magnetic coils, Rev. Sci. Instrum. 93 (2022)
093508.
[4] A. V. Melnikov 2D distributions of potential and density mean-values and
oscillations in the ECRH and NBI plasmas at the TJ-II stellarator et al 2022 Plasma
Phys. Control. Fusion 64 054009.
[5] P. Pons-Villalonga, A. Cappa, and J. Martinez-Fernandez, Exploring the operational
limits of Mirnov coil arrays in stellarators by means of a synthetic diagnostic, Sep.
2024. arXiv: 2409.04221 [physics]. [Online]. Available: http:/
/arxiv.org/abs/2409.04221.
[6] D. A. Spong, R. Sanchez, A. Weller, "Shear Alfvén continua in stellarators," Phys.
Plasmas 10 (2003) 3217–3224.
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