Speaker
Description
For operation of future tokamak scenarios, a high pressure gradient favours the bootstrap mechanism, but also makes the plasma prone to MHD instabilities [1]. One of these instabilities is the fishbone instability, which is a 𝑚=𝑛=1 internal kink mode driven unstable by energetic particles [2]. It has been reported by both experimental and
numerical studies that the fishbone instability is not only redistributing EPs in the tokamak core but can also lead to increased ion temperature gradients via the suppression of background turbulence [3, 4]. On ASDEX Upgrade, the fishbone instability have been observed before the Internal Transport Barrier (ITB) formation [5]. In order to understand the detailed physics mechanisms and conditions for this effect, a global, kinetic non-linear approach is necessary that allows us to describe the cross-coupling between this non-perturbative global instability and the electromagnetic micro-scale fluctuations.
In this poster, simulations with the gyrokinetic particle-in-cell code ORB5 [6] are performed, to first find the triggering condition for the fishbone instability. The fraction of energetic particles is scanned across three different magnetic equilibria, including reversed shear. Two energetic particle distribution functions are compared, a
Maxwellian-like distribution and a strongly anisotropic one with 𝑣 ∥ ≈ 0, which maximizes the trapped-particle fraction. For the Maxwellian distribution function, increasing the fraction of energetic particles increases the growth rate of the mode, which appears to be a toroidal Alfvén eigenmode (TAE). The anisotropic distribution function with 𝑣 ∥ ≈ 0 causes a reduction in growth rate, the TAE mode is lost, and an internal kink mode is recovered, where the growth rate decreases with EP density. The results presented show how the observed modes vary between
the scanned parameters. Further scans will investigate the triggering conditions for the fishbone instability as a preparation for comprehensive studies under experimentally relevant conditions.
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom
Research and Training Programme (Grant Agreement No 101052200 – EUROfusion). Views and opinions expressed are however those of the
author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the
European Commission can be held responsible for them.
References
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Astrophysics, May 2013.
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1125, Mar 1984.
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S. D. Pinches, P. Liu, J. H. Nicolau, and H. Lütjens. Saturation of fishbone instability by self-generated zonal flows in tokamak plasmas.
Phys. Rev. Lett., 132:075101, Feb 2024.
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Physical Society. ECA Vol. 25A (2001) 57-60.
[5] S. Günter, A. Gude, J. Hobirk, M. Maraschek, S. Saarelma, S. Schade, R.C. Wolf, and ASDEX Upgrade Team. Mhd phenomena in advanced
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[6] E. Lanti, N. Ohana, N. Tronko, T. Hayward-Schneider, A. Bottino, B.F. McMillan, A. Mishchenko, A. Scheinberg, A. Biancalani, P. An-
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