Speaker
B. Centurión
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1061.pdf
MHD equilibria with magnetic islands in TJ-II using SIESTA
B. Centurión1 , J. J. Martinell1 , A. López-Fraguas2 , J.M. Reynolds3 and D. López-Bruna2
1 Instituto de Ciencias Nucleares, UNAM, A. Postal 70-543, México D.F., Mexico
2 Laboratorio Nacional de Fusión, As.EURATOM-CIEMAT, 28040 Madrid, Spain
3 Universidad Carlos III, Leganes 28021, Spain
Experiments in the TJ-II heliac show a correlation between the position of magnetic rational
surfaces and a modification of the electron temperature profile [1], measured using Electron
Cyclotron Emission (ECE) in low density Electron Cyclotron Resonance heated discharges. On
plasma discharges heated using Neutral Beam Injection, ECE cannot be used due to the high
density; however, using the heliac’s flexibility, different rational surfaces can be swept along the
minor radius by varying the helical current and it was found that the transport was reduced at
the position of the rational surfaces. Bolometry studies have also shown a correlation between
transport barriers appearing on rational surfaces and MHD activity [2]. This may also lead to
transitions of the L-H type. These observations suggest that the plasma confinement can be im-
proved by strategically placing rational surfaces, which in in turn give rise to a transport barrier.
Since magnetic islands are likely to form at rational surfaces because the magnetic perturbations
are resonant there, investigation of the presence of magnetic islands is quite interesting in the
context of confinement improvement.
Using SIESTA code we calculated MHD equilibria with magnetic islands in TJ-II to deter-
mine the properties of the islands. The starting equilibrium state is the one obtained by the
VMEC code with nested magnetic surfaces. Standard computations use the heliac’s toroidal
periodicity of 4 periods to reduce the code runtime, but this, when used in SIESTA, limits the
toroidal periodicity of the islands to multiples of 4. To solve this limitation we tailored the input
parameters to run VMEC without 4-period symmetry. In this way, running SIESTA with those
equilibria, magnetic islands of any periodicity are obtained, including those resonant at ι = 3/2
whose magnetic islands were previously absent from the simulations. The results also show 2D
pressure profiles that match the location of the magnetic islands observed in the Poincaré plots
of the field lines.
References
[1] D. López-Bruna et al., Plasma Phys. Control. Fusion 53, 124022 (2011)
[2] D. López-Bruna et al., Nuclear Fusion 53, 073051 (2013)
