Open AccessModel for current drive induced crash cycles in W7-X
Author(s) -
K. Aleynikova,
S. R. Hudson,
P. Helander,
Arunav Kumar,
J. Geiger,
M. Hirsch,
Joaquim Loizu,
C. Nührenberg,
K. Rahbarnia,
Zhisong Qu,
Yu Gao,
H. Thomsen,
Y. Turkin,
M. Zanini,
the W-X Team
Publication year - 2021
Publication title -
nuclear fusion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.774
H-Index - 120
eISSN - 1741-4326
pISSN - 0029-5515
DOI - 10.1088/1741-4326/ac2ab9
Subject(s) - sawtooth wave , physics , magnetohydrodynamics , stellarator , plasma , wendelstein 7 x , magnetic reconnection , magnetohydrodynamic drive , electron cyclotron resonance , computational physics , nuclear physics , computer science , computer vision
In the Wendelstein 7-X (W7-X) stellarator, the vacuum rotational transform, ι , has a flat radial profile and does not cross any major rational resonance. Nevertheless, during plasma operation the ι ‐profile can be strongly modified by electron cyclotron current drive in such a way that the resulting ι -profile passes through low-order rational values, and this can trigger magnetohydrodynamic (MHD) events. Indeed, W7-X plasmas are sometimes subject to repetitive collapses of core confinement, which can be observed regardless of the direction in which the EC current is driven. Even though the origin of these MHD instabilities is under investigation, the crashes may be connected to the formation of magnetic islands and magnetic reconnection. In the present work, we try to shed light on the dynamics of different events happening during the course of sawtooth cycles in W7-X by proposing a model that combines a slow current diffusion with a recipe for fast relaxation that conserves the corresponding helical flux (Kadomtsev 1975 Fiz. Plazmy 1 710–15). We also propose a simple model based on Taylor relaxation (Taylor 1974 Phys. Rev. Lett. 33 1139), (Taylor 1986 Rev. Mod. Phys. 58 741) to predict the nonlinear redistribution of plasma current caused by the largest of the observed events.