Speaker
Katsuji Ichiguchi
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1046.pdf
Causality Study of MHD Events in LHD Plasmas
K. Ichiguchi1,2 , B.A. Carreras3 , S. Sakakibara1,2
1 National Insititute for Fusion Science, Toki, 509-5292, Japan
2 SOKENDAI, The Graduate University of Advanced Studies, Toki, 509-5292, Japan
3 Universidad Carlos III 28911, Madrid, Spain
In high beta experiments in the Large Helical Device (LHD), partial collapse phenomena
caused by the interchange mode can be observed[1]. These phenomena occurs in two types
of discharge condition. One is the inward shift of the vacuum magnetic axis. This condition
enhances the magnetic hill, where the driving force is enhanced.. The other is ramp-up of the
net toroidal current that increases the rotational transform. This condition reduces the magnetic
shear, where the stabilizing effect is reduced. In both cases, the growth of the interchange modes
leads to the partial collapse of the electron temperature profile. In some of the collapses, clear
correlation between the decay of the mode frequency and the growth of the mode is seen. That
is, the disappearance of the mode frequency and the beginning of the mode growth look to syn-
chronize. The mode frequency is mainly due to the ExB rotation and the diamagnetic rotation
of the plasma, and the former is usually dominant [2] Therefore, these results suggest that the
plasma rotation play a significant role in the collapse phenomena. In the nonlinear simulation
for the interaction of the interchange mode and the ExB shear flow, we obtained that the flow
has a stabilizing contribution against the mode growth [3].
However, the causality of the events is not still clear, whether the plasma rotation stopping
causes the mode growth or the mode growth stops the rotation.. van Milligan et al., [4] show
that the transfer entropy [5] is a powerful tool to investigate such causality. They examine the
transfer entropy for the causality studies in the fluctuations observed in the TJ-II experiments.
Thus, we apply this method to the time evolution of the magnetic fluctuations and the mode
frequency. Utilizing the results, we also discuss the similarity and the difference in the collapse
property between the cases of the hill enhancement and the shear reduction in the aspect of the
causality.
References
[1] S. Sakakibara, et al., Nucl. Fusion, 55, 083020 (2015).
[2] Y. Takemura, et al., 2013, Plasma and Fusion Res. 8, 1402123.
[3] K. Ichiguchi et al., Proc. IAEA FEC 2016 Kyoto, TH/P1-4.
[4] B. Ph. van Milligan, et al,. Nucl. Fusion 54, 023011 (2014).
[5] T. Shreiber, Phys, Rev. Letters, 85, 461 (2000).
