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(THP1/15) Influence of External Toroidal Flux on Low-Aspect-Ratio Toroidal Plasma

   
S. Ikuno  and M. Natori 
Institute of Information Sciences and Electronics, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Ibaraki 305-8573, Japan
 
A. Kamitani 
Department of Electrical and Information Engineering, Faculty of Engineering, Yamagata University, Johnan 4-3-16, Yonezawa, Yamagata 992-8510, Japan

Abstract
In the HIST  device, the external flux is generated by two kinds of currents: the current $I_{\rm s}$ flowing along the symmetry axis and the bias coil current $I_{\rm D}$. The influence of the external flux on the MHD equilibrium and stability of the low-aspect-ratio toroidal plasma in the HIST device is investigated numerically. Equilibrium configurations of the low-aspect-ratio toroidal plasma in the HIST device are numerically determined by means of the combination of FDM and BEM. The influence of $I_{\rm s}$ and $I_{\rm D}$ on their stability is also investigated by using the Mercier criterion. The results of computations show that the Mercier limit decreases to zero with increasing $I_{\rm s}$ and with decreasing $I_{\rm D}$. Moreover, either a further increase in $I_{\rm s}$ or a further decrease in $I_{\rm D}$ raises the Mercier limit  considerably. Besides, the equilibrium configuration in the HIST device changes its state from spheromak  through ultra-low q to tokamak with increasing $I_{\rm s}$ and with decreasing $I_{\rm D}$.

 

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