MHD Equilibrium of Heliotron J Plasmas

Yasuhiro Suzuki, Yuji Nakamura, Katsumi Kondo, Noriyoshi Nakajima1), Takaya Hayashi1)

Graduate School of Energy Science, Kyoto University, Uji, Kyoto 611-0011 Japan
1) National Institute for Fusion Science, Toki, Gifu 509-5292, Japan

MHD Equilibrium properties of Heliotron J plasmas are investigated by the HINT code. Heliotron J device is a L=1/M=4 Helical-Axis Heliotron (HAH), which has large flexibility of the magnetic configuration[1]. By changing the current of several coils, magnetic field components can be controlled. In previous studies, MHD equilibria are obtained mainly from the VMEC code with the fixed boundary constraint for simplicity. However, for Heliotron J plasmas, its equilibrium has large Shafranov shift of the plasma column because the magnetic shear is weak and the effective toroidicity εt is not small. Therefore, it is necessary to study MHD equilibrium under the free boundary constraint. Furthermore, for a low shear configuration like Heliotron J plasmas, the island formation in finite pressure equilibria is very important. To study free boundary MHD equilibria, the HINT code is applied to Heliotron J plasmas. The HINT code is a three-dimensional MHD equilibrium code based on the relaxation method and it does not a priori require the nested flux surface. The original scheme of the pressure relaxation in HINT is not efficient for the calculation of low shear stellarators but it is modified to improve the convergence of the pressure relaxation[2]. Using the modified HINT code, the "standard configuration" of the Heliotron J plasma is studied. The shape of the last closed flux surface is clearly changed. The magnetic island is generated by the resonance corresponding to ι= 4/7 and the edge field is ergodized by the finite pressure effects. In this study, properties of the MHD equilibrium are investigated more extensively. For the finite pressure equilibrium, the rotational transform is increased near the magnetic axis and decreased in the edge region. The width of the magnetic island is changed by the magnetic shear. When the profile of the plasma pressure is specified to the peaked profile, the magnetic shear is strong at the island and the width of islands is suppressed. On the other hand, for equilibria with the broad pressure distribution, the position of the resonance is shifted to the edge region because of small Shafranov shift and the magnetic island is grown in the region. In addition, configurations which is changed the position of the magnetic axis by varying the vertical field are studied.

The net toroidal current is observed by the magnetic diagnostics experimentally. Effects of net currents to the MHD equilibrium are very important in helical system plasmas. MHD equilibria with some prescribed profile of net currents are studied. Since the rotational transform is influenced sensitively, the width of the island is changed widely. Furthermore, the magnetic field structure is ergodized by the currents.

References

[1] M. Wakatani et al. , Nucl. Fusion 40 (2000) 569
[2] Y. Suzuki et al. , Plasma Phys. Control. Fusion 45 (2003) 971