The Effect of Non-Axisymmetry of Magnetic Configurations on Radial Electric Field Transition Properties in Helical Systems

M.Yokoyama, K.Ida, M.Yoshinuma, T.Shimozuma, K.Y.Watanabe, S.Murakami1), A.Wakasa2), S.Kubo, Y.Takeiri, K.Narihara, S.Morita, K.Tanaka, Y.Yoshimura, T.Notake, M.Osakabe, K.Itoh, A.Komori, O.Motojima and LHD experimental G.

National Institute for Fusion Science, Toki 509-5292, Japan
1)Department of Nuclear Engineering, Kyoto Univ., Kyoto 606-8501, Japan
2)Graduate School of Engineering, Hokkaido Univ., Sapporo 060-8628, Japan

The transition properties of the radial electric field (Er) have been theoretically and experimentally examined in the Large Helical Device (LHD) to grasp inter-relationships between magnetic configuration characteristics and Er properties [1,2]. The Er is calculated based on the ambipolar condition with the neoclassical flux estimated by the analytical formulae [3]. The effective helicity, used for flux calculations is estimated with the GIOTA code [4]. The Er transition utilizing the non-axisymmetry of magnetic configurations in helical systems is focused, which is possible through the nonlinear dependence of plasma transport on Er. This study is valuable to utilize Er transition for confinement improvement in non-axisymmetric configurations.
The plasma parameter region for realizing electron root has been predicted to differ according to the non-axisymmetry of a magnetic configuration (controlled by Rax) [5]. This has been successfully experimentally verified [6] by finding the difference of the density threshold for realizing electron root among different Rax configurations. The larger Rax (outward-shifted) makes the threshold density larger (or, in other words, electron root is possible even at higher collisionality). This is physically corresponding to effectively deeply located in 1/ν regime for the same real-collisionality in a case of a larger non-axisymmetry.
This agreement has given the proof that the Er transition properties in LHD are predominantly determined based on the neoclassical transport. Based on this, possible experiments in other helical devices will be proposed to generalize this finding. Also, the dependence of the threshold ECH power on configuration variation for establishing the electron internal transport barrier [7] would also be the interesting subject based on this study.

References

[1] K.Ida et al., Phys. Rev. Lett. 86(2001)5297.
[2] M.Yokoyama et al., Nucl. Fusion 42(2002)143.
[3] K.C.Shaing, Phys. Fluids 27(1984)1567; L.M.Kovrizhnykh, Nucl. Fusion 24(1984)435.
[4] N.Nakajima et al., NIFS LHD Technical Report 1, p.288 (in Japanese).
[5] M.Yokoyama et al., J.Plasma and Fusion Res. (to appear in August, 2003).
[6] M.Yoshinuma et al., presented at the H-mode workshop (2003).
[7] T.Shimozuma et al., Plasma Phys. Controlled Fusion (to be published).


This work has been supported by a grant-in-aid for young scientists (B) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan to one of the authors (MY).