H-mode Characteristics with Hot-Cathode-Biasing in Tohoku University Heliac

Yutaka Tanakaa), Sumio Kitajimaa), Hiromi Takahashia),
Hiroyasu Utoha), Mamiko Sasaoa) and Masakazu Takayamab)

a) Department of Quantum Science and Energy Engineering, Tohoku University, Aramaki, Aoba, Sendai, Miyagi, 980-8579, Japan
b) Department of Electronics and Information Systems, Akita Prefectural University, Ebinokuchi, Tsuchiya, Honjyo, Akita, 015-0055, Japan

Many kinds of improved mode are now discovered both in tokamaks and helical devices. H-mode like behaviors accompanying the formation of a strong negative radial electric field have been observed in hot-cathode-biasing experiments in Tohoku University Heliac (TU-Heliac) [1,2]. While the central electron temperature diminishes to some extent, a strong increase of central density, reduction of density and potential fluctuations, and overall improvement of plasma performance are clearly seen.
Reduction of Hα light emission has been known as an important signature of improved mode, indicating the reduction of edge particle transport. In the present paper, experimental results on Hα behaviors during L to H, and H to L transitions on central and edge chords measured by a 25cm monocrometer on TU-Heliac are reported. Because the origin of Hα emission is mainly electron impact excitation in plasmas of TU-Heliac, the Hα intensity is normalized by its reactivity to n=3 levels to obtain the radial particle flux. The radial flux increases immediately after the H mode transition, and then decreases towards the L mode level. On the other hand, it decreases immediately after the L mode transition. It is different from the usual H mode characteristics .The positive bias experiments are planned and comparison with the negative bias experiment will be made.

References

[1]S. Kitajima et al., Int. J. Appl. Electromagnetics and Mechanics, 13, 381 (2002)
[2]S. Kitajima et al., IEEE Conference Record of 2003 IEEE International Conference on Plasma Science, 465 (2003)