Fluctuation Measurement in Electron Penetration Experiment on Helical Magnetic Configuration

H. Wakabayashi, H. Himura, M. Fukao, M. Isobe1), Z. Yoshida, and CHS group1)

Department of Advanced Energy, Graduate School of Frontier Sciences, The University of Tokyo, 7-3-1 Bunkyo Ward, Tokyo 113-0033, Japan
1)National Institute for Fusion Science, Toki, Gifu 509-5292, Japan

Experimental studies on collisionless penetration into a helical magnetic field have been performed. It is found that the space potential φs has a weak dependence on the injection angle α of electrons except for a narrow `window' region in which φs significantly drops. A calculation shows that because of the electric field of the electron gun (e-gun), the emitted electrons are always launched quasi-parallel to the helical magnetic field, regardless of α. This seems to agree with the observation. The `window' seen in the data may be attributed to a current-driven instability which might result in the insufficient electron penetration or the degradation of electron confinement in the magnetic surface [1]. In order to investigate the instability and the mechanism of the collisionless penetration, electrostatic fluctuation has been measured in the stochastic magnetic region. FFT analyses of the data show that apparent fluctuations of about 3.5 MHz appear during the electron penetration. Moreover, the frequency strongly depends on the electron energy and density, implying an instability related with the electron injection in the stochastic region.

References

[1] H. Wakabayashi et al., "Parameter Dependence of Inward Diffusion on Injected Electrons in Helical Nonneutral Plasmas", will be appeared in Non-neutral Plasma Physics V (American Institute of Physics) (2003).


This work is performed under the auspices of the NIFS LHD Project Research Collaboration.