Transport Simulation of Electron ITB Formation on LHD

H.Funaba, N.Ohyabu, K.Yamazaki, K.Ida, S.Murakami1), K.Y.Watanabe, S.Kubo,
T.Shimozuma, K.Narihara, Y.Takeiri, M.Yoshinuma, K.Tanaka, B.J.Peterson, M.Osakabe,
Y.Nagayama S.Inagaki, M.Yokoyama and the LHD Experimental Group

National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
1) Department of Nuclear Engineering, Kyoto University, Kyoto 606-8501, Japan

Electron temperature profiles with ITB (Internal Transport Barrier) were observed on the Large Helical Device (LHD) when the centrally localized ECH power exceeded a certain threshold level [1]. In such an electron ITB (eITB) region, the radial electric filed, Er, in the electron root regime is important for the reduction of the electron transport. The ambipolar condition, ΣZjΓjna = 0, is a mechanism which determines the neoclassical radial electric filed, Ernc, where Γjna represents the non-axisymmetric part of the neoclassical flux. In this study, the eITB formation is simulated including the effect of Er by using the one dimensional transport code, PROCTR, and the results are compared with the experimental data.
The experimental electron thermal diffusivity, χeexp, is derived based on the power balance of the steady state. The absorbed power density of ECH is calculated by the ray-tracing method and the NBI power deposition profiles are calculated by a three-dimensional Monte Carlo simulation code. In order to simulate the eITB formation, it is necessary to reproduce the following processes by the time-dependent transport calculation : (1) the Te rise due to the increment of the heating power, (2) positive Ernc formation by the achievement of high Te, (3) the reduction of χe by Ernc. In the case of the inward shifted magnetic configuration on LHD, the anomalous transport is dominant in the electron transport. That is, χean >> χenc, where χenc and χean are the electron thermal diffusivities by the neoclassical and anomalous transport, respectively. For example, at the foot point of the eITB, χeexp is about 2m2/s, while χenc with the calculated Ernc is about 0.2m2/s. The value of χenc become smaller when the multi-helicity effect is involved. Thus, because χeexp is larger than χenc, it is difficult to account for the χe reduction only by the effect of Ernc on χenc. Therefore, some functions are assumed in order to express any dependence of χean on Ernc. For the comparison with the experimental data, the temporal evolution of Te is calculated and compared with the results of the Thomson scattering or ECE measurements.

References

[1] T.Shimozuma, S.Kubo, et.al., Plasma Phys. Control. Fusion 45(2003)1183.