Evidence of Stochastic Region near a Rational Surface in Core Plasmas of LHD

Full text: Clear evidence of stochastization of the magnetic surfaces near a rational surface is observed in the core plasma with weak magnetic shear in the Large Helical Device (LHD) by applying heat pulses driven by modulated electron cyclotron heating (MECH). The stochastization of the magnetic surfaces is confirmed by the observation of 1) flattening of electron temperature profiles, 2) very fast propagation of the heat pulse, which is in contrast to the slow heat pulse propagation observed in the electron temperature flat region of the nested magnetic island. One piece of evidence of stochastization of the magnetic surfaces is the flattening of Te profile, however, the flattening can occur due to the lack of heat flux crossing the magnetic flux as seen in the magnetic island even if the magnetic flux surface is nested inside. In this paper, experimental confirmation of stochastization of the magnetic surface is carried out by the analysis of heat pulse propagation driven by modulated electron cyclotron heating (MECH) with a frequency of 39 Hz and cold pulse propagation driven by TESPEL in the plasma with a flat electron temperature profile, which is observed when the magnetic shear at the rational surface measured with motional stark effect (MSE) spectroscopy drops below a critical value (0.2 ∼ 0.5). When the direction of the NBI is switched from balanced to counter to equivalent plasma current, the magnetic shear at a rational surface at ι =0.5 decreases, because the rotational transform near the plasma edge decreases due to non-inductive current drive by NBI and the central rotation transform even increases transiently due to the inductive current. When the magnetic shear drops below the critical values, the large core flattening of electron temperature up to half of plasma minor radius and partial flattening at one-third of plasma minor radius are observed. Since the power deposition of MECH is localized at the magnetic axis, a heat pulse propagates from the plasma center toward the plasma edge (outward) when there is no magnetic island. However, the heat pulse propagates rapidly in the electron temperature flat region (no time delay within the accuracy of the measurements), which is a clear evidence stochastization of magnetic surface. (author)