Fluctuation measurement across the broad range of the low-field side edge plasmas in the TST-2 spherical tokamak

This paper presents initial results from experimental observation of edge turbulence structures in ohmically heated plasmas on Tokyo Spherical Tokamak-2 (TST-2). The experiment was conducted using two-dimensionally movable Langmuir probes. With monitoring floating potential Vf and ion saturation current Iis, the radial and poloidal structures of the edge turbulence are obtained. The high coherence in fluctuations at 10-100kHz frequency was observed by the use of the Langmuir probes 90deg toroidally and ∼75deg poloidally distant from each other, suggesting that the fluctuations have long range correlation along a magnetic field line. To investigate nonlinear structures of the turbulence, bispectral analysis was used, and the observed auto-bicoherence pattern suggests the presence of the coupling and energy transfer between different modes. (author)