An increase in turbulent Reynolds stress preceding an L-H transition in the Compact Helical System (CHS) was observed. A positive increase in the Reynolds stress is associated with a negative jump in the floating potential. The relationship of signs is consistent with the momentum balance equation. Therefore, this observation supports the hypothesis that the Reynolds stress plays an important role in triggering the L-H transition in CHS plasmas. Understanding of the mechanism that triggers the transition to high-confinement plasmas [1] is important for realizing fusion plasmas. In some tokamak operations, Hmode transitions are triggered by a sawtooth crash. However, such events have not been observed in H-mode transitions in helical plasmas. In this rapid communication, we present a direct observation of turbulent Reynolds stress (RS) preceding an L-H transition in Compact Helical System (CHS) [2]. Compression of RS could be a source of shear flows and is a candidate mechanism for the formation of a radial electric field, leading to H-mode plasmas [3].The CHS is a low-aspect ratio middle size stellarator with a major radius R 0 = 1 m, minor radius a = 0.2 m, number of helical windings l = 2, and toroidal period number N t = 8. In these experiments, the magnetic axis is located at R = 92.1 cm (R axis ), and the toroidal magnetic field B t = 0.9 T. Two neutral beam injection (NBI) units were used to make reproducible H-mode plasmas. Edge fluctuations were measured with the hybrid probe (HP) [4]. The HP has six electrodes by which the floating potential signals were measured. The RS is calculated from the floating potential measured with two pairs of electrodes separated radially and poloidally, respectively. The HP is movable in the poloidal plane, and the two-dimensional RS structure was obtained, neglecting temperature fluctuations.Figure 1 (a) shows the time evolution of twodimensional maps of the RS. Positive RS indicates radially outward transport of velocity in the electron diamagnetic drift direction. The maps are reconstituted from shot-byauthor's e-mail: nagashima@k.u-tokyo.ac.jp shot scan data from the HP. Timings of the H α drop are used as the standard to synchronize different shot data in the same time series in two-dimensional maps. The RS starts to increase at 78 ms, indicated by red square (1) in Fig. 1 (a), has a maximum at 86 ms (2), and vanishes at 94 ms (3). We have discovered an increase in the RS preceding the H α drop (the L-H transition). At the beginning of the increase, the RS is localized near the outermost surface of the plasma at Z = 0 m. However, around the period when the RS has a maximum (84 ms), a large RS is distributed along the plasma surface, and the radial gradient of the RS is strong. The RS can transfer poloidal momentum in the radial direction and/or radial momentum in the poloidal direction. The RS gradient is a source/sink for momentum. Therefore, the finite radial gradient observed here can drive poloidal shear flow.The momentum balance equation determines the correspond...
