The SOL turbulence characteristics in the first experimental campaign on W7-X

Turbulence is considered to play an important role in the edge cross field heat and particle transport in fusion devices. The SOL turbulence characteristics were measured by the combined probe mounted on the fast manipulator during the first experimental campaign of W7-X. The combined probe head, consisting of triple probe, Mach probe and two sets of 3D magnetic coils, are capable to measure edge profiles of Te, ne, floating potential, parallel Mach number, local magnetic field, electrostatic and electromagnetic fluctuations. With this setup of probe head, the turbulence correlation length, decorrelation time, propagation behaviour along both radial and poloidal directions can be obtained. The poloidal cross correlation spectrum S(kθ, f), representing the dispersion relation in kθ and f space, shows that the dominant fluctuations have a frequency below 60 kHz, and their phase velocity and group velocity shift to the direction of ion diamagnetic drift with decreasing the distance to the last closed flux surface (R-RLCFS) in the limiter plasma on W7-X. Both the poloidal and radial correlation lengths are in the range of 5-10 mm. A low frequency mode around 7 kHz has been observed by both Langmuir probes and magnetic probes, and it can propagate to the far SOL (about R=6.15 m). The amplitude of this mode has strong dependence on magnetic topology.