Scrape-off layer flows in the Tore Supra tokamak

Near-sonic parallel flows are systematically observed in the scrape-off layer (SOL) of the limiter tokamak Tore Supra, as in many X-point divertor tokamaks. The poloidal variation of the Mach number of the parallel flow has been measured by moving the contact point of a small circular plasma onto limiters at different poloidal angles. The resulting variations of flow are consistent with the existence of a poloidally nonuniform core-to-SOL outflux concentrated near the outboard midplane. Strong variations of the SOL width up to a factor of 10 suggest that this localized outflux is due to enhanced radial transport. The plasma that gets ejected into the SOL can expand radially to the wall if magnetic field lines have long connection lengths and pass unobstructed across the outboard midplane.