Suprathermal electron beams and large sheath potentials generated by RF - antennas in the scrape - off layer of Tore Supra

In Tore Supra detailed radial-poloidal mapping of antenna-SOL interaction zones are made in a single shot using a retarding field analyzer (RFA) or Mach probe. A retarding field analyzer was used during LH current drive experiments to provide direct measurements of the particle and power fluxes of suprathermal electrons emanating from the region in front of the LH grill. When one of the active wave-guide rows is magnetically connected to the RFA, a strong particle flux due to suprathermal electrons is observed. A fraction of the electrons have energies greater than 1000 eV. Theory predicts that waves with high refractive index should be totally absorbed within at most 5 mm from the grill, and that the electron current should be stationary in time. In reality, electron current is measured at least 4-5 cm in front of the grill, and it exhibits a highly intermittent temporal evolution with a characteristic burst rate in the 10 kHz range. A Mach probe was used to map the SOL plasma parameters on flux tubes connected to both the standard and the new ITER-like ICRH antennae. A 1 cm layer of large positive floating potential (about one order of magnitude higher than the local electron temperature) is observed on flux tubes that graze the leading edges of the lateral protection limiters. Preliminary measurements of the true sheath potential using the RFA show that it is slightly more positive than the floating potential.