High energy electron deposition within vertical ports, during lower hybrid current drive on Tore Supra

Unexpected hot spots were observed around the edges of vertical ports on Tore-Supra, caused by fast electrons, accelerated by the lower hybrid waves used to drive the current, and trapped in the local ripple wells. Trajectory calculations, with the magnetic fields alone, show that no electrons should reach such locations. However, electrostatic potentials, at a kilovolt level, can induce a toroidal pinch of the trajectories, allowing particle deposition in these normally shadowed areas. Their origin is attributed first to a strong increase of the sheath potential at the port entrance, due to the fast electron flux itself, and second to its reduction within the port, when the sheath width becomes larger than the port width, due to the too low value of the local density. An increase of the capability of the cooling panels within the port, to cope with these additional fluxes, has been implemented.