Quasistationary plasma structure at the periphery of discharges in the TF-1 tokamak

Measurements have been made of the electron temperature and density distributions in the poloidal cross section of the TF-1 tokamak using movable Langmuir probes. They revealed that in a narrow radial layer close to the boundary between the shadow of the limiter and the bulk plasma a time-independent nonuniformity in the plasma parameters is present in the form of a dip in the Te profiles. The resulting topology of the contours of constant temperature implies that there are closed contours representing localized cells with enhanced or reduced electron temperature. At the average level Te ≅ 12-15 eV the time-independent spatial variation amounts to 25-30%. Within these temperature cells no variation in the electron density profiles was observed. Superposing an external perturbing radial magnetic field on the plasma had no effect on the formation or the typical size of the cells. The spatial disposition of the nonuniform structures is insensitive to variation of the global discharge parameters Bz, I, q and is highly reproducible from shot to shot. The structure found to be attached to the limiter cannot be explained in terms of magnetic islands