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New experimental evidence of the correlation between edge sheared E× B flow and reduction of turbulence has been measured in the CASTOR tokamak ( R = 0.4 m, a = 0.085 m, BT = 1 T). A biasing electrode is placed at the separatrix in a configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement. A set of movable electrostatic probes (rake, Langmuir, Gundestrup, and rotating Mach) provide redundant, simultaneous measurements of poloidal flow, toroidal flow, electron temperature, density, and radial electric field with high temporal resolution and at the same poloidal location. Particular effort has been made in the optimization of the Gundestrup probe collector geometry in order to reduce the relative uncertainty of Mach number measurements in plasmas with weak flow ( M∥, M⊥ < 0.1). The measurements from the rake, Gundestrup, and rotating Mach probes give three independent radial profiles of E× B shear in ohmic and biased modes. Good agreement is obtained both for the profile shape and its absolute magnitude. The plasma flows, especially the poloidal E× B drift velocity, are strongly modified in the sheared region, reaching Mach numbers as high as half the sound speed. The corresponding shear rates (≈5×106 s-1) derived from both the flow and electric field profiles are in excellent agreement and are at least an order of magnitude higher than the growth rate of unstable turbulent modes as estimated from fluctuation measurements. In addition, we compare the measured E× B ion mass flow with the phase velocity of fluctuations moving poloidally across the Gundestrup collectors. Given the poloidal separation of the collecting plates and the sampling frequency (5 MHz), the maximum detectable phase velocity turns out to be rather modest compared to the measured bulk poloidal flow speed. Therefore the two quantities are only compared when the poloidal speed is low; in that case, they show similar behaviour in response to the applied bias. [ABSTRACT FROM AUTHOR]

Injection of test impurities into the edge plasma of tokamak discharges by erosion probes containing these impurities is described. By applying a short bias pulse to the probe inserted into the sol-plasma, the plasma density and temperature at the probe location can be deduced and the time of injection during the discharge can be controlled. By locating the probe at an appropriate position it is possible to release defined quantities of the material and to influence the fraction which penetrates into the core plasma. The injection of Li into hydrogen discharges of the smallsized tokamaks Castor and MT-1 is demonstrated. The nature of the main erosion process (ion sputtering or arcing) has been found to depend on the radial position of the probe and the probe potential. The lithium amount released by sputtering is determined, while in the case of arcing only an estimate can be given. The temporal evolution and the radial penetration of the Li influx into the plasma have been observed by monitoring the neutral emission line using a grating spectrometer and a CCD camera. In addition, Li transported through the plasma was collected on solid samples. An estimate is given on the fraction of the impurity efflux from the core plasma which is collected on the samples. [ABSTRACT FROM AUTHOR]