On the determination of the ion drift speed perpendicular to the magnetic field by means of a probe

In an attempt to determine the flow speed of ions perpendicular to the magnetic field the effective area of the probe pins has to be known. Its value depend on the value of the electric field in the presheth of the pin and can be significantly larger than the geometrical area. It is demonstrated that without considering the effect of the presheath even the determined direction of the flow can be wrong. Probe signals obtained by means of the Rotating Double Cylinder Probe give an indication of the importance of the presheath effect in a plasma with cross field flow. A new probe head design is presented which might be suited to determine the cross field flow speed. First results are discussed

On the determination of the drift perpendicular to the magnetic field by means of a probe

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Investigation of E x B transport with a multi-electrode probe in the plasma boundary of textor : first results

A movable multi-element Langmuir probe was implemented in TEXTOR in order to study properties of the edge and scrape-off plasma. The probe has five graphite electrode pins allowing the simultaneous measurement of main plasma parameters such as plasma densities, electron temperatures, floating potentials, poloidal and radial electric fields. Both time-averaged and fluctuating quantities have been considered in order to evaluate the DC and turbulence-driven cross-field particle fluxes. The spectral analysis of the fluctuating floating potentials at spatially separated probe pins allows to determine the velocity associated with the rotations of the boundary plasma. The investigations have been focused on the variations of plasma, boundary properties in plasmas with pure ohmic heating as well as auxiliary heating (ICRH). Special attention has been paid to the change of transport properties with the transition to a detached plasma. In particular, a significant reduction of the poloidal phase velocity at the limiter edge has been observed for detached plasmas. Preliminary data on physical effects near the plasma boundary, whichoccur when the toroidal belt limiter (ALT-II) is biased, are reported

Properties of "detached" plasmas

The phenomenon "detached"-plasma is described and analyzed, based on detailed experimental studies in the TEXTOR tokamak, in particular by applying comprehensive edge diagnostics. The major parameters responsible for the transition to a "detached"-plasma are identified (heating power, electron density, impurity level) and can be related to a power loss via "100% radiation cooling" from low-Z impurities at the edge. The relevance for plasma-wall interaction, as evidenced e. g. by drastic changes in heat loads to the limiter, increase of particle flux to the wall, thereby enhancing the impurity release, is discussed. The great importance of considering the "detached"-plasma properties in scaling studies and additional heating effects is demonstrated by the impact of this plasma state on particle and energy confinement times and by ICRF-heating induced transitions