Enhanced edge particle confinement: Edge biasing
The edge region of RFX features a double ExB velocity shear layer: the first shear layer, across the plasma boundary, is similar to that observed in tokamaks, whereas the second, more internal layer is related to the radial electric field becoming positive in the plasma core.
The second shear layer has been found to be in a marginal situation for electrostatic turbulence decorrelation to take place. Accordingly, an edge biasing experiment has been designed in order to try to increase the velocity shear and to affect the electrostatic turbulence, which has been found to be responsible of most of the edge particle transport.
The equipment consists of two carbon fiber electrodes designed so that they can drive up to 10 kA each.
The biasing experiments, performed with an electrode insertion into the plasma of 7.5 cm, have shown that the velocity profile can be substantially affected by this technique. In particular, the negative biasing has allowed to increase the velocity shear in the second layer.
As a consequence, a substantial reduction of the particle flux driven by the electrostatic turbulence has been observed over a large portion of the edge region. The reduction is mainly due to a change in the cross phase between density and plasma potential fluctuations, in analogy to what has been observed in similar experiments performed in tokamak plasmas.