The influence of the poloidal variation of the density on the locally measured velocities induced by biasing experiments

The plasma edge plays an important role in the physics of improved confinement. In this region, the shear of the radial electric field and the related rotation are thought to be responsible for the suppression of turbulence. We have developed a consistent set of experiments and theory to analyse these important phenomena. An electric field is set up with a biasing electrode. The resulting rotation velocities are measured with an inclined Mach probe. The measurements are then compared with the predictions of a theoretical fluid model. In this model, parallel viscosity and neutral friction were already identified as important components to explain the very important and localised electric fields. In this paper we focus on compressibility effects and show that it is necessary to take the poloidal variation of the density into account to explain the measured rotation velocities.