Direct measurement of the plasma potential in the edge of ASDEX Upgrade using a self emitting probe

Abstract In this paper we present first measurements of the plasma potential close to the separatrix in ASDEX Upgrade using a self emitting tip. The probe was inserted into the edge plasma of AUG using the midplane manipulator. Assuming Maxwellian plasmas, the observations agree with the predicted voltage drop in the plasma sheath, V pl ps − V fl = 2.5 T e , where V pl ps is the plasma potential at the presheath boundary and V fl the floating potential. Applying this technique a rapid change of the plasma potential was observed close to the separatrix during Ohmic discharges. From the gradient we derive a radial electric field E r of about −5 kV / m close to separatrix. Further out the field strength changes sign and we find up to +7 kV / m in the SOL.