Langmuir probe measurements in the lower x-point vicinity of the ASDEX Upgrade divertor

Abstract Measurements made with a reciprocating Langmuir probe which crosses both divertor legs of the ASDEX Upgrade tokamak just below the lower x -point are presented. The electron temperature, density and pressure profiles obtained in ohmic discharges are similar to previous measurements but the detailed structure, especially near the nominal separatrix, is rather complex and differs from earlier divertor configurations. Observations made in H-mode discharges near the L–H power threshold indicate that the probe tip influences the ELM signature when crossing the vicinity of the inner and outer leg separatrices, causing a temporary reduction of the ELM signature amplitude accompanied by an increase in the frequency. We briefly discuss possible mechanisms responsible for this effect.