Plasma potential and electron temperature evaluated by ball-pen and Langmuir probes in the COMPASS tokamak M Dimitrova, Tsv K Popov, J Adamek et al. Abstract. The first derivative probe technique was applied to study the ISTTOK tokamak plasma. This technique employs the electron part of the Langmuir probe current-voltage (IV) characteristic and yields information on the plasma potential and the electron energy distribution function (EEDF). The IV characteristic was measured with new electrical probes mounted on a horizontal manipulator, one oriented in parallel and the other perpendicularly to the magnetic field lines. Using the first-derivative probe technique, the plasma potential and the EEDF at different radial positions were acquired. We show that, in the vicinity of the last close flux surface (LCFS), the EEDF is non-Maxwellian and can be approximated by a biMaxwellian one with a dominant cold electron population and a minority group of hot electrons. In the limiter shadow, the EEDF obtained is Maxwellian.The comparison of the plasma parameters evaluated using the first-derivative probe technique for both probes shows a satisfactory agreement.