INSTITUTE OF PHYSICS PUBLISHING PLASMA PHYSICS AND CONTROLLED FUSION Plasma Phys. Control. Fusion 44 (2002) 567–578 PII: S0741-3335(02)31687-7 Measurements with an emissive probe in the CASTOR tokamak Roman Schrittwieser 1,6 , Jiˇ r´ ı Ad ´ amek 2 , Petru Balan 1 , Martin Hron 2 , Codrina Ioni˛t˘ a 1 , Karel Jakubka 2 , Ladislav Kryˇ ska 2 , Emilio Martines 3 , Jan St¨ ockel 2 , Milan Tichy 4 and Guido Van Oost 5 1 Institute for Ion Physics, University of Innsbruck, Innsbruck, Austria 2 Institute of Plasma Physics, Association EURATOM/IPP.CR, Academy of Sciences of the Czech Republic, Prague, Czech Republic 3 Consorzio RFX, Associazione EURATOM-ENEA sulla Fusione, Padova, Italy 4 Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech Republic 5 Department of Applied Physics, Ghent University, Ghent, Belgium E-mail: roman.schrittwieser@uibk.ac.at Received 7 December 2001 Published 22 April 2002 Online at stacks.iop.org/PPCF/44/567 Abstract An emissive probe has been used in the edge region of the CASTOR tokamak in order to test the possibility of direct measurements of the plasma potential. The difference between the floating potential of a cold probe and that of an emissive probe has been found to be approximately 1.3 times the electron temperature, which is less than predicted by the probe theory. Several possible reasons to explain this discrepancy are offered, such as secondary electron emission, uncertainties in the ion temperature, different collecting areas for electrons and ions, etc. The possible impact of a space charge formed by the emitted electrons is also discussed. 1. Introduction Particle and energy confinement in magnetized plasmas are strongly influenced by electric fields and their fluctuations [1]. Electric fields are determined by gradients of the plasma potential. In the edge region of fusion devices, the plasma potential is usually deduced from the floating potential measured by Langmuir probes. The difference between the floating potential V fl and the plasma potential  is a function of electron and ion temperatures and possibly also other parameters: V fl =  − f (T i ,T e , . . .). (1) This technique is comparatively simple and provides the local information with a satisfactory temporal resolution. 6 Author to whom correspondence should be addressed. 0741-3335/02/050567+12$30.00 © 2002 IOP Publishing Ltd Printed in the UK 567