journalef nuclear materials ELSEVIER Journal of Nuclear Materials 241-243 (1997) 988-992 Locked modes induced plasma-wall interactions in RFX M. Valisa *, T. Bolzonella, L. Carraro, E. Casarotto, S. Costa, L. Garzotti, P. Innocente, S. Martini, R. Pasqualotto, M.E. Puiatti, R. Pugno, P. Scarin Gruppo di Padova per Ricerehe sulla Fusione, Corso Stati Uniti 4, 1-35127 Padua, ltah' Abstract In the RFX reversed field pinch (R = 2 m, a = 0.45 m) the plasma-wall interaction has been characterised in all of the discharges by the presence of the locking both in phase and in the laboratory frame of unstable MHD modes. These locked modes cause a helical deformation of the magnetic surfaces driving large power fluxes onto the wall, The plasma edge is severely perturbed: a power density loading as high as 100 MW m -2 may locally be exceeded, the surface of the plasma facing components reach the sublimation temperature, the impurity release and the power radiated locally increase by a factor of ~ 100, large fluctuations in the electron density develop and halo currents flow in the inconel vacuum vessel. It is estimated that the locked modes are responsible for losses that may amount to up to about 30% of the total power input. Keywords: RFX; Reversed field pinch; Impurity source; Energy balance 1. Introduction The control of the interaction of magnetically confined plasmas with the first wall is a key issue in thermonuclear fusion experiments. Several tokamaks have demonstrated the importance of minimising the impurity production and controlling the recycling of the fuelling gas in order to obtain enhanced confinement modes [1]. Analogous con- clusions have been drawn in reversed field pinch (RFP) configurations where wall conditioning procedures lead to an improved performance [2,3]. In RFP's the edge region plays a crucial role in the confinement properties of the plasma, being characterised by relatively well defined magnetic surfaces in contrast with the plasma core, where the presence of overlapping magnetic islands implies a large degree of field line stochasticity and ultimately an enhanced radial transport [4,5]. Moreover, in the RFP edge a dynamo-like process generates a large parallel current density to sustain the toroidal magnetic flux and therefore a low plasma resistivity is a premium [6]. In RFX the presence of locked MHD modes in all of the discharges results in a large kink perturbation that leads to a strong enhancement of the plasma-wall interac- * Corresponding author. Tel.: +39-49 8295 031; fax: +39-49 8700 718; e-mail: valisa@pdigi3.igi.pd.cnr.it. tion. This paper describes the phenomenology associated with such perturbation and analyses the related impact on the confinement properties of the plasma. 2. The RFX experiment The plasma facing components in RFX are constituted by 2016 graphite tiles (10 × 18 cm 2) which protect the inconel vacuum vessel almost completely, with only a l cm interspace left between them. The protection has been quite effective and the concentration of metals in the plasma has never significantly contributed to Zef f, though evidence of erosion by direct power deposition and arc formation on the vessel has been found. No limiters are present and the plasma should lean evenly onto the first wall. As a matter of fact the presence of errors in the magnetic configuration introduces asymmetries in the plasma-surface interaction. The vacuum vessel is made by two nested inconel sheets 2 and 1 mm thick, respectively, separated by 20 mm and strengthened by internal inconel corrugations and 72 poloidal rings onto which the tiles are fixed. A 5 cm thick aluminium shell surrounds the vessel at b/a = 1.17 with the purpose of stabilising MHD modes. The shell has two poloidal and two toroidal gaps. Recently one poloidal and the external toroidal gaps have been short-circuited to reduce the associated field errors. 0022-3115/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PH S0022-3 115(96)00645-9