Influence of convective heat losses on the H-mode power threshold 1 D. Kalupin, 1 M.Z. Tokar, 1 B. Unterberg, 1 X. Loozen, 2 R.Zagorski, 1 D.Reiser, 3 Y.Andrew, 3 A.Korotkov, 1 S.Wiesen, TEXTOR and * JET contributors. 1 Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, EURATOM Association, D-52425 Jülich, Germany, partner in Trilateral Euregio Cluster 2 Institute of Plasma Physics and Laser Microfusion, EURATOM Association, 00-908 Warsaw, Poland 3 Euratom/UKAEA Fusion Asociation, Culham Sciense Centre, Abingdon, OX14 3DB, UK * Annex 1 of Pamela J. et al. 2005 Overview of JET results OV-1/1.2 Proc. 20 th IAEA Fusion Energy Conf. (Vilamoura, Portugal, 1-6 Nov 2004), Nucl Fusion 45 S63-85 e-mail: d.kalupin@fz-juelich.de In order do trigger the transition to the high confinement mode (H-mode), the power lost through the separatrix into the scrape-off layer (SOL), P L , estimated as the sum of the Ohmic and auxiliary heating power from which the time derivative of the plasma stored energy is substructed, should exceed some threshold value P th . A simple scaling for P th has been deduced from the experimental data collected on several tokamaks with divertors [1], 94 . 0 78 . 0 64 . 0 042 . 0 S B n P e th = , (1) where e n is the line averaged electron density in 10 20 m -3 , S is the plasma surface area in m 2 , and B is the toroidal magnetic field in T. 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 P sep / P th edge density, 10 13 cm -3 P L / P th low density limit Fig.1 Deviation of the L-H threshold power from the one predicted by the scaling law (1) at low densities measured in JET At the same time, there are many experimental situations where the conditions for the H-mode onset can not be described by the Eq.(1) [2-4]. In the present contribution the mechanisms of such deviations will be analyzed by means of predictive transport modelling for the cases of the L-H transition at low plasma densities and in limiter configurations. In both situations the power required to establish the H-mode is significantly lager than the one predicted by the multi- machine scaling (1). As an example, Fig. 1 shows the variation of P L normalized to the value predicted by Eq.(1) with the edge density measured in JET discharges with MKIIGB septum divertor [4]. When the density is below a limit value of 1.3×10 13 cm -3 , the H-mode power threshold exceeds the scaling prediction by a factor up to 2. The existence of the minimum in the L-H power threshold at low density is a phenomenon observed in many tokamaks. The low density limit blow which the power threshold increases with decreasing density can be related to the condition when the penetration depth of neutrals recycling from divertor plates is of the order of 0.1*κa, where κ is the elongation 33rd EPS Conference on Plasma Phys. Rome, 19 - 23 June 2006 ECA Vol.30I, P-2.160 (2006)