Density dependence of the tearing mode threshold

The excitation threshold of the m/n=2/1 tearing mode has been studied by applying the external perturbation field induced by the Dynamic Ergodic Divertor (DED) on TEXTOR. A sudden drop in the threshold dependence on the density has been observed for the first time. Present scalings of the error field mode threshold do not explain the observed behaviour. Introduction - The TEXTOR tokamak is equipped with the Dynamic Ergodic Divertor (DED) which consists of 16 helical coils mounted at the high-field side of the torus. The DED can generate either static or rotating perturbation fields with frequency up to 10 kHz with the funda- mental mode numbers m/n =12/4,6/2 or 3/1, here m and n are the poloidal and toroidal mode numbers. Previous experimental results show that the DED in the 3/1 configuration is a good tool to study the excitation of the 2/1 tearing mode due to the presence of a large 2/1 sideband (1). Additionally, two neutral beam injectors provide momentum input in co-current as well as in counter-current direction allowing to disentangle rotation and beta effects. The established scaling of the locked mode threshold scales proportional to the density with a slightly different exponential depending on the machine, e.g. 0.58 at JET (2) or 1 at Alcator C-Mod (3). The error field correction coil system on ITER is based on the dependency of the mode threshold. Experiment - Recently the dependencies of the threshold of the 2/1 tearing mode excitation on the plasma density, rotation velocity and beta have been studied systematically by applying the DED on TEXTOR. The m/n=2/1 mode excitation threshold has been determined by a current ramp-up of the DED in an ohmic heated plasma with densities between 0.8 and 4·1019m 3. The experimental results from this density scan show a sudden drop of the mode threshold at an electron density of ne = 2·1019 m 3 indicated in Fig. 1. The first branch at lower density is comparable to the scaling of JET in L-mode plasmas at low density (2). The second branch at higher density, the linear rise, resembles the results from Alcator C-Mod (3). In between both regimes is a sudden drop in the excitation threshold. The dependence of the excitation threshold on the density has been investigated in detail by adding small amounts of co- and/or counter neutral beam injection while keeping a constant total input power of 250 kW, thus only