Dynamics of tearing modes during strong electron cyclotron heating on the FTU tokamak.

The localized electron cyclotron resonance heating power that can suppress sawteeth reconnection often drives m 2 tearing modes in a tokamak operating at constant current. The dynamics of mode onset and coupled mode evolution is described in detail and compared with a nonlinear theoretical model that identifies the effects of mode coupling, finite inertia of the rotating islands, and wall braking. PACS numbers: 52.35.Py The observation of enhanced magnetohydrodynamic (MHD) fluctuations of the tearing type affecting the plasma energy transport in additionally heated tokamaks stimulates great interest in gathering experimental knowledge and understanding of the detailed mechanisms of these instabilities for finding a possible strategy for their active control in resistive and neoclassical regimes. In this Letter, we report the experimental evidence on the FTU tokamak and interpretation of two significant types of response of low order resistive MHD perturbations to the effects of temperature profile changes induced by localized heating. In one case the sawteeth instability is suppressed and an isolated m 2 rotating tearing mode is driven; in another case m 1 and m 2 tearing modes are driven. We identify for the first time the detailed mechanism governing the rotation of toroidally coupled and uncoupled magnetic islands, associated to finite inertia and wall braking. High power electron cyclotron resonance heating (ECRH) is used for controlling the MHD activity by fine adjustments of the position of the rf power absorbing layer [1,2]. The fundamental resonance scheme at 140 Ghz was used. A power P EC 800 kW was injected in plasmas with major radius R0 0.97 m, minor radius a 0.27 m, toroidal magnetic field B 4 6 T, low plasma current (Ip 350 kA) and high values of the safety factor qa 6. At these qa values, sawtooth relaxations are small or absent, while MHD oscillations with poloidal number m 2 are observed in many cases. As a consequence of a moderate reshaping of the current density profile and of a substantial increase of the plasma pressure induced by ECRH near q 1, MHD oscillations near q 2 are strongly amplified (Fig. 1). The oscillations are detected by a fast electron cyclotron emission (ECE) multichannel polychromator, by an array of detectors for soft-x-ray tomography, and by a set of Mirnov coils. Cross-correlation analysis shows that the poloidal and toroidal periodicity of these fluctuations is m 2, n 1 across most of the plasma section. In some cases, an m 1, n 1 component is detected in the central region. As shown in Fig. 2, the poloidal pattern of softx-ray emissivity has an even m periodicity that is confirmed by Mirnov coils signals to be m 2. The phasing between the oscillations in the ECE and soft-x-ray diagnostics, placed at different toroidal angles, is consistent with an n 1 toroidal order. The fast measurements of the electron temperature profile (Fig. 3), taken at different times during the island rotation, show a change in the slope around rq2 as the measurement happens to be alternatively on or near the “X” point (t 0.66035 and t 0.6592) and the “O” point during rotation. A