Tearing mode stabilisation by ECRH

Tearing modes are systematically observed in FTU plasmas with high edge safety factor and low density. The increase of plasma pressure that can be induced by ECRH close to the plasma centre allows studying the dynamics of these modes as a function of the poloidal beta and to compare the results with the predictions based on neoclassical MHD models. Furthermore, the flexibility of ECRH launching system allows to deposit the ECRH power near the magnetic island in order to act on the mode stability. Furthermore, toroidal beam steering allows current drive effects to be studied.

Time traces of central electron temperature, poloidal magnetic field fluctuations and ECRH power during tearing mode stabilisation. The ECRH is deposited near the magnetic island (r=13 cm). As the tearing mode is suppressed (dashed vertical line), a substantial increase of central heating is observed, and the sawtooth activity sets in. In comparison discharges with more central heating, the tearing mode amplitude stays constant or increases, resulting in a lower central temperature.

Main results:

- The amplitude of magnetic islands increases with plasma poloidal beta. The analysis of the mode dynamics (evolution of the mode rotation frequency and amplitude) shows a good agreement with the predictions of neoclassical tearing modes theory.

- Islands with different helicity (m/n=2/1 and 1/1) have been observed simultaneously in some cases, and the mechanisms of mutual coupling and wall braking have been identified.

-Tearing modes with m/n=2/1 have been stabilised by heating near the magnetic island; a subsequent improvement of central confinement has been observed. Stabilisation has been found to be very sensitive to ECRH location (within 1-2cm) and is obtained above a power threshold of about 100kW.

- Experiments using current drive gave preliminary indications that heating is the dominant effect for stabilisation.

- These experiments indicate that ECRH can be effectively used for neo-classical tearing mode stabilisation.

Experimental Reports