Changes in the density profile due to the m=2 tearing mode in ASDEX

Resistive MHD tearing modes can develop magnetic islands near the rational magnetic surfaces, where q=m/n. In some situations, depending on the shape of the current profile and on the radial position of the q=2 surface, the (m=2, n=1) rotating structure of growing islands can come to rest ('mode locking'); in ASDEC, the so-called locked modes can be found for several plasma scenarios, and generally they are precursor to disruptions. The magnetic structure of rotating tearing modes in a tokamak plasma is quite well known from the measurements of the modulations of the poloidal magnetic field. We have studied the effects on plasma density of large amplitude m=2, n=1 modes in ASDEX, using broadband microwave reflectometry. (orig.)