Evidence for High- m Secondary Islands Induced by Large Low- m Islands in a Tokamak Plasma

Small-scale structures with high poloidal mode numbers ($m=10--20$) have been observed in the TEXTOR tokamak plasma with pulsed radar reflectometry and an electron cyclotron emission diagnostic, in conjunction with large $2/1$ and $1/1$ islands. The small islands have a peaked density profile, similar to that of the simultaneously observed large-scale $2/1$ islands. This together with the observation that high-frequency density and temperature fluctuations are very pronounced near the $X$ points of the large islands hints to a strongly perturbed magnetic topology around the $X$ points.