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.

Figure 1

(a) Schematic diagram depicting how a $2/1$ island, with a peaked internal density profile, is observed by the pulsed radar reflectometer. Because the mode rotates with the plasma, the microwave pulses are reflected alternating by the critical density layer in the island and by a density layer behind the island, causing a typical garland structure in the flight time measurements. (b) Density and temperature profiles measured with multiposition Thomson scattering in the same shot. The gray bars indicate the radial position of the $m=2$ island.

Figure 2

Top trace: measurements with a pulsed radar reflectometer system (operating at 39 GHz in $O$ mode). The reflections by the $2/1$ island are indicated by arrows. The other reflections are originating from a layer behind the main island. It is clearly seen that the time-of-flight diagram has a pronounced garland structure as in the case of the main island. This proves the presence of a chain of small structures with a peaked density profile. Bottom trace: temperature variation of the 142 GHz channel that is probing the layer just behind the $2/1$ mode. On top of the modulation of the large $2/1$ mode, one can see a much faster modulation caused by a chain of small structures.

Figure 3

Measurement with the pulsed radar reflectometer of a rotating $2/1$ island in the TEXTOR plasma. The white curve depicts the flight time of the radar pulses as a function of time. The colored spectrum gives the fluctuations in the flight time (indicative of density fluctuations) as a function of time.

Figure 4

Wavelet power spectra of the temperature fluctuations measured by means of ECE in the vicinity of the $O$ point and of the $X$ point of the $m/n=2/1$ islands.

Figure 5

Frequency spectrum of ECE measurements as a function of time. The red colors depict high activity, whereas blue/green colors depict low activity. The blue trace shows the ECE signal at 142 GHz (identical to the curve of Fig. 3). One can see (blue trace) that the rotation of the $2/1$ island reduces, while the frequency of the small-scale structures increases (the orange-yellow cloud). The circles indicate the frequency measurements with pulsed radar reflectometry. Because the pulsed radar reflectometer cannot see the small islands when the main $2/1$ mode is in front of it, only discontinuous measurements can be made.