Observations of Multiple Magnetic Islands in the Core of a Reversed Field Pinch

We describe in this Letter the first measurement of multiple islands in the core of a reversed field pinch (RFP). These islands appear with current profile modification leading to magnetic fluctuation reduction in the Madison symmetric torus RFP. Magnetic island widths decrease to an unprecedented level, reducing the overlap of adjacent islands and allowing distinct islands to appear. The structures are observed in multichord measurements of soft-x-ray emissivity. The soft-x-ray data is validated with Poincaré reconstructions of the magnetic field structure in the core.

Figure 1

Safety factor profile for an RFP discharge. Dots indicate the location of some $m=1$ mode resonant surfaces.

Figure 2

Normalized amplitude of the two innermost resonant $m=1$ magnetic perturbations, $b_{\theta ,(1,6)}$ vs $b_{\theta ,(1,7)}$, for the PPCD shots in which one (red squares) or two (blue dots) SXR structures are present. Dashed lines represent constant mode energy (quadratic sum) contours. The grey area represents the region corresponding to standard discharges.

Figure 3

(a) Position of the simulated SXR hot region in a rotation period, in a poloidal cross section of MST. Four chords, relative to the SXR signals of panel (b), are shown. The cross (+) represent the magnetic axis. (b) Measured (black) and simulated (red) high-pass-filtered SXR signals for a plasma with a single central structure. Numbers refer to the SXR structure positions in frame (a).

Figure 4

(a) Wavelet spectra (in arbitrary units) of the SXR signal integrated along line of sight A and (b) line of sight B. Solid lines represent the frequency of the dominant magnetic mode, $f_{1,6}$, while dashed lines represent the first harmonic $2f_{1,6}$.

Figure 5

Magnetic perturbation amplitudes (a) and poloidal rotation frequencies (b). Band-pass-filtered signal of chord C in a frequency range around the dominant frequency (c) and the first harmonic (d).

Figure 6

Poloidal Poincaré plot. The inner island (red) is $m=1,n=6$, and the outer one (blue) is $m=1,n=7$.