Observation of Confined Current Ribbon in JET Plasmas

We report the identification of a localized current structure inside the JET plasma. It is a field-aligned closed helical ribbon, carrying current in the same direction as the background current profile (cocurrent), rotating toroidally with the ion velocity (corotating). It appears to be located at a flat spot in the plasma pressure profile, at the top of the pedestal. The structure appears spontaneously in low density, high rotation plasmas, and can last up to 1.4 s, a time comparable to a local resistive time. It considerably delays the appearance of the first edge localized mode.

Figure 1

Time traces showing a long-lived outer mode, starting from 14 s to 15.38 s, and its effect on plasma behavior: (a) rise in $D\alpha$; (b) rise in power to outer target; (c) drop in pedestal temperatures; (d) slower pedestal density rise, rising core density; (e) stationary energy; (f) toroidal rotation frequency at top and middle of pedestal; (g) fast Fourier transform of magnetic signal.

Figure 2

Mirnov coil signals from toroidal array (outboard), showing toroidal propagation of magnetic feature in cocurrent direction. The lowest plot is the integrated signal from the coil at 3°.

Figure 3

Comparison of simulated signals (colored/grey) with data (black) in Mirnov coils located at the plasma outboard (top row), and three inboard coils located above, near, and below the plasma midplane, respectively. Simulated signals are generated with rotating current ribbons located at $q=4$ (red), $q=3$ (green), and $q=2$ (blue) flux surfaces. The currents quoted in each case are adjusted to match calibrated signal amplitude.

Figure 4

$q=4$ spinning current ribbon that provides best fit to magnetic signals from Mirnov coil set, with 100–300 A.

Figure 5

Profiles of toroidal rotation frequency, $n_e$, $T_e$, $T_i$ at start (top three plots) and end (lower three plots) of OM. Vertical line mark position where mode and plasma rotation frequencies are equal.

Figure 6

Heat flux contours at outer strike, measured with IR, and integrated magnetic signal showing correlation of heat pulses with rotation of current structure.