Typical plasma discharge disruption. Temporal variation of (a) plasma current Ip , (b) loop voltage VL , (c) intensity of line, (d) electrode current, and (e) applied bias voltage. Without bias voltage (narrower line) and with bias voltage at −70 V (wider line).
Power spectra of typical Mirnov probe fluctuations at three phases: (a) pre-precursor phase (3–5 ms), (b) precursor phase (5–9 ms), and (c) disruption phase (13–17 ms) without bias. Precursor phase (lower power) is also shown in panel (c) for comparison.
Time evolution of frequency-filtered Mirnov coil signals at four poloidal locations in precursor phase (5–9 ms) with no bias. (a) Range 8.1–9.2 kHz indicates m = 3 and (b) range 12.5–13.2 kHz indicates m = 2 from spatial phase differences at same time instant (indicated by the vertical lines). (c) Mode numbers for cases (a) and (b) as the slope of the line that best fits the signal phases against the poloidal angle.
Mirnov signal phase as a function of poloidal angle when filtered at frequency of 9–10 kHz in the disruption phase with no bias. The slope of the best-fit line indicates a mode number m = 3.
Time evolution of frequency-filtered (9–10 kHz) Mirnov coil signals at two toroidal locations ( apart) but in same poloidal locations ( ) in disruption phase with no bias indicates toroidal mode number n = 1 from spatial phase differences at same time instant (indicated by the vertical line).
Model radial profiles for normalized current density and q(r) in precursor (dashed line) and disruption (dotted line) phases with no bias and for biased case (continuous line), showing the location of q = 2, 3 surfaces for each case and corresponding current.
Comparison of power spectra of typical Mirnov coil fluctuations in disruption phase (13–17 ms) with no bias (upper panel) and with −80 V bias (lower panel) for the same time interval. Notice the large difference in amplitudes of the peaks.
Time evolution of frequency-filtered (9–10 kHz) Mirnov probes signals at four poloidal locations with −80 V bias at (a) time interval of 9–17 ms, coincident with disruption phase when there is no bias, (b) time interval of 17–21 ms (phase differences at vertical lines indicate that the poloidal mode number is m = 2), and (c) plots the phases of the signals versus poloidal positions to get mode number through the slope of best fit line.
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