Magnetic fluctuation measurements in the thin resistive shell OHTE device operated as a reversed field pinch

A small set of magnetic pickup coils inside the thin resistive shell and an extensive set outside are used to monitor high frequency (3-50 kHz) MHD activity in a reversed field pinch discharge of duration long compared to the shell time constant. The MHD activity is dominated by nearly equal amplitude m = 0 and m = −1 poloidal modes both of whose frequency spectra peak near the drift frequency, i.e. about 8 to 10 kHz. The m = 0 and m = −1 activities are uncorrelated and exhibit strong turbulence; the autocorrelation times for both are about 40 μs, and the toroidal correlation lengths are about equal to the 20 cm shell minor radius b. The toroidal variation of the m = 0 activity at a given time when Fourier analysed in toroidal mode n has a power spectrum peaking at n = 4-5 or nb/R o ≈ 1. The n spectrum for m = −1 is nearly zero up to n = 10 and substantial at 11 ≤ n ≤ 16, i.e. those modes which are pitch resonant in the plasma interior. The aperiodic development of a localized helical kink, the slinky mode, stabilizes, i.e. turns off, in turn the m = −1 high frequency activities of the n = 11 and 12 modes. This is postulated to be due to the slinky mode flattening the gradient in μ near the axis (μ is the ratio between current density parallel to the magnetic field and the magnetic field strength).