Non-linear interactions of drift-waves in quadrupole geometry

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Abstract
The UMIST quadrupole magnetic confinement system shows a well-developed spectrum of drift waves, which are anti-symmetric between opposite sides of the machine (they have one full wavelength around a closed line of force). The authors consider non-linear interactions of these modes, concluding that these should include a nearly resonant interaction of two drift-waves to give a symmetric drift-wave (with two full wavelengths around a closed line of force) at double the frequency. They show that the observed power-spectrum does show a frequency-doubling peak, and from observations of the bispectrum that it is linked by non-linear interactions with the drift-waves. The authors use the bispectrum as a probe of the eigenfunction at the frequency-doubled peak, and show from this that the resonant interaction described above can contribute only a small part of the total power, and that it must be suppressed relative to non-resonant interactions with other modes. They discuss possible reasons for this, and conclude that considerable caution must be exercised in identifying non-linear interactions, however plausible they may appear.