We describe measurements of non-linear interactions between drift waves and law-frequency flute-like modes in the UMIST quadrupole. We present experimental measurements of the bispectrum, and a l s ~ results from a technique of "amplitude correlation" which we believe provides a more direct way of determining the direction of the energy flow. On the basis of the results we propose a model olenergy flaw in the drift-wave spectrum: the observed spectrum contains regions af growth at high frequency and damping at low frequency, with flow dawn the spectrum via a cascade mediated by the flute-like modes.

We give a more extended disirsion of the method of 'amplitude correlalions' introduced by Crossley e! of for the study of nonlinear interaaions in plasma turbulence, and in panicularfordetermining thesenseofenergy flow throughaturbulentspe. Fora particular case, the'frequencydoubling' intbactiondrift wavesinthemagneticquadrupoleGOLUX, which tansfen energy from driving to driven modes, we present a rigorous jistification of the method: the driven mode amplitude lags that ofthe driving modes, as the intuitive interpretation woulil lead us to expect. We compare the theory with experimental results, and show that the method can, in principle, lead U) estimates of the damping rate of the driven mode and the nonlinear energy throughpi rare. A full determination of these quantities, however, requires funher information on the dispersion c u m of the driven mode, which is obrainable in principle from wave propagation experiments. Finally we show that the method requks many fewer data and less computational effort than the conventional method using the bispeer" and bimherence