Saturation and energy flow of drift waves in a steady-state plasma

The drift-wave spectrum excited in the plasma of the UMIST Quadrupole forms an ideal system for testing theories of non-linear saturation and energy flow. We present measurements of the bispectrum, and results from a new technique of 'amplitude correlations'. These are all consistent with a unidirectional flow of energy towards lower frequency through a series of decay interactions with low-frequency flute-like modes also present. We show that the shape of the spectrum, and, in particular, the unexpected stabilisation of high-frequency drift modes, find a possible explanation in this scheme. However, theoretical considerations predict that the non-linear interaction should be reversible, leading to a bi-directional energy flow to both higher and lower frequency. To agree with experiment we must postulate that the dominant interaction is irreversible, based on non-linearity in dissipative terms