Experimental and theoretical study of drift-waves in a quadrupole

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Abstract
Spontaneous oscillations observed in the UMIST steady-state quadrupole are identified as drift waves with one full wave-length around a closed line of force, and with the potential perturbation anti-symmetric about the maximum field point. Existing theories do not properly describe this mode, for which trapped particles are important. The authors develop the theory of the mode and solve the resulting eigenvalue equation to obtain the dispersion curve; the result agrees with experimental evidence within experimental error. The excitation is identified as due to the 'dissipative trapped particle' mechanism, and the corresponding theoretical growth rate also agrees closely with experiment. In their conditions the excitation is due to electron-neutral rather than electron-ion collisions; as a result the temperature gradient is destabilising rather than stabilising, and they show that this also agrees with experiment.