Evidence of Kelvin Helmholtz instabilities in the UMIST linear quadrupole

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The authors present observations of a new instability within the 'private' flux regions of the UMIST linear quadrupole. The private flux (pf) regions of all multipole configurations have in the past been universally observed as quiescent, but in the UMIST device they have observed low frequency (f 10 kHz) 10-30% density fluctuations. They believe these result from the plasma injection mechanism which leads to discontinuties in the plasma drift velocity across the pf layer, and thus to the formation of Kelvin-Helmholtz fluid instabilities. They present experimental results of a flute-like instability propagating along a shear layer perpendicular to the magnetic field. Dispersion analysis reveals two branches to the dispersion curve with the mode's maximum intensity occurring at their intersection. The lower 'forward' branch is linear and is identified as the primary mode, whilst the upper 'reverse' branch (secondary mode), displays a reciprocal relation. Theoretical studies suggest the pf regions to be stable toward the classical MHD flute instability. However, an estimate of the effects of the stabilizing field curvature and destabilizing velocity shear indicate a marginal stability towards a combined K-H flute mode.