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Experimental verification of the low-frequency drift instability in a magnetized ring discharge

The two-dimensional radial and azimuthal electron temperature and density distributions in a magnetized ring discharge (B = 100 - 190 mT, P(He) = 3 - 6 Pa, are obtained from measured intensity ratios of the HeI lines 447.15 nm, 471.31 nm, 492.19 nm and 587.56 nm, using a calibrated ICCD camera system. Dynamical plasma emission imaging (DPEI) represents a new diagnostic tool for studying not only the stationary parameters but also the electron temperature and density oscillations induced by a coherent low-frequency instability. The phase shift between temperature and density oscillations, as well as the radial profiles of the oscillation amplitudes, may be determined. These features characterize a prominent type of drift instability arising in an inductively coupled rf discharge with an axial magnetic field superimposed where the radial electron temperature and density gradients are in opposite directions. Numerical simulations recurring to the measured plasma densities and temperatures help to clarify the anomalous transport leading to a degradation of plasma confinement.