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Schematic of CHT.
Sequence of camera images (color added) showing a spoke of increased light emission propagating in the direction.
The frequency-wavenumber spectrum is measured using two azimuthal probes separated by 90°. (a) 5 mm from the anode. (b) 17.5 mm from the anode.
(a) At a threshold in the keeper current of the hollow cathode, drops sharply. The spoke is visible for (filled circles), and is not visible at (open circle). The reduced at corresponds with a somewhat less coherent spoke, indicating nonmonotonic behavior of the discharge with increasing keeper current. Results shown are typical. (b) Using a filament cathode, drops sharply at a threshold in the filament current. The spoke is visible at (filled triangle), and is not visible for (open triangles). Results shown are typical. (c) Power spectral density of the discharge current. High frequency oscillations of order are excited at high keeper current when the hollow cathode is used and high filament current when the filament cathode is used. In this separate experiment, the spoke is visible at and is not visible at ; the difference in threshold is due to variations in filament and filament location.
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