Plasma instabilities in the plume of a hollow cathode
Plasma instabilities are spatially characterized in the plume of a 20 A LaB6 hollow cathode using ion saturation probes. The wave measurements are analyzed using a continuous wavelet transform to decompose the signal into three types of oscillations: 50 kHz, 0.1-0.5 MHz, and 0.8-1.2 MHz. Measurements of the wave amplitude of the 50 kHz oscillation with position show that this instability is localized in the plasma plume and is interpreted as the source for the other instabilities in the plasma. The 0.1-0.5 MHz waves are shown to be dominant near the cathode and are interpreted using ion-acoustic soliton theory. The analytical description of a soliton is in good qualitative agreement the measurement; however, estimates of the width are 1 to 2 orders of magnitude smaller than the observations. Lastly, the waves from 0.8-1.2 MHz are shown to be qualitatively well described by a Gaussian wave packet, though the velocity measured from the dispersion of these waves is too large to be an ion-acoustic wave packet. Together, these measurements support the notion that a localized instability in the cathode plume gives rise to propagating instabilities composed of the natural modes of the plasma including solitons and wave packets.
