Practical Application of Wide Bandwidth Floating Emissive Probes and Wavelet Analysis to the X2 Nested Hall Thruster

Floating emissive probe plasma potential maps of the X2 dual nested channel Hall thruster across a triad of operating conditions (inner, outer, and dual channel) at 150 V discharge voltage are presented. The maps extend from the thruster exit plane and thruster centerline to 3.5 mean outer channel radii (OCR) axially and 2.8 OCR radially. Together with measured floating potential maps, electron temperatures and temperaturecorrected plasma potentials are also estimated over a slightly smaller domain, about 1.5 OCR axially by 1.5 OCR radially. Construction techniques for a compact quad emissive probe array with aspect ratio >100 are presented to address emissive probe mortality, and the design and characterization of a high bandwidth probe heating and measurement circuit with linear probe frequency response up to 100 kHz and sensitivity up to 1 MHz are given. These techniques are discussed in detail. Time-resolved fluctuation measurements show raw plasma potential oscillations (uncorrected for electron temperature) with RMS values from 10-30 V in the very near field over both discharge channels. These fluctuations largely die down within 0.5 OCR of the exit plane. The high bandwidth plasma potential measurements are used together with wavelet analysis to detect plasma potential oscillation spectra in the near field of the outer X2 discharge channel that match those corresponding to the breathing and rotating spoke modes detected by high speed imaging. The spoke oscillation spectral peaks are stronger in the very near field than the breathing mode peak, though downstream the breathing mode oscillation is dominant. Finally, a simple finite element model for emissive probe temperature as a function of heating current is included to assist in experiment design.