Characteristics of plasma fluctuations at the frequencies 10-100 kHz in Hall thrusters were investigated since the fluctuations might affect not only its performance but also its lifetime. Measured fluctuation characteristics varied with magnetic flux density of the applied field and they were categorized into the four regimes. Since the electron mobility strongly depends on applied field strength, it was thought that electrons would play an important role on this oscillation. Therefore, the oscillation model derived from ionization instability considering electron mobility was proposed. The predicted stable/unstable operating condition map agreed well with the measured one.
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