This work examines a long duration Hall thruster start transient caused by the vacuum chamber environment. During operation of a cluster of four Hall thrusters, large anode discharge fluctuations, visible as increased anode current and a more diffuse plume structure, occur in an apparently random manner. For single thrusters, the transient appears as a smoothly decaying elevated anode current with a diffuse plume which persists for less than 500 seconds. The start transient is characterized by severe 18 kHz oscillations which dominate the anode discharge. This contrasts with typical steady state behavior of a strong DC component overlaid with a low amplitude 25 kHz component. The main discharge chamber has been previously determined to be the source of this behavior. The work shows that transient appears to be a result of the release of water previously hydrate d on to on the surface layer of the boron nitride acceleration channel insulator.
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