An analytical model has been developed for the lifetime estimation of a 3-grid ion engine grid system. In this model, grid system failure is considered to occur by electron backstreaming which is brought by the enlargement of accel grid inner radius by charge-exchange ion erosion. The electron backstreaming was modeled using measurable three parameters: the beam current, accel grid voltage and accel grid current. The characteristics of the model is that lifetime exhibits very strong dependence on beam current and the dependence is in inverse proportion to the third up to the fourth power of beam current. A computer program was used to validate the model and it was shown that the model is accurate and very easy-to-use for lifetime estimation.
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