A Grid Lifetime Model for a 3-Grid Ion Engine

* Presented as Paper IEPC-01-84 at the 27 International Electric Propulsion Conference, Pasadena, CA, 15-19 October, 2001. † Copyright © 2001 by the Electric Rocket Propulsion Society. All rights reserved. A grid lifetime model is presented for the lifetime estimation of a 3-grid ion engine system. The failure mechanism considered in the model is electron backstreaming due to charge exchange ion erosion, which is fatal to the operation of the ion engine. The process which will be ended with electron backstreaming is described as follows: 1) charge exchange ion erosion enlarges accel grid hole diameter, 2) the enlarged accel grid hole diameter decreases the negative potential between the screen and accel grids, and 3) the negative potential becomes too small to expel the neutralizing electrons away from the discharge chamber. The above process is modeled with the space-charge-limited current density law and a newly introduced parameter called extended effective acceleration length. The characteristic of the model is that lifetime strongly depends on beam current and accel grid voltage. An evaluation was made to validate the model using a computer program and showed that the model agreed quite well with simulations and gave good scaling for grid lifetime.