1992-08-03

Development of a Self-Consistent Kinetic Plasma Model of Thermionic Energy Converters 929427

The present work is aimed at developing a computational model of the interelectrode phenomena in thermionic energy converters which will be accurate over a very wide range of plasma conditions and operating modes. Previous models have achieved only moderate degrees of accuracy and, in a limited range, of validity. This limited range excludes a number of advanced thermionic devices, such as barium-cesium converters. The model under development promises improved accuracy in prediction of conventional devices and extension of predictive capability to advanced devices.
The approach is to adapt the “Converted Scheme”, or CS method, to the cesium vapor plasma diode. This method, developed at the University of Wisconsin- Madison, is an extremely efficient algorithm for the solution of charged-particle kinetic equations and has been successfully used to simulate helium RF glow discharges.
A challenge in making the CS practical for this application is the wide variation in characteristic time scales under converter conditions. Ordinarily the method would be limited to time steps on the order of the plasma period (∼0.1 to 10 nanoseconds), which is impractically small since about 10 microseconds will be required to reach steady-state. Currently, two methods are being employed to remedy this. The first incorporates an ‘Implicit Scheme’ which allows a basic time step of tens of plasma periods. The second is the so-called ‘Scale-Up’ procedure which is a method of extrapolating toward the converged steady-state condition. Both of these methods have been shown to be effective for helium RF discharges.
Work is proceeding to identify the best available values for cesium collision cross-sections and incorporate them into the model. The complex multi-stage ionization and recombination process in cesium is being modeled and will include the effect of diffusion of excited-state atoms. An efficient algorithm to treat charged-particle Coulomb collisions is also discussed.
Expected results include realistic calculated volt-ampere characteristics which will be compared with experimental data.

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