Kinetic Study of Current-Voltage Characteristics and Plasma Behavior in Thermionic Energy Converters

Thermionic Energy Converters (TECs) are static devices that directly convert thermal energy into electrical energy, showing promise for space power and waste heat recovery applications. This study employs 1d3v Particle-In-Cell/Monte Carlo Collision (PIC/MCC) methods to investigate current-voltage characteristics and plasma kinetic behavior in both vacuum and cesium TECs. For vacuum TECs, we validated the PIC method accuracy by comparing with analytical solutions from Child-Langmuir law and Langmuir space charge theory. Analysis of electron phase space distributions under different operating modes (accelerating, flat-band, decelerating) revealed virtual cathode formation mechanisms and their impact on electron transport. For cesium TECs, we established a collisional-radiative model (CRM) with 52 excited states coupled with PIC/MCC framework, successfully simulating ignited mode operation. Results show cesium TECs exhibit "S-shaped" I-V curves with three modes: unignited (diffusion), transition, and ignited (arc). In unignited mode, the system behavior resembles that of vacuum TECs, primarily limited by space charge. During transition, electric field reversal occurs at the emitter surface, with the sheath transforming from electron sheath to ion sheath. In ignited mode, high-density plasma effectively neutralizes space charge, achieving plasma densities of 10²⁰ m^-3 and current densities approaching 30 A/cm², far exceeding that of vacuum mode. The arc initiation process under ignited conditions follows a similar process, with the Schottky effect further enhancing thermionic emission and transport current density. This research provides crucial theoretical guidance and numerical tools for TEC design optimization, establishing foundations for performance prediction and parameter control of thermionic energy conversion systems.