Understanding thermionic current enhancement and discharge mode transitions driven by ion trapping in virtual cathodes

Recent simulations [1] and experiments [2] show that ion trapping in nonplanar virtual cathode (VC) potential wells can raise the transmitted electron current above the traditional space charge limit, closer to the full available emission. Optimizing the trapped ion current enhancement could be valuable to technologies that rely on thermionic cathodes. To this end, a self-consistent hot cathode sheath model with trapped ions is formulated [3]. Our poster will explore the role of various conditions on current enhancement and offer testable predictions for further experimental validation. A key prediction is for a small emitting cathode in a large pre-existing plasma with fixed upstream density, the current as a function of emitted flux does not saturate when a VC forms but can reach far higher steady values and eventually become oscillatory. If the hot cathode is responsible for upstream plasma generation, such as in a hot filament discharge, then new couplings between the upstream plasma and trapped ion region occur because they feed the growth of each other while competing for space [4a,b].



[1] G. R. Johnson and M. D. Campanell, PSST 30, 015003 (2021). [2] C.-S. Yip, et al., Phys. Scripta 98, 025012 (2023). [3] Z. L. Idema and M. D. Campanell, arXiv:2501.00662. [4] (a) M. Sengupta and M. D. Campanell, PoP 32, 063503 (2025) and (b) J. App. Phys. 137, 173304 (2025).