Thermodynamic performance optimization for an irreversible vacuum thermionic generator Lingen Chen, Zemin Ding, Junle Zhou, Wenhua Wang and Fengrui Sun Eur. Phys. J. Plus, 132 7 (2017) 293 Published online: 04 July 2017 DOI: 10.1140/epjp/i2017-11561-2