Fundamental research in the field of plasma energetics provides a new opportunity to achieve high efficiency of thermionic converters (TIC). The interelectrode gap of such TICs contains the condensate of excited states (CES), consisting of up to 1000 excited Cs atoms in the form of a plasma crystal (Rydberg matter). In a laboratory TIC with Cs CES the efficiency of ~25 % was registered by emitter’s tem- perature ТЕ ~1600 К and collector’s temperature ТС ~700 К. A unique combination of low TE and high efficiency of the system provides a prospect to implement such systems in small nuclear energetics. The results were obtained with SSC RF – Institute for Physics and Power Engineering (Obninsk).