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A review is given of experimental work on the study of properties of a condensate of excited states (Rydberg matter) of cesium by electrophysical and optical methods. The work has been performed by the author with colleagues from the SSC RF-IPPE since 2002 up to the present, the results having been published in various domestic scientific editions. The findings are compared with theoretical data of various authors and experimental results of a group of L. Holmlid from the University of Gothenburg (Sweden), the only group except a group of V. Yarygin working in the field of experimental studies of Rydberg matter (RM) of cesium and other alkali metals. A concept is discussed of a practical use of RM in operation of a low-temperature thermal to electric energy thermionic converter (TIC) at the emitter temperature lower than 1,600 K, the collector temperature lower than 700 K, the generalized index of an energy loss in a TIC (so-called barrier index) V B *1.6 eV.Keywords Arc discharge of a thermal to electric energy thermionic converter Á Clusters Á Condensate of excited states of cesium Á Low-temperature cesium plasma Á Rydberg matter

A thermionic energy converter with a nickel collector and cesium vapor as a working gas was studied, and an abnormally low value of the surface work function of ≈1 eV was obtained if the collector was covered by a thin carbon layer. Scanning electron microscopy x-ray microanalysis data of the elemental composition of the collector's surface after its long exposure to plasma indicate that the carbon structure was intercalated with cesium atoms, and this change to surface structure can be a reason for the anomalously low work function ∼1 eV. The thermionic energy converter with such a collector demonstrated high heat-to-electric power conversion efficiency up to ∼20%.

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A preliminary experimental investigation is performed of a thermionic diode with a solid insulating medium between the electrodes. Aluminum, gallium, and indium oxide powders are used as the insulator. The current-voltage characteristics of the metal-insulator-metal structures, obtained by using metal oxides and in the presence of heat flow through the structure, are studied. Features due to electricity generation are observed on the experimental curves of the current versus the voltage. The influence of the type of conduction in the insulator on the position of these regions in the current-voltage plane is determined. Analysis confirms that the data obtained agree with the theory of current flow through a metal-insulator-metal structure.The efficiency of thermionic converters can be increased substantially by switching to small gaps. A converter with a collisionless cesium plasma in an interelectrode gap ~10 -3 cm is closest to adoption in practice. Making the gap smaller and switching to a vacuum regime could result in even greater improvement of the output electric characteristics, but a converter with a gap less 10 -4 cm is still difficult to make.An idea for producing a converter with a solid interelectrode gap by inserting a continuous insulator between the electrodes has been advanced in [1,2]. Since the insulating layer is thin (<10 -4 cm), the electrons will pass through it without scattering. The work function for electrons leaving the metal into the insulator, equal to the chemical potential of the insulator, will be less than the work function for electrons leaving the metal into vacuum. Thus, the problem is transferred into a new plane -development of a metal-insulator-metal structure with a ~10 -5 cm insulating layer.High-current structures of this type can be used in conventional thermionic converters, secondary-emission converters of fission-product energy [3], and in solid-state converters for converting shock-wave energy [4].The purpose of the present work was to make a preliminary experimental investigation of metal-insulator-metal structures.Experimental Arrangement and Procedure. The setup for determining the thermionic properties of metal-insulator-metal structures is shown in Fig. 1. A flat stationary electrode (emitter) is equipped with a heater. A cold electrode (collector) in the form of a small sphere, which moves in the vertical plane, is placed above it. A changeable insulating medium is placed in the gap between the emitter and collector. The electrodes are coated with a platinum foil in the zone of contact with the insulator. The temperature of the electrons was monitored with chromel-alumel thermocouples.Three insulating media were used in the experiments: aluminum oxide monohydrate Al 2 O 3 ·H 2 O in the form of conglomerates consisting of ~1 mm long and 20-50 nm in diameter fibers, gallium oxide Ga 2 O 3 and indium oxide In 2 O 3 in the form of powders with characteristic grain size ~200 nm. The impurity content in the insulating media was ≤10 -4 mass%.