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A model of the direct photoelectric conversion of concentrated solar radiation in a plasma ignited in a heat pipe filled with a mixture of sodium vapor and krypton is developed. The model considers the non-homogeneous distribution of the alkali atom density in the heat-pipe volume and the thermionic effect of a cathode. The model treats a hot plasma core in a local thermal equilibrium (LTE) state and takes into account nonequilibrium layers near the converter walls. The model is employed to calculate an opencircuit voltage, a plasma resistance, a short-circuit current, an energy flux of positive ions directed toward the cathode, and a conversion efficiency of the solar radiation. Two different approaches were used to estimate a value of the electron temperature in the ionization non-equilibrium layer near the cathode. We assumed within the framework of an isothermal approximation that the electron temperature in the ionization layer near the cathode is equal to the temperature of the LTE plasma. This isothermal model predicted a rather low value (approximately 3 %) for the conversion efficiency. We found within the framework of a two-temperature model that the reduction of the electron temperature by 20 % compared with the LTE plasma temperature took place at the outer boundary of the ionization layer near the cathode. This non-isothermal model predicts a rather high value (approximately 33 %) for the conversion efficiency for a 3009 solar radiation concentration ratio.

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