A design for a thermionic converter is described which utilizes a permeable heat shield and a homogeneous magnetic field normal to the cathode and anode surfaces to minimize the radiant power loss without appreciably decreasing the electrical power output. The permeable heat shield consists of thin, close- spaced foils perpendicular to the cathode and anode surfaces. Radiant power transfer is expressed as a function of the spectral emissivities of the cathode and anode surfaces and the interfoil and interelectrode spacings. The heat shield is biased positive with respect to the cathode and anode surfaces to prevent positive ion loss, and, in conjunction with the normal homogeneous magnetic field, channels the electrons and ions to the anode. The required heat- shield positive bias is deduced and given as a function of the parameters of the converter. The energy conversion efficiencies attainable with this converter appear to be close to the Carnot efficiency. (auth)