This study includes the design of a fully demountable alkaline‐metal‐plasma machine. The plasma is produced by contact ionization of alkaline metal vapor on a refractory metal emitter. The properties of this plasma depend on three parameters: emitter temperature, cesium vapor pressure, and potential of an electrode which sets the electric boundary conditions for the medium. Three movable probes analyze the plasma. Measurements are made and results are discussed in terms of the parameters. The first part of the study is devoted to a description of the machine and the probes. The second part analyzes its different operating modes. There are two fundamental modes. The first covers the case where collisions are predominant. The second is a flux mode in which interactions are very improbable. The machine permits continuous transition from the first to the second mode. In the transition zone complex phenomena arise. The study is primarily concerned with an examination of the inflection potentials given by the second derivative of the current‐voltage characteristics of the probes. Interpretation of this inflection potential will differ greatly from one operating mode to another.