The method of contact ionization is used to produce a three‐component plasma (Cs+, Cl−, e−) in theQ‐device BARBARA using a beam of Cesium chloride. The end‐plate temperature controls the concentration of Cl−, which can be varied in a wide range. The density of negative ions is obtained from the difference between the positive ion density given by a Langmuir probe and the electron density given by a microwave resonator. A high concentration of negative ions is achieved.