Measurement of the electrical properties of a flowing plasma, including a critical comparison of probe experiment and theory

The primary quantities characterizing the electricaJ carriers of a flowing plasma in a low density wind tunnel have been determined from measurements using electrostatic probes immersed in the plasma. With the exception of the ion temperature, the plasma parameters have been obtained from the current—voltage characteristics of two types of single electrode probe. The probes consist of a cylinder, the major axis of which is aligned parallel to the flow of the plasma, and a disk, the exposed surface of which is normal to the direction of flow. Experiments with a double electrode probe consisting of a disk-shaped collector electrode which is screened from direct exposure to the plasma by a fine wire mesh, grid electrode, made it possible to obtain current—voltage characteristics with the ion and electron components separated from one another. From the current—voltage characteristic corresponding to collection of ions, using the screen grid probe, values of the ion temperature and drift velocity have been obtained. The measurements have been made at various points along the centre line of flow, for one particular value of the flow rate using argon as the test gas. For a given position of the probes, one value of the ion temperature has been evaluated, together with two independent values of each of the other primary quantities characterizing the electrical carriers of a flowing plasma. Each pair of values agree satisfactorily amongst themselves, good agreement being generally obtained between probe theory and experiment.