THE ADVERSE EFFECT OF PERPENDICULAR ION DRIFT FLOW ON CYLINDRICAL TRIPLE PROBE ELECTRON-TEMPERATURE MEASUREMENTS

The cylindrical triple probe method is an attractive technique for measuring electron temperatures (T(e)) and electron number densities (n(e)) in a variety of plasmas sources. In practice, however, the cylindrical triple probe can be sensitive to sources of error that affect all Langmuir probe techniques. In particular, the presence of an ion drift velocity component,that is perpendicular to the probe axis has been known to result in erroneous measurements of n(e). Less obvious, however, is that ion flow perpendicular to the probe has a significant effect on the indicated T(e). The purpose of this note is to make researchers aware of such an effect and to demonstrate a technique which can mitigate it. The approach taken to investigate this phenomenon was to make T(e) measurements in the plume of a 20 kW magnetoplasmadynamic thruster with the probe oriented at several angles with respect to the local ion flow.