The Origin of the Negative Resistance Found in Langmuir Probe Characteristics

Abstract

Electrostatic (Langmuir) probes continue to be one of the most important and most widely used tools for making measurements in plasmas. Langmuir probes are employed routinely in order to measure plasma density, Ne, electron temperature, Te, and plasma potential, V,. A typical Langmuir probe consists of a bare metal electrode which can be inserted into the plasma and electrically biased with respect to the machine ground as shown in Figure 1. The theory of current collection for biased electrodes is well understood in the limit of a magnetic field free plasma (Hoyaux, 1968; Laframboise and Parker, 1973; Schott, 1968). As the probe is biased more and more negatively the (predominant) electron current which is collected can be observed to decrease. The plasma electron temperature is obtained from the bias voltage required to reduce the probe current by a factor l/e whereas the plasma electron density is proportional to the magnitude of the current itself. A typical Langmuir probe current versus voltage curve is shown in Figure 2. Occasionally however, one or more regions of negative resistance are seen in Langmuir probe characteristic curves. These perturbations are of interest because they were not predicted by theory and can distort the shape of the probe curves making them unusable. Examples are shown in Figure 3. The earliest explanation of this perturbation invoked magnetic field effects (Dote and Amemita, 1964) but it was shown subsequently that similar effects could be obtained in magnetic field free plasmas as well (Jones, 1983).