Electrostatic Probe Characteristics in a Magnetic Field

Abstract

A wide variety of electrostatic probes are found to retain their exponential current-voltage characteristic in the presence of a strong magnetic field. Electrostatic (Langmuir) probes continue to be one of the most important and most widely used tools for making measurements in plasmas. Langmuir probes are routinely employed in order to measure plasma density, Ne, electron temperature, Te, and plasma potential, V,. A typical Langmuir probe consists of a bare metal electrode electrically biasable with respect to the machine ground as shown in Figure 1. The theory of current collection for biased electrodes of various different shapes (i.e., flat metal discs, cylindrical wires, or conducting spheres) is well understood in the limit of a magnetic field free plasma (Hoyaux, 1968; Laframboise and Parker, 1973; Schott, 1968). Langmuir probes are also routinely employed in strongly magnetized plasmas (Brown et al., 1971; Buchenauer and Molvik, 1988; Stangeby, 1982). Our understanding of current collection by biased electrodes in a magnetoplasma is in a much more primitive state. There has been a considerable amount of theoretical work aimed at describing the current flow across a magnetic field to a probe electrode in the presence of classical coulomb and neutral particle collisions (Niyogi and Cohen, 1973; Dote and Amemiya, 1964; Sanmartin, 1970; Sato, 1972; Laframboise and Rubenstein, 1976). Unfortunately, it is widely agreed that turbulent rather than classical processes are likely to dominate charged particle motion across a magnetic field (Bohm et al., 1949a; Stangeby, 1982). As a consequence of this, the classical probe theory in a magnetic field is highly suspect (Stangeby, 1982).