Langmuir Probes

Abstract

A Langmuir probe is small conductor that can be introduced into a plasma to collect ion or electron currents that flow to it in response to different voltages. The current vs. voltage trace, called the I–V characteristic, can be analyzed to reveal information about n, T _e, V _s (space potential), and even the distribution function f _e(v), but not the ion temperature. Since the probe is immersed in a harsh environment, special techniques are used to protect it from the plasma and vice versa, and to ensure that the circuitry gives the correct I –V values. The probe tip is made of a high-temperature material, usually a tungsten rod or wire 0.1–1 mm in diameter. The rod is threaded into a thin ceramic tube, usually alumina, to insulate it from the plasma except for a short length of exposed tip, about 2–10 mm long. These materials can be exposed to low-temperature laboratory plasmas without melting or excessive sputtering. To avoid disturbing the plasma, the ceramic tube should be as thin as possible, preferably < 1 mm in diameter but usually several times this. The probe tip should extend out of the end of the lube without touching it. so that it would not be in electrical contact with any conducting coating that may deposit onto the insulator.