Langmuirprobes

A Langmuir probe is a diagnostic electrode used to measure key properties of a plasma, such as electron temperature, electron density, and plasma potential. It consists of a conducting wire or small metal tip inserted into the plasma and biased with a variable dc voltage relative to the surrounding plasma.

The measurement relies on the current-voltage I-V characteristic that results when the probe is biased across a range that spans negative to positive voltages relative to the plasma potential. At large negative biases, ions are collected (ion saturation current); at large positive biases, electrons are collected (electron saturation current). The floating potential is the bias where the net current to the probe is zero. The slope and shape of the I-V curve near the transition region provide information about the electron temperature, while the magnitude of the currents in the saturation regions, together with geometric factors, gives an estimate of the electron density. In practice, often a swept voltage is applied and the measured current is fitted to standard probes models to extract Te and ne.

Variants include the single-probe, double-probe, and triple-probe configurations, as well as planar, cylindrical, and miniature forms. In magnetized or high-frequency plasmas, or in RF discharges, specialized techniques and compensation methods are employed to mitigate perturbations and to obtain reliable measurements. Langmuir probes are widely used in laboratory plasmas, fusion devices, space plasma experiments, and industrial plasma processing.

Limitations include perturbation of the local plasma by the probe, the need for the Debye length to be small compared with probe dimensions, and sensitivity to plasma conditions such as collisions, magnetic fields, and RF noise. Proper interpretation requires careful calibration and consideration of the local plasma environment.