Measurement principle
The Langmuir probe is a probe consisting of a conductive electrode that is inserted into a plasma in order to obtain the current-voltage characteristic curve by applying different voltages and measuring the resulting current. This characteristic curve can then be used to calculate the electron density, electron temperature, and plasma and floating potential, as the measured currents change with the applied voltage and particle movement in the plasma.
The setup includes a conductive electrode. In our product, the standard is a 50 µm tungsten wire. However, any conductive material can be used. A second electrode measures the floating voltage and, in high-frequency plasmas, also compensates for the high frequency together with the rest of the setup. An insulator separates the electrodes and the rest of the setup. Finally, the voltage is applied and the current is measured.
Since plasma consists of charged particles (ions and electrons), a current flows to the probe, which changes depending on the applied voltage. At very low (negative) voltages, electrons are repelled by the probe and only the heavier ions are collected. As the voltage is increased (positive), more and more electrons can reach the probe. Finally, the probe reaches a potential at which the electrons no longer repel the probe and the current reaches its saturation value.
The measured current-voltage characteristic curve forms the basis for calculating the plasma parameters.
The Langmuir probe is an important diagnostic tool that enables detailed characterization of a plasma thanks to its simple handling but complex evaluation.