Langmuir probe
The Langmuir probe , named after Irving Langmuir , is an experimental arrangement that is used to characterize a plasma . They can be used to determine electron density , electron temperature , floating and plasma potential . The method was first described in 1923 and is still the subject of probe research in the field of plasma diagnostics .
construction
In general, a Langmuir probe consists of a wire made of tungsten or molybdenum encased in a ceramic. Only the probe tip is not insulated.
functionality
The probe is inserted into the plasma at one point and a voltage is applied. In practice this is a sawtooth voltage . The resulting current is measured using a measuring resistor .
As a result, a characteristic curve of the plasma can be recorded and conclusions can be drawn from this about the potential distribution in the plasma. The electron and ion density, the electron temperature, floating and plasma potential as well as the electron energy distribution can be calculated from the characteristic curve . However, the probe represents a local disturbance of the plasma, which is why the measured values do not correspond to the exact parameters at this point.
If the probe is strongly negatively biased, positively charged ions are attracted and electrons cannot reach the probe. This voltage range is called the ion saturation range, in the ideal case the current is constant here. The ion current can be determined here.
If the negative voltage of the probe becomes smaller, more and more hot electrons (which due to their small mass are much faster than the inert and cold ions) can reach the probe. The I (U) curve merges into an exponentially growing area (if the electron gas has a Maxwell-Boltzmann distribution ), the so-called electron start-up area. At a characteristic voltage, the so-called floating potential, the ion and electron currents are equal, so the current is zero overall. This would be the voltage to which the probe tip would charge itself in the plasma. Above this voltage the ion current is practically zero, the electron current increases exponentially. The electron temperature can be calculated from the exponent of this increase.
At another, characteristic voltage, the exponential increase in the electron current breaks off. The electron saturation area begins here. The characteristic voltage is called plasma potential and corresponds to the electrical potential of the plasma at this point. In the ideal case, the current would be constant again from here onwards; in reality, the course of the characteristic curve, especially in the electron saturation range, depends heavily on the probe geometry (spherical probe, cylinder probe, flat probe).
The plasma parameters can be obtained by fitting a model to the I (U) characteristic. There are a number of such models that are more or less valid depending on the high frequency power, plasma density and gas pressure.
literature
- M. Kaufmann: Plasma Physics and Fusion Research. 1st edition. Vieweg + Teubner Verlag, 2003, ISBN 978-3-519-00349-6 , pp. 237-239.