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Measurements with electric probes belong to the oldest as to the most often used procedures of the low-temprature plasma diagnostics.The method has been developed by Langmuir and his co-workers in the twenties.since then it has been subject of many extensions and of further developments in order to extend its applicability to problems with more general conditions as presumed by Lungmuir.the method of the Langmuir probe measurements is based on the estimation of the current voltage characteristics-thew so-called probe chracteristics-of a circuit consisting of two metallic electrodes that are both immersed into the plasma under study.Two cases are of interest: (a)the surface areas of both the electrodes being in contact with plasma differ by several orders of magnitude. (b)the surface areas of both the electrodes being in contact with plasma are very small in comparison with the dimensions of the vessel containing plasma and approximately equal to each other. case (a) is called 'the single probe method',case (b) 'the double probe methode'.

The range of the conventional Langmuir probe theory covers the conditions of the collisionless movement of charge carriers in the space charge sheath around the probe.further it is assumed that the sheath boundary is well-defined and that behined this boundary the plasma is completely undisturbed by the presence of the probe.this means that the electric field caused by the difference between the potential of the probe and the plasma potential at the place where the probe is located is limited to the volume inside the probe sheath boundary and does not penetrate it. The general theoretical description of the current to a Langmuir probe requires the simultaneous solution of the following fundamental equations:

• the poisson equation
• the collision-free Boltzmann(vlasov) equation
• the continuity equation
with regard to the respective boundary conditions at the probe surface and in the plasma.At a larger distance from the probe such solution must be identical to the solution which corresponds to the undisturbed plasma....

```M.Sc. student and researcher in Plasma Physics Research Centre,Tehran,Iran