Influence of the Langmuir Probe Shaft on Measuring Plasma Parameters

Abstract

AbstractBecause of their good spatial and temporal resolution Langmuir probes are often used for plasma diagnostics. One has to keep in mind, however, that the presence of the probe can substantially perturb the neighboring plasma.Apart from the probe tip, in particular the probe shaft may affect the measured values of temperature and density. We have studied this latter effect at the linear plasma generator PSI‐2 which provides a stationary plasma column of 2.6 m in length and 8 cm in diameter.Two single probes differently arranged were used in these experiments. In the first case the probes were in the same plane but tilted to each other in azimuthal direction. In the second arrangement the probes were separated by a distance along the magnetic field lines at the same azimuthal angle. One probe was kept at a fixed (but variable) position while the other was scanning the plasma radially.It is found that the electron temperature is hardly influenced by the presence of the second probe, but the electron density is decreased up to 30%. The results can approximately be described in the frame of a global particle model treating the probe shafts as additional plasma sinks. Most striking is the observation that large global reductions of the density are accompanied by only moderate local changes. This means that the perturbations caused by the probe are characterized by a large scale effect, possibly indicating anomalous diffusion. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)