Interpretation of Langmuir, heat-flux, deposition, trapping and gridded energy analyser probe data for impure plasmas

The impurity level in the central plasma of fusion devices can be almost entirely controlled by the properties of the edge plasma. Progress on better understanding and control of the impurity problem requires improved diagnosis of the edge, including probes. The impurity fraction in the edge plasma is likely to be substantially greater than in the central plasma. The influence of high impurity levels on the interpretation of the probe data is examined. It is found that: (a) measurements of n e and T e using a Langmuir probe are not very greatly affected even by high levels of the type of impurities likely to be present in the edge plasma; (b) measurements of ion temperature using the combined Langmuir probe/heat flux probe technique can be compromised by the presence of impurities; (c) measurements of ion and electron temperature using a gridded energy analyser usually should not be compromised by the presence of impurities; (d) the measurement of hydrogenic density n DT , using a trapping probe should not be greatly affected by even large impurity levels provided that the hydrogen is not released during exposure by sputtering or surface heating; (e) increased sputtering due to the presence of impurity ions can pose practical problems.