Application of triple probes to magnetized plasmas

Triple probes are often exposed to the outer layers of fusion devices as well as to other plasmas, mainly to make use of the better time resolution if compared with ramped single probes. But this type of probe puts its tips on fixed potentials with respect to the floating potential and, therefore, the evaluation of plasma parameters must rely on a certain functional structure of the underlying current-voltage characteristic. Furthermore, the interpretation of triple probe measurements is usually based on the assumption that all three tips are exposed to the same plasma. If this is not the case (as expected for strong gradients) or if the characteristics are distorted from the assumed form (as in strongly magnetized plasmas) the parameters derived are no longer what they are supposed to be. Several typical deviations from the simple interpretation of triple results are discussed, indicators for a malfunction are identified, and corrections are proposed, at least for some cases. (orig.)