Predicted effects of parallel temperature gradients on the overestimation of TCV divertor target Langmuir probe Te measurements

During high recycling conditions and divertor detachment in the TCV tokamak, divertor target Langmuir probes never measure electron temperatures much below ∼5 eV. Code simulations using B2–EIRENE of these plasmas indicate that Te≈1 eV. This inconsistency has also been seen in other tokamaks. In this paper, the influence on a theoretical Langmuir probe VI-characteristic of a strong Te gradient parallel to the magnetic field is examined by using B2–EIRENE simulated Te and ne profiles and adopting a simple numerical approach to compute divertor target electron energy distribution functions. Standard fitting of these theoretically generated VI-characteristics results in a temperature very close to the predicted value at the highest densities corresponding to divertor detachment in TCV. At intermediate densities, the computed temperature at the inner target is found to be very sensitive to assumptions regarding the magnitude of the assumed electron collision mean-free-path.