The ratio of electron and ion saturation currents measured by probes embedded in the plasma-facing components (PFCs) in magnetised plasmas is typically lower than predicted by Langmuir theory. In the past, several works targeted various effects influencing the ion saturation current, which can be larger than expected, especially if the magnetic field is at a grazing angle with respect to the probe surface. In this contribution, we focus on the electron current, which can be reduced when the magnetic field line intercepting the probe passes through the magnetic pre-sheath of the nearby PFC. In such a case an effective potential well forms and repels a fraction of the electrons incoming from the plasma. Experimental results obtained by a tiltable limiter at the DITE tokamak are revisited and reproduced by means of 2D3V particle-in-cell simulations. The reduction of the electron current is indeed observed both in simulations and in experiment. This mechanism may also explain why some divertor biasing experiments did not produce flows in the divertor region.
Improved understanding of the ball-pen probe through particle-in-cell simulations