Description
In this contribution, we investigate the mechanisms of collection of deuterium ions collected by charged surfaces, such as tokamak divertor tiles or electric probes. Especially in the case of probes, these effects usually influence the amount of collected current due to a phenomenon commonly known as the sheath expansion. Empirical counter-measures, employing four-parameter fits [1] or increasing the probe collecting area [2] are widely accepted solutions, but their operational space is usually limited [3] and although the interplay between electric and magnetic fields within the Debye and Chodura sheath is well understood, the quantification of the influence, especially the location of the transition between and the distribution of the potential drop between these two sheath regions, relies mostly on results given by a 1D fluid approximation [4].
Employing particle-in-cell modelling, we can take the opportunity to track individual particles, especially at the moment when the collection by the surface becomes inevitable. Identifying this location while knowing the exact state of the plasma within the simulation brings new insights to conditions that lead to the collection process. Summarizing results of a wide parametric scan that covered not only parameters of existing devices but also those of tokamaks underway, such as COMPASS Upgrade, we present new scalings of Chodura sheath potential drop as well as the Debye sheath thickness caused by the sheath expansion for grazing field angles down to 0.4°. Preliminary results also indicate that these corrections can be directly applied to analysis of probe measurements in order to approve accuracy of values of measured ion saturation currents.
Additionally, a section of the contribution is dedicated to the latest updates of the in-house developed SPICE2 model used in the calculations. This work covers especially reduction of memory footprint of the simulation enabling high-resolution simulations not only within this contribution, but also in related Czech Science Foundation and EUROfusion projects.
[1] Bergmann, Rev. Sci. Instrum. 89, 043512 (2018)
[2] A. Q. Kuang et al., Rev. Sci. Instrum. 89, 043512 (2018)
[3] M. Komm et al, Nucl. Fusion 62, 096021 (2022)
[4] P. C. Stangeby, Nucl. Fusion 52, 083012 (2012)