Numerical reconstruction of Langmuir probe measurements obtained from the negative ion source for ITER (SPIDER)

The results of numerical studies on the plasma parameters in Source for the Production of Ions of Deuterium Extracted from Radio-frequency (RF) plasma (SPIDER) source are presented. The analysis was performed using the numerical code Fluid Solver for SPIDER in 2D (FSFS2D), which provides a self-consistent two-dimensional description of the source, including neutral gas flow, plasma chemistry, RF coupling in the driver, and plasma transport through the magnetic filter. The different particle species were described using separate continuity equations. The electron temperature is governed by the electron energy equation, whereas the plasma potential is described by the Poisson equation. The FSFS2D code was used to investigate the SPIDER source by performing several numerical scans. The results of the simulations were validated using the existing experimental databases of Langmuir probe measurements. The code not only reproduced the plasma profiles but also provided some information on the response of the plasma parameters to specific scans of the operational quantities. It was found that neutral depletion increased with an increase in the magnetic field in the filter, which is in qualitative agreement with the experimental data from SPIDER. Studies of the negative ion performance showed that an increase in almost all the considered operational parameters leads to an enhancement of the negative ion current. The degradation of the negative ion performance was observed only in the case of the RF power scan. However, the degradation of the negative ion current disappeared at sufficiently large plasma grid potentials. The first simulation results in cylindrical geometry indicate that rectangular geometry is more appropriate for reproducing experimental results from SPIDER. However, recent developments in the model led to results close to those of the experiment in the cylindrical case. A strong influence of the neutral and ion models on the results was observed, which calls for the inclusion of new equations into the model (ion and neutral temperature models and momentum equation for the gas dynamics).