Computationally studying H- extraction and beamlet formation: the impact of the plasma model

The ITER NBI requires H-/D- beamlets with a low divergence, to maximize the transmitted power through the beamline. Ion extraction and beamlet formation are typically studied with gun-type codes such as IBSimu, which do not treat the plasma explicitly. These codes neglect many physical processes, such as the surface production of negative ions, particle drifts in the plasma as a result of magnetic fields, and sheath formation at surfaces. 3D PIC simulations show that the Debye sheath, which forms between the plasma and the plasma grid, has an impact on the optics of negative ions extracted from the plasma. In this paper, a repelling potential around the plasma grid is implemented ad-hoc in the IBSimu plasma model. The impact of the sheath is investigated for the BUG-MLE grid system for negative ions that come from the plasma. An electron repelling sheath increases the H- extraction probability since fewer particles are lost on the plasma grid, however the extracted current density at the edge of the aperture is reduced. The sheath increases the perpendicular energy distribution of the extracted H-, because particles are reflected towards the extraction region. In the underperveant regime, the sheath decreases the divergence, since it prevents H- from being extracted at the edge of the aperture, and these particles are highly divergent in the underperveant regime. The divergence optimum is shifted towards lower extracted current density with the sheath. The sheath leads to a higher minimum divergence due to the increased perpendicular energy distribution in the extraction region and the changed current density distribution.