Influence of large biased surfaces on the plasma parameters in the extraction region of a negative hydrogen ion source

The neutral beam injection system in the upcoming fusion reactor ITER will use radio-frequency (RF), low-pressure, and high-power negative hydrogen/deuterium ion plasma sources. The extraction of the negative ions from the plasma is always accompanied by co-extraction of undesirable electrons, which can limit the source performance. This paper aims to investigate the effect of biased surfaces in the vicinity of the extraction on the plasma parameters towards the reduction of the co-extracted electrons. For this purpose, a 2D fluid plasma model is employed on a prototype plasma ion source where the biases of two different surfaces are varied independently. Since the self-consistent RF fluid plasma models can be computationally heavy and time-demanding, the study of the effect of the biased surfaces is used a simplified model where the RF heating is replaced with a power deposition profile, set as an input parameter. This procedure reduces the degree of freedom with more than one order of magnitude and, respectively, the computational time from tens of hours to several for one run of the model. To justify the method, the simplified model is first compared with a benchmarked RF self-consistent model on a smaller geometry. Both models produce similar results for the plasma parameters in the region of interest, namely, close to the extraction, which gives the confidence to use the simplified model for the purpose of the study. Applying a positive bias to big surfaces in the vicinity of the extraction region provides a higher electron density and plasma potential as well as a change in the electron flux orientation. However, the difference between the surface and the plasma potentials decreases more than 2 times and in combination with the reversed orientation of the flux, the electron current collected by the biased electrodes increases. The modelling results for the currents collected by the biased surfaces indicate similar behaviour as the experimental ones.