Langmuir Probes as a Tool to Investigate Plasma Uniformity in a Large Negative Ion Source

The Source for the Production of Ions of Deuterium Extracted from an RF plasma (SPIDER) experiment is the full-size test-bed of the negative ion sources of ITER neutral beams. It aims to extract and accelerate more than 350 A/m $^{2}$ of negative hydrogen ions from 1280 apertures, distributed over a surface of approximately 2 m $^{2}$ . To assess plasma uniformity, SPIDER is equipped with bi-dimensional arrays of Langmuir probes, covering the plasma grid (PG) and the bias plate (BP) plasma-facing surfaces. These sets of diagnostics are routinely used during the ion source operation and allow to investigate the plasma properties in the so-called source expansion and extraction region. The dependencies of plasma parameters on source operation parameters such as RF power, pressure, magnetic filter field, PG, and BP biases are reported and used to evaluate the uniformity of the plasma, showing the presence of plasma drifts due to the magnetic field. Finally, the correlation between plasma and beam uniformity is assessed.