Langmuir probes as a tool to investigate plasma uniformity in a large negative ion source

The SPIDER experiment is the full size prototype of ITER heating neutral beam. It is equipped with two bi-dimensional arrays of Langmuir probes, covering the Plasma Grid (PG) surface and the Bias Plate (BP). These sets of diagnostics are routinely used during SPIDER operation and allow to investigate the properties of the plasma in the expansion region. The measurements are typically influenced by RF induced source oscillations, and a technique to extract average values is described. The dependencies of plasma parameters on power, pressure, magnetic filter field, plasma grid and bias plate biases are reported and used to assess the uniformity of the plasma, showing the presence of plasma drifts due to the magnetic field.