Electrostatic potential normalised by potential of the positive outer ring (+30 V) along cylindrical axes, ρ (left) and z (right). For the vacuum case (blue line), the ring radii fraction, a, is identical to the SPIS model, and the absolute potential of the disks are taken from the output of the reference SPIS simulation (red circles). The associated analytical solution for the barrier potential, U_M, is marked with a dashed black line. The other SPIS simulations were simulated at identical conditions except with fixed potentials on the rings, and with different plasma Debye lengths (modulated by changing the plasma density or electron temperaturein each simulation).