3D visualisation of electrostatic potential structure for the SPIS concentric disks (a = 1.17 m, b = 1.25 m) simulation with V_B = +75 V, coloured by electrostatic potential. To illustrate the potential in the volume, we plot the potential along the X–Z plane, as well as 10 equipotential surfaces from −30 V to +25 V, cut in the X-Y plane. The potential of the outer disk is −45.1 V and 29.9 V for the inner disk.