Vacuum potential and electric field pattern in the ρ−z plane around a thin circular disk of radius b as sketched in Fig. 7b. The disk potential inside ρ = a is V_in = −10 V while the potential V_out in the annulus a < ρ ≤ b varies as stated above each panel. In all cases, a = 0.98 b. Numerically integrated electric field lines are plotted in white. The 0 V equipotential is shown in thick red; the potential is zero also at infinity. Black curves indicate equipotentials at every integer value (in volts), with the background colour further highlighting the potential. The magenta star indicates the location of minimum electron barrier height.