Light ion diodes for producing 1–100 TW ion beams are required for inertial confinement fusion. The theory, numerical simulations, and experiments on a self‐magnetically insulated ion diode are presented. The treatment is from the point of view of a self‐magnetically insulated transmission line with an ion loss current and differs from the usual treatment of the pinched electron beam diode. The simulations show that the ratioV/IZ0=0.25 in such a structure with voltageV, local total currentI, and local vacuum wave impedanceZ0. The ion current density is enhanced by a factor of approximately 2 over the simple space‐charge limited value. The simulation results are verified in an experiment. An analytical theory is then presented for scaling the results to produce a focused beam of protons with a power of up to 1013W.