Research in light‐ion‐driven inertial‐confinement fusion is emphasizing high‐voltage, nonprotonic ions to obtain improved magnetic stiffness. Target preheat by coaccelerated proton impurities is a serious concern. A method is described which should essentially eliminate protonic preheat for applied magnetic field, magnetically insulated (applied‐B) ion diodes. Briefly, the charge state of the principal ion species is changed by passing through a thin foil in the applied magnetic field. Due to the change in canonical angular momentum, the ions’ closest approach to the axis of an axisymmetric diode is different from that of the proton contaminant. This difference provides effective separation of protons from the principal species.